Understanding and repairing the power supply from a 1969 analog computer

We recently started restoring a vintage1 analog computer. Unlike a digital computer that represents numbers with discrete binary values, an analog computer performs computations using physical, continuously changeable values such as voltages. Since the accuracy of the results depends on the accuracy of these voltages, a precision power supply is critical in an analog computer. This blog post discusses how this computer's power supply works, and how we fixed a problem with it. This is the second post in the series; the first post discussed the precision op amps in the computer.

The Model 240 analog computer from Simulators Inc. was a "precision general purpose analog computer" for the desk top, with up to 24 op amps. (This one has 20 op amps.)

Analog computers used to be popular for fast scientific computation, especially differential equations, but pretty much died out in the 1970s as digital computers became more powerful. They were typically programmed by plugging cables into a patch panel, yielding a spaghetti-like tangle of wires. In the photo above, the colorful patch panel is in the middle. Above the patch panel, 18 potentiometers set voltage levels to input different parameters. A smaller patch panel for the digital logic is in the upper right.

The power supply

The computer uses two reference voltages: +10 V and -10 V, which the power supply must generate with high accuracy. (Older, tube-based analog computers typically used +/- 100 V references.) The power supply also provides regulated +/- 15 V to power the op amps, power for the various relays in the computer, and power for the lamps.

The power supply in the bottom section of the analog computer. The transformer/rectifier section is on the left and the regulator card cage is on the right. Wiring harnesses on top of the power supply connect it to the rest of the computer.

The photo above shows the power supply in the lower back section of the analog computer. The power supply is more complex than I expected. The section on the left converts line-voltage AC into low-voltage AC and DC. These outputs go to the card cage on the right, which has 8 circuit boards that regulate the voltages. The complex wiring harnesses on top of the power supply provide power to the five analog computation modules above the power supply as well as the rest of the computer.

With a vintage computer, it's important to make sure the power supply is working properly, since if it is generating the wrong voltages, the results could be catastrophic. So we proceed methodically, first checking the components in the power supply, then testing the power supply outputs while disconnected from the rest of the computer, and finally powering up the whole computer.

The transformer / rectifier section

We started by removing the power supply from the computer, and disconnecting the two halves. The left half of the power supply (below) produces four unregulated DC outputs and a low-voltage AC output. In contains two large power transformers, four large filter capacitors, stud rectifiers (upper back), smaller diodes (front right), and fuses. This is a large and very heavy module because of the transformers.2 The smaller transformer powers the lamps and relays, while the larger transformer powers the +15 and -15 volt supplies as well as the oscillator. Presumably, using separate transformers prevents noise and fluctuations from the lamps and relays from affecting the precision reference supplies.

This section of the power supply reduces the line-voltage AC to low-voltage DC and AC.

One concern with old power supplies is that the electrolytic capacitors can dry out and fail over time. (These capacitors are the large cylinders above.) We measured the capacitance and resistance of the large capacitors (using Marc's vintage HP LCR meter) and they tested okay. We also checked the input resistance of the power supply to make sure there weren't any obvious shorts; everything seemed fine.

We removed all the cards from the card cage, cautiously plugged in the power supply, and... nothing at all happened. For some reason, no AC voltage was getting to the power supply. The fuse was an obvious suspect, but it was fine. Carl asked about the power switch on the control panel, and we figured out that the switch was connected to the power supply via the socket labeled "CP" (below). We added a jumper, powered up the supply, and this time found the expected DC voltages from the module.

The side of the power supply has three twist-lock AC sockets labeled "FAN", "DVM-LOGIC", and "CP" (control panel). The "DVM-LOGIC" socket powers a 5-volt supply for the digital logic, which we still need to repair.

The regulator cards

Next, we tested the power supply's various cards individually. The power supply has four regulator cards generating "lamp voltage", "+15", "-15", and "relay voltage". The purpose of a regulator card is to take an unregulated DC voltage from the transformer module and reduce it to the desired output voltage.

We hooked up the regulator cards using a bench power supply as input to make sure they were working properly. We tweaked the potentiometer on the +15 V regulator to get exactly 15 V output. The -15 V regulator seemed temperamental and the voltage jumped around when we adjusted it. I suspected a dirty potentiometer, but it settled down to a stable output (narrator: this is foreshadowing). We don't know what the lamp and relay voltages are supposed to be, and they're not critical, so we left those boards unadjusted.

One of the voltage regulator cards. A large power transistor is attached to the heat sink.

The photo above shows one of the regulator cards; you might think it has a lot of components just to regulate a voltage. The first voltage regulator chip was created in 1966, so this computer uses a linear regulator built from individual components instead. The large metal transistor on the heat sink is the heart of the voltage regulator; it acts kind of like a variable resistor to control the output. The rest of the components provide the control signal to this transistor to produce the desired output. A Zener diode (yellow and green stripes on the right) acts as the voltage reference, and the output is compared to this reference. A smaller transistor generates the control signal for the power transistors. In the lower right, a multi-turn potentiometer is used to adjust the voltage output. The larger capacitors (metal cylinders) filter the voltage, while the smaller capacitors ensure stability. Most power supply of just a few years later would replace all of these components (except the filter capacitors) with a voltage regulator IC.

The chopper oscillator

The precision op amps in the analog computer use a chopper circuit for better DC performance, and the chopper requires 400 Hertz pulses. These pulses are generated by the oscillator board in the power supply (called the gate for some reason). We powered up the board separately to test it, and found it produced 370 Hz, which seemed close enough.

The gate card provides 400 Hertz oscillations to control the op amp choppers.

The circuitry of this card is somewhat bizarre, and not what I was expecting on an oscillator card. The left side has three large capacitors and three diodes, powered by low-voltage AC from the transformer. After puzzling over this for a bit, I determined it was a full-wave voltage doubler, producing DC at twice the voltage of the AC input. I assume that the chopper pulses needed to be higher voltage than the computer's +15 volt supply, so they used this voltage doubler to get enough voltage swing.

The oscillator itself (right side of the card), uses one NPN transistor as an oscillator, and another NPN transistor as a buffer. It took me a while to figure out how a single-transistor oscillator works. It turns out to be a phase-shift oscillator; the three white capacitors in the middle of the board shift the signal 180°; inverting it causes oscillation.

The op amps

Calculations in the analog computer are referenced to +10 volt and -10 volt reference voltages, so these voltages need to be very accurate. The regulator cards produce fairly stable voltages, but not good enough. (While testing the regulator cards, I noticed that the output voltage shifted noticeably as I changed the input voltage.) To achieve this accuracy, the reference voltages are generated by op amp circuits, built from two op amp boards and a feedback network card.

An op amp card. This card has a single input on the right. It uses a round metal-can op amp IC, but the chopper circuitry improves performance.

Somewhat surprisingly, the op amp cards used in the power supply are exactly the same as the precision op amps used in the analog computer itself. Back in 1969, op amp integrated circuits weren't accurate enough for the analog computer, so the designers of this analog computer combined an op amp chip with a chopper circuit and many other parts to create a high-performance op ap card. I described the op amp cards in detail in the first post, so I won't go into more detail here.

The network card

The network card has two jobs. First, it has precision resistors to create the feedback networks for the power supply op amps. Second, it has two power transistors (circular metal components below) that buffer the reference voltages from the op amp for use by the rest of the computer.

The network card. The two connectors on the left are attached to the op amp inputs.

One of the problems with an analog computer is that the results are only as accurate as the components. In other words, if the 10 volt reference is off by 1%, your answers will be off by 1%. The result is that analog computers need expensive, high-precision resistors. (In contrast, the voltages in a digital computer can drift a lot, as long as a 0 and a 1 can be distinguished. This is one reason why digital computers replaced analog computers.) Typical resistors have a tolerance of 20%, which means the resistance can be up to 20% different from the indicated value. More expensive resistors have tolerance of 10%, 5%, or even 1%. But the resistors on this board have tolerance of 0.01%! (These resistors are the pink cylinders.) The two large resistors on the left are 15Ω "Brown Devil" power resistors. They protect the voltage outputs in case someone plugs the wrong wire into the patch panel and shorts an output, which would be easy to do.

The network card receives an adjustment voltage from the control panel, and also has multi-turn potentiometers on the right for adjustment (like the regulator cards). The green connectors are used to connect the network card to the op amp cards. (The op amps have a separate connector for the input, to reduce electrical noise.)

Powering it up and fixing a problem

Finally, we put all the power supply boards back in the cabinet, put the power supply back in the computer, and powered up the chassis (but not the analog computer modules). Some of the indicator lights on the control panel lit up and the +15 V supply showed up on the meter. However, the -15 V supply wasn't giving any voltage, and the op amp overload lights were illuminated on the front panel, and the reference voltages from the op amps weren't there. The bad -15 V supply looked like the first thing to investigate, since without it, the op amp boards wouldn't work.

I removed the working +15 regulator and failing -15 regulator from the card cage and tested them on the bench. Conveniently, both boards are identical, so I could easily compare signals on the two boards. (Modern circuits typically use special regulators for negative voltage outputs, but this power supply used the same regulator for both.) The output transistor on the bad board wasn't getting any control signal on its base, so it wasn't producing any output. Tracing the signals back, I found the transistor generating this signal wasn't getting any voltage. This transistor was powered directly from the connector, so why wasn't any voltage getting to the transistor?

A regulator board was failing due to loose screws (red arrows). The circuit was powered through the thick bottom PCB trace and then current passed through the heat sink from the lower screw to the upper screw.

I studied the printed circuit board and noticed that there wasn't a PCB trace between the transistor and the connector! Instead, part of the current path was through the heat sink. The heat sink was screwed down to the PCB, making a connection between the two red arrows above. After I tightened all the screws, the board worked fine.

The analog computer with the plugboard and sides removed to show the internal circuitry. The power supply is in the lower back section. One module has been removed and placed in front of the computer.

We put the boards back in, powered up the chassis, and this time the voltages all seemed to be correct. The op amp overload warning lights remained off; the warning light went on before because the op amps couldn't operate with one voltage missing. The next step is to power up the analog circuitry modules and test them. We also need to repair the separate 5-volt power supply used by the digital logic since we found some bad capacitors that will need to be replaced. So those are tasks for the next sessions.

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Notes and references

1. The computer's integrated circuits have 1968 and 1969 date codes on them, so I think the computer was manufactured in 1969. ↩

2. Most modern power supplies are switching power supplies, so they are much smaller and lighter than linear power supplies like the one in the analog computer. (Your laptop charger, for instance, is a switching power supply.) Back in this era, switching power supplies were fairly exotic. However, linear power supplies are still sometimes used since they have less noise than switching power supplies. ↩

Reliable after 50 years: The Apollo Guidance Computer's switching power supplies

We recently restored an Apollo Guidance Computer, the revolutionary computer that helped navigate to the Moon and land on its surface.1 At a time when most computers filled rooms, the Apollo Guidance Computer (AGC) took up just a cubic foot. This blog post discusses the small but complex switching power supplies that helped make the AGC compact enough to fit onboard the spacecraft.

Inside the Apollo Guidance Computer. The power supplies are the tangles of wires on the far left.

The photo above shows the Apollo Guidance Computer after separating its two trays. Tray A on the left holds the logic and interface modules, while Tray B on the right has the memory circuitry. The AGC has two power supplies in Tray A on the far left: a +4V power supply and a +14V power supply; the power supplies look like a tangle of wires in the photo. The logic circuitry, entirely built from NOR gates, was powered by 4 volts. The interface circuitry and memory used the 14 volt supply.

The spacecraft generated 28 volts from fuel cells, which combined hydrogen and oxygen to produce both water and electricity.3 The task of the power supplies was to convert the spacecraft's 28 volts into the 4 and 14 volts required by the computer.2 The 4-volt power supply could output about 10 amps (i.e. 40 watts) while the 14-volt power supply could output about 5 amps (i.e. 70 watts).4 Thus, the power supplies are roughly equivalent to laptop chargers (although a laptop charger deals with more challenging AC line voltages).

The power supply module in front of the AGC. The module in position A30 provides +14 volts, while the (identical) module in position A31 provides +4 volts.

Cordwood construction

The power supplies, like the AGC's other non-logic modules, were built with cordwood construction. In this high-density technique, cylindrical components were inserted into holes in the module, passing through the module, with their leads exiting on either side.6 The left side of the photo below contains resistors, capacitors, and diodes. Because of the cordwood construction, the components are not visible except for the ends of their leads poking through holes. Point-to-point wiring connected the components with welded connections. (The other side of the module is similar, connecting the other ends of the components.) The shiny rectangle on the right is a relay, used to shut off power for standby operation. The ends of large filter capacitors are visible below the relay.

Cordwood construction in the power supply. On the left, components are mounted vertically through the module, with welded wiring on both sides. The metallic box on the right is a relay. Underneath the relay, the ends of filter capacitors are visible.

Cordwood construction was used for high density in applications from aerospace to Cray's CDC 6600 computer. For flight, the AGC's cordwood wiring was encased (potted) in epoxy, protecting it from vibration.

How the power supplies worked

Because the power supplies needed to be lightweight and efficient, they were switching power supplies, an unusual technology for the time. Most computers back then used linear power supplies, which were simpler but much too inefficient for the AGC since excess voltage is turned into waste heat.5 A switching power supply, on the other hand, switches the input voltage on and off at a high frequency. This yields the desired output voltage with very little wasted energy.

The AGC's power supplies used a common switching circuit called a buck converter, which converts an input voltage to a lower voltage. The diagram below shows the key components: a switch (transistor), inductor, diode, and capacitor. The key idea is that if the switch is closed for more time, more of the input voltage will appear across the load. Thus, the output voltage is controlled by the switch timing. The inductor stores energy and releases it when the switch is open, producing a relatively stable output.

A buck converter rapidly switches between the on state and the off state. When on, current flows from the voltage source (V) through the switch and inductor to the load (right). When the switch is open, stored energy in the inductor continues to provide current to the load, through the diode. (Source: Cyril Buttay, CC BY-SA 2.5).

A switching power supply requires a complex control mechanism to switch on and off at the right time. The AGC used a technique called PWM (pulse-width modulation), where power is switched on and off at a fixed frequency (e.g. 20 kilohertz), but changing the fraction of the time the power is on to regulate the voltage.

The schematic below shows the AGC's power supply. (Don't worry about reading the details; click for a larger version.) The buck converter itself (outlined in the lower right) has the expected switching transistor, diode, inductor, and capacitors. However, the power supply has many more components to implement the PWM control circuitry.

Schematic of the AGC's power supply. The main signals are highlighted: 28-volt input (red), 4-volt output (orange), reference voltage (green), comparator output to control the PWM (purple), and PWM output (brown). (source)

To summarize the power supply's operation, 28 volts (red) is supplied at the upper left and filtered. The buck converter in the output circuit (right) reduces the voltage to 4 volts (orange) On the control side (left), the output voltage is used for feedback. A two-transistor comparator (lower left) compares the output voltage with a reference voltage (green) set by a Zener diode and resistor network. The output of the comparator (purple) goes through the PWM control circuit where it modifies the width of the pulses (brown) produced by the PWM circuit. These pulses drive the switching transistor in the buck converter, closing the feedback loop. The computer's clock signal providing timing for the PWM circuit.7

Astronauts interacted with the AGC through the Display/Keyboard (DSKY). The STBY button (lower right) put the computer in standby mode, which was indicated via the STBY light (left). Photo from Virtual AGC.

The power supply also included a standby circuit. By pressing the STBY key on the display/keyboard (DSKY), a relay would disconnect most of the computer's power. This reduced energy consumption when the computer wasn't needed.8

The diagram below shows the top of the power supply module with the major components labeled. Note the large size of the transistors, inductors and filter capacitors compared to the tightly-packed cordwood circuitry on the left. The switching transistor for the buck converter is almost an inch in diameter.

The major components of the AGC's power supply. The components for the buck converter are much larger than the control circuitry.

The transistors of the 1960s were barely able to support switching power supplies since they required a power transistor that could operate at both high speed and high current, which was difficult at the time. (Modern transistors (MOSFETs) are cheap and can handle much higher voltages, leading to ubiquitous low-cost phone and laptop chargers that run off an AC outlet.) The switching transistor required a high-current control signal, which was provided by three drive transistors (in a "complementary Darlington" configuration).

Closeup of transistors in the power supply. The large transistor on the right is the high-current switching transistor. Driving it required the three transistors on the left.

Testing the power supply

We extensively tested the AGC's components before powering up the system. For the power supply, we first checked all the tantalum capacitors since tantalum capacitors are prone to shorting out. We found that the capacitors were all in good shape with the proper capacitances. This is in contrast to modern capacitors, which often leak or fail after a few years. NASA used expensive aerospace-grade capacitors and X-rayed each one to test for faults, and this made a large difference.

Wiring up each power supply for testing (below) was more complex than you might expect. The AGC used two identical power supplies that supplied 4 or 14 volts. The output voltage was selected by backplane wiring that connected different resistors in the feedback resistor network. We reproduced these connections on a breadboard, as well as connecting up the input and output. Some high-wattage resistors (lower right) served as the load.

The setup we used to test the power supply. Connections were made to the pins on the bottom of the module. These pins connected the module to the rest of the AGC. In this view you can see the white wires on the side of the module that connected the circuitry on top of the module to the pins on the bottom.

We powered-up the AGC modules with 28 volts using a current-limited supply to limit potential damage from any faults. We took measurements and found that 4V power supply produced 4.09 volts while the 14V power supply produced 14.02 volts. The quality of the power was good, with about 30mV of ripple. We were somewhat surprised that both power supplies worked flawlessly after 50 years.

Conclusion

The Apollo Guidance Computer used advanced switching power supplies that were lightweight and efficient. While switching power supplies were exotic in the 1960s, improved semiconductors have made them cheap and ubiquitous. Now the switching transistor, a high-precision voltage reference, and the control logic can be combined on a single chip. The modern equivalent of the AGC's power supply is a tiny 5A buck converter for $1.50 on eBay (below). While I wouldn't trust this converter to get to the moon, let alone still work 50 years from now, it illustrates the dramatic improvements in switching power supply technology. (I've written more about the history of switching power supplies.)

A modern 5A buck converter is compact and costs $1.50.

To learn more about our AGC restoration, see Marc's series of AGC videos; the video below shows us testing the power supplies. I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. Thanks to Mike Stewart for photos.

Notes and references

1. The AGC restoration team consists of Mike Stewart (creator of FPGA AGC), Carl Claunch, Marc Verdiell (CuriousMarc on YouTube) and myself. The AGC that we're restoring belongs to a private owner who picked it up at a scrapyard in the 1970s after NASA scrapped it. For simplicity, I refer to the AGC we're restoring as "our AGC". ↩

2. The first version of the Apollo Guidance Computer was known as Block I. The AGC was extensively redesigned to produce the Block II version that was flown. The Block I used +3V and +13V power supplies, while the Block II used +4V and +14V. The Block I power supply is documented here in section 4-8.7. The Block II power supply is documented here in section 4-5.9. ↩

3. The power systems were different between the command/service module and the lunar module. On the command/service module the 28 volts was fed to the different parts of the spacecraft using two buses (Main A and Main B) for redundancy. Main A bus was connected to the A31 power supply module, while the Main B bus was connected to the A30 power supply module (schematic). The two buses were tied together inside the AGC after passing through power rectifiers, so either bus could power the AGC.

   (You may recall from Apollo 13: "Houston, we've had a problem. We've had a Main B Bus undervolt". When the oxygen tank exploded, the voltage from the fuel cells dropped, triggering the low voltage alarm.)

   The lunar module used batteries for its 28 volt supply, rather than fuel cells. Instead of Main Bus A and Main Bus B, the lunar module had a Commander bus (CDR BUS) and a Lunar Module Pilot bus (LMP BUS). The AGC on the lunar module was only connected to the CDR BUS, so there wasn't redundancy. ↩

4. I estimated the wattage of the power supplies by looking at the current-limit feature. The power supplies have two 0.12Ω current-sense resistors. The voltage drop across these resistors will turn on transistor Q13, which will reduce the PWM output and thus the power supply's output voltage. The 4V power supply has the two resistors in parallel (connected by external wiring). Assuming the transistor turns on at 0.6V, this corresponds to a current of 0.6V / 0.06Ω = 10 A. The 14V power supply uses one current-sense resistor, so it will be limited to around 0.6V / 0.12Ω = 5A. ↩

5. Some calculations show the problem with using a linear power supply. The AGC's power supply produced 4 volts at 10 amps, which is 40 watts. A linear power supply would dissipate 24 volts (of the 28 volts) at 10 amps, i.e. 240 watts. The linear power supply would be 14% efficient, wasting 86% of the energy. When you need tanks of liquid hydrogen and oxygen to provide the energy, wasting 86% is unacceptable. In addition, disposing of waste heat on a spacecraft is difficult, so an additional 240 watts would be a problem. ↩

6. In the power supplies, the cordwood components are mounted differently from the other cordwood modules. Most AGC modules had components running from one side to the other as shown below, while components in the power supply went from top to bottom, parallel to the pins. This allowed the use of longer components, in particular, the large filter capacitors.

   ↩

   Most of the cordwood modules, such as this interface module, had components running from side-to-side through the module.

7. One interesting thing about the power supply is that the PWM circuit was driven by the computer's oscillator. But the oscillator was powered by the power supply, raising a chicken-and-egg problem of how the system started up. The solution was that the PWM circuit would self-oscillate at 20 kilohertz if there was no external clock signal, so it would still produce the correct output voltage. Once it provided power to the oscillator module and the oscillator produced a clock signal, the power supply synchronized to this clock signal (50 kilohertz for the 4V supply and 100 kilohertz for the 14V supply). ↩

8. The standby (STBY) key on the DSKY was changed to PRO (proceed) on later versions of the DSKY and the functionality was changed somewhat. ↩

Glowing mercury thyratrons: inside a 1940s Teletype switching power supply

We recently started restoring a Teletype Model 19, a Navy communication system introduced in the 1940s.14 This Teletype was powered by a bulky DC power supply called the "REC-30 rectifier". The power supply uses special mercury-vapor thyratron tubes, which give off an eerie blue glow in operation, as you can see below.

The thyratron tubes in the Teletype REC-30 power supply give off a blue glow. The orange light is a neon bulb used as a voltage reference.

The power supply is interesting, since it is an early switching power supply. (I realize it's controversial to call this a switching power supply, but I don't see a good reason to exclude it.) While switching power supplies are ubiquitous now (due to cheap high-voltage transistors), they were unusual in the 1940s. The REC-30 is very large—over 100 pounds—compared to about 10 ounces for a MacBook power supply, demonstrating the amazing improvements in power supplies since the 1940s. In this blog post, I take a look inside the power supply, discuss how it works, and contrast it with a MacBook power supply.

What is a Teletype?

A Model 19 Teletype. Image from BuShips Electron, 1945.

Teletypes are a brand of teleprinter, essentially a typewriter that could communicate long distances over a wire. You may be familiar with Teletypes from old newsroom movies, where they chatter out news bulletins, or you may have seen computers that used an ASR33 Teletype as a terminal in the 1970s. (Much of the terminology used by serial ports on modern computers dates from the Teletype era: start and stop bits, baud rate, tty, and even the break key.) Teletypes could also store and read characters by punching holes in paper tape, using a 5-bit code2 (below).

"Teletype is here to stay." This image shows the 5-hole paper tape used by Teletypes. Image from BuShips Electron, 1945.

Teletypes were introduced in the early 1900s. In that pre-electronic era, character selection, serialization, and printing were accomplished through complex electromechanical mechanisms: cams, electromagnets, switches, levers and gears. Pressing a Teletype key closed a combination of switches corresponding to the character. A motorized distributor serialized these bits for transmission over the wire. On the receiving side, electromagnets converted the received data bits into movement of mechanical selectors. The selector pattern matched the notches on one of the typebars, causing that typebar to move and the correct character to be printed.1

Partially disassembled Model 19 Teletype.

The current loop

Teletypes communicated with each other using a 60 milliamp current loop: if current is flowing, it's called a mark (corresponding to a hole in the paper tape), and if current is interrupted, it's called a space. Each character was transmitted by sending a start bit, 5 data bits, and a stop bit. (If you've used serial devices on your PC, this is where the start and stop bit originated. And the baud rate is named after Émile Baudot, inventor of the 5-bit code.) The REC-30 power supply produced 900 milliamps at 120V DC, enough current for a room of 15 Teletypes.
You might wonder why Teletypes don't just use voltage levels instead of the strange current loop. One reason is that if you're sending signals over a wire to the next city, it's hard to know what voltage they are receiving because of voltage drops along the way. But if you're sending 60mA, they'll be getting the same 60mA (assuming no short circuits). 3 The hefty current was necessary to drive the electromagnets and relays in Teletypes. Later Teletypes often used a 20 mA current loop instead of 60 mA.

Why a switching power supply

There are several ways of building a regulated power supply. The most straightforward is a linear power supply, which uses a component such as a tube or transistor to regulate the voltage. The component acts as a variable resistor, dropping the input voltage to the desired output voltage. The problem with linear power supplies is they are generally inefficient, since the extra voltage turns into waste heat.
Most modern power supplies, instead, are switching power supplies. They rapidly switch on and off, making the voltage averages out to the desired output voltage. Because the switching element is either on or off, not resistive as in a linear power supply, switching power supplies waste very little power. (Switching power supplies are usually much smaller and lighter too, but apparently the designers of the REC-30 didn't get that memo.4 The REC-30 is over two feet wide.) Most of the power supplies you'll encounter, from your phone charger to the power supply in your computer, are switching power supplies. Switching power supplies became popular in the 1970s with the development of high-voltage semiconductors, so tube-based switching power supplies are a bit unusual.

REC 30 Teletype power supply in its case painted Navy gray. The power cords exit the top. The tubes are behind the door on the right.

Inside the REC-30 power supply

The photo below shows the main parts of the REC-30 power supply. AC power enters at the left and is fed into the large autotransformer. The autotransformer is a special single-winding multi-tap transformer that converts the input AC voltage (between 95V and 250V)6 into a fixed 230V AC output. This allows the power supply to accept a variety of input voltages, simply by connecting a wire to the right autotransformer terminal. The 230V output from the autotransformer feeds the plate drive transformer, which outputs 400 volts AC to the thyratron tubes.5 The thyratron tubes rectify and regulate the AC into DC, which is then filtered by capacitors (not visible in photo) and inductors (chokes), to produce the 120V DC output.

REC-30 power supply, showing the main components.

Ignoring the switching for a moment, the AC-to-DC conversion in the REC-30 power supply uses a full-wave rectifier and center-tapped transformer (the drive transformer), similar to the diagram below. (The thyratron tubes provide rectification rather than the diodes in the diagram.) The transformer windings provide two sine waves, out of phase, so one will always be positive. The positive half goes through one of the thyratron tubes, producing pulsed DC output. (In other words, the negative half of the AC waveform is flipped to produce a positive output.) The power supply then smooths out these pulses to provide steady voltage, using inductors (chokes) and capacitors as filters.

A full wave rectifier circuit (center) converts AC (left) to pulsed DC (right). Image by Wdwd, CC BY 3.0.

Unlike the diodes in the diagram above, the thyratron tubes in the power supply can be controlled, regulating the output voltage. The basic idea is to turn the thyratron on for a fixed part of the AC cycle, as shown below. If it is on for the full cycle, you get the full voltage. If it is on for half the cycle, you get half the voltage. And if it is only on for a small part of the cycle, you get a small voltage.7 This technique is called phase angle control because it turns the device on at a particular phase angle (i.e. a particular point between 0° and 180°in the AC sinusoid). (This is very similar to a common light dimmer switch, which uses a semiconductor TRIAC rather than thyratron tubes.11)

Diagram of phase control. Top shows the fraction of the pulse used. Bottom shows the point at which the thyratron switches on. Image by Zureks, CC BY-SA 2.5.

The thyratron tubes in the power supply resemble vacuum tubes, but have argon and mercury vapor inside their glass shell (unlike vacuum tubes which not surprisingly contain a vacuum). These thyratron tubes are constructed from three components: the filament, the plate, and the grid. The filament, kind of like a light bulb filament, heats up and gives off electrons. The plate, connected to the top of the tube, receives electrons, allowing current to flow from the filament to the plate. Finally, a control grid between the filament and the plate can block the electron flow. When electrons flow to the plate, the mercury vapor in the tube ionizes, turning on the tube and producing the blue glow you can see below. (In contrast, a regular vacuum tube has a flow of electrons, but nothing to ionize.) The ionized mercury provides a highly conductive path between the filament and the plate, allowing a large (1.5 amp) current to flow. Once the mercury ionizes, the grid no longer has control over the tube and the thyratron remains on until the voltage between the filament and plate drops to zero. At this point, the ionization ceases and the tube shuts off until it is turned on again.

REC-30 Teletype power supply, showing the thyratron tubes with their blue glow and the neon bulb voltage reference glowing orange. The timer/relay is visible in the upper left.

The grid voltage on a thyratron controls the tube. The negative voltage on the grid repels the negatively-charged electrons, preventing electron flow between the filament and the plate. But when the voltage on the plate gets high enough, electrons will overcome the grid repulsion, causing the tube to turn on. The important factor is that the more negative the grid, the more repulsion and the higher the plate voltage needs to be for the tube to turn on. Thus, the grid voltage can control the point in the AC cycle at which the tube turns on.
The control circuit regulates the power supply's output voltage by changing the grid voltage and thus the thyratron timing.9 I used the power supply's adjustment potentiometer to show below how changing the timing changes the voltage. I could set the output voltage (blue) between 114 and 170V. The regulation circuit changed the grid voltage (pink), resulting in the thyratron timing (cyan and yellow) changing accordingly.10 The oscilloscope trace is a bit tricky to interpret; see the footnote for details.12 The main thing to notice is how the ends of the cyan and yellow curves move back as the voltage increases, indicating the thyratrons fire earlier.

Change in phase angle as Teletype REC-30 power supply is adjusted from 130V to 170V output. Yellow and cyan are the voltage across the thyratrons. Pink is the grid control signal. Blue is the (inverted) output voltage.

The schematic below shows the circuitry of the REC-30 power supply (larger schematic here). The AC input circuit is highlighted in green, with the autotransformer adjusting the input voltage to 230V and feeding the drive transformer. These thyratron tubes have the interesting requirement that they must be heated up before use to ensure that the mercury is vaporized; a bimetallic timer waits 20 seconds before powering up the drive transformer.8 On the secondary side of the drive transformer, the 400V drive voltage is in red, the regulated output voltage from the thyratrons is orange, and the low side of the output is blue.13 The regulation circuit (at the bottom) is a bit more complicated. The grid control tube (a 6J6 pentode) provides the control voltage to the grids of the thyratrons, controlling when they will turn on. The grid control tube takes a feedback voltage (pin 5) from the output via a potentiometer voltage divider. The output from this tube (pin 3) sets the thyratron grid voltage to keep the output voltage regulated. The voltage drop across the neon bulb is almost constant, allowing it to act as a voltage reference providing a fixed voltage for the control tube's cathode (pin 8).

Schematic of the REC-30 Teletype power supply. For some reason, the schematic indicates ohms with a lower-case ω rather than the usual upper-case Ω.

Comparison with a MacBook power supply

It's interesting to compare the REC-30 power supply to a modern MacBook power supply, to see how much switching power supplies have improved in 70 years. An Apple MacBook power adapter is roughly comparable to the REC-30 power supply, producing 85 watts of DC power from an AC input (versus 108 watts from the REC-30). However, the MacBook power supply is about 10 ounces, while the REC-30 is over 100 pounds. The MacBook supply is also considerably less than 1% the size of the REC-30 power supply, showing the incredible miniaturization of electronics since the 1940s. The bulky thyratron tubes to switch the power have been replaced by compact MOSFET transistors. The resistors have shrunk from the size of a finger to smaller than a grain of rice. Modern capacitors are smaller, but haven't miniaturized as much as resistors; capacitors are some of the largest components in the MacBook charger, as you can see below.

Inside an Apple MacBook 85W power supply. Despite its small size, the MacBook power supply is much more complex than the REC-30. It contains a Power Factor Correction (PFC) circuit to improve power line efficiency. Multiple safety features (including a 16-bit microcontroller) monitor the power supply, shutting it down if there is a fault.

Most of the weight reduction in the Macbook charger comes from replacing the enormous autotransformer and plate drive transformer with a tiny high-frequency transformer. The MacBook power supply operates at about 1000 times the frequency of the REC-30, which allows the inductors and transformers to be much, much smaller. (I wrote more on the MacBook charger here and more on power supply history here.)
The following table summarizes the differences between the REC-30 power supply and the MacBook power supply.

	REC-30	MacBook 85W
Weight	104.5 lb	0.6 lb
Dimensions	25" x 8" x 11" (1.3 ft^3)	3 1/8" x 3 1/8" x 1 1/8" (0.006 ft^3)
Input AC	95-250V AC, 25-60 Hz	100-240V 50-60Hz
Output	108W: 120V DC at 0.9A	85W: 18.5V DC at 4.6A
Idle (vampire) power consumption	60W	<0.1W
Harmful substances inside	Mercury, lead solder, probably asbestos wire insulation	No: RoHS certified
Output control	Bimetallic timer / relay	16-bit MPS430 microcontroller
Switching elements	323 thyratron tubes	11A N-channel power MOSFETs
Voltage reference	GE NE-42 neon glow discharge bulb	TSM103/A bandgap reference
Switching control	6F6 pentode tube	L6599 resonant controller chip
Switching frequency	120 Hz	~500 kHz

I measured the quality of the REC-30's output voltage (below). The power supply provides much higher quality output than I expected, with only about 200 mV of ripple (the waves in the horizontal blue line) which is close to Apple-level quality. There are also narrow spikes (the vertical lines) of about 8 volts when the thyratrons switch. These spikes are fairly large compared to an Apple power supply but still better than a cheap charger.

Output from the REC-30 power supply, showing a small amount of ripple and switching spikes.

Conclusions

The REC-30 power supply provided over 100 watts of DC power for Teletype systems. Introduced in the 1940s, the REC-30 was an early switching power supply that used mercury-filled thyratron tubes for efficiency. It was a monstrously large unit for a 100 watt power supply, weighing over 100 pounds. A comparable modern power supply is under 1% of the size and weight of this unit. Despite its age, the power supply worked flawlessly when we powered it up, as you can see in Marc's video below. The power supply is beautiful in operation, with a blue glow from the thyratrons and orange from the large neon bulb.

I announce my latest blog posts on Twitter, so follow me at @kenshirriff for future articles. I also have an RSS feed. Thanks to Carl Claunch and Marc Verdiell for work on the power supply.

Notes and references

1. For more information on how Teletypes operate, see this page. For comprehensive information, see Fundamentals of Telegraphy (Teletypewriter), Army Technical Manual TM 11-655, 1954. More REC-30 schematics are here and documentation is here. ↩
   
2. In the 1870s, Émile Baudot invented the 5-bit Baudot code. A different 5-bit code was created by Murray in 1901 and standardized as ITA2. Both codes look like the characters are in random order; the original Baudot code used a Gray code, while the Murray code was optimized to use the fewest holes for common characters, reducing wear and tear on the machinery. (It wasn't until ASCII in the 1960s that putting the alphabet in binary order became a thing.) ↩
   
3. Note that in contrast to voltage-based signals, the components of the current loop must form a topological loop for the current to flow. Removing a device will break the circuit unless provisions are made to close the loop. As a result, the Teletype system is full of jacks that short when you unplug a component, to keep the loop intact. ↩
   
4. The main reason the REC-30 power supply is so heavy and bulky compared to modern switching power supplies is that it switches at 60 Hz (and even down to 25 Hz), while modern power supplies switch at tens of kilohertz. Since the transformer's EMF is proportional to the frequency, a high-frequency transformer can be much smaller than the corresponding low-frequency transformer (details). ↩
   
5. Isolation between the AC input and the DC output is a key safety feature in most power supplies, from chargers and PC power supplies to the REC-30, preventing a shock from the DC output. In the REC-30, the plate drive transformer has the critical role of providing isolation. (Note that the autotransformer doesn't provide any isolation protection because it has a single main winding; touching its output is like touching the AC input.) The rest of the circuitry is carefully designed so there is no direct path between the AC input and the output: the control circuitry is all on the secondary side, the filaments are powered by isolated windings off the autotransformer, and the relay provides isolation in the timer. Also note that for safety the 120V DC output is floating, rather than grounding either side; this means you'd need to touch both sides to get a 120V shock. ↩
   
6. The power supply accepts a wide variety of input voltages (95, 105, 115, 125, 190, 210, 230, 250V AC) as well as multiple frequencies: 25, 40, 50, and 60 Hz. While modern switching power supplies can automatically adjust to handle the input voltage, the REC-30 required a wire to be moved to the proper autotransformer tap to support a different voltage. 25 Hertz might seem like a strange frequency for a power supply to support, but many parts of the United States used 25 Hertz power in the 1900s. In particular, Niagara Falls generated 25 Hertz electricity due to the mechanical design of its turbines. In 1919, more than two thirds of power generation in New York was 25 Hertz and it wasn't until as late as 1952 that Buffalo used more 60 Hertz power than 25 Hertz power. Because of the popularity of 25 Hz power, many of IBM's punch card machines from the early 1900s could also operate off 25 Hertz (details). ↩
   
7. Modern switching power supplies use pulse-width modulation (PWM) schemes to switch on and off thousands of times a second. This results in a smaller power supply and gives smoother output than switching once per AC cycle, but requires more complicated control systems. ↩
   
8. In the REC-30 power supply, the 20 second delay before powering up the tubes is accomplished by a timer and relay. The timer uses a bimetallic strip with a heater. When you turn on the power supply, the filaments are powered immediately to heat up the tubes. Meanwhile, a heater inside the timer warms the bimetallic strip; eventually the strip bends enough to close the contacts and energize the tubes. At this point, the relay latches the contacts closed. ↩
   
9. Initially, I assumed that as the load increased, the thyratrons would switch on for longer periods of time to provide more current. However, I did oscilloscope measurements under varying load and found no phase shift. This turns out to be the expected behavior; a transformer provides essentially constant voltage regardless of the load. Thus, the thyratron timing remains essentially the same as the load changes and the transformer just provides more current. You can see the thyratrons brightening as the current increases in this video. ↩
   
10. Under low load, the power supply sometimes skips entire AC cycles, rather than switching the thyratrons mid-cycle. This is visible as the thyratrons start to flicker rather than glow steadily. I'm not sure if this is a bug or a feature. ↩
    
11. The modern solid-state equivalent of the thyratron is the silicon controlled rectifier, also known as the SCR or thyristor (combining "thyratron" and "transistor"). The SCR has four semiconductor layers (rather than a 2-layer diode or 3-layer transistor). Like the thyratron, the SCR is normally off until triggered by the gate input. It then remains on, acting like a diode, until the voltage drops to 0, at which point is switches off. A TRIAC is a semiconductor device similar to a SCR, except it can pass electricity in either direction, making it more convenient for AC use. ↩
    
12. In the oscilloscope trace, the yellow and cyan curves are the voltage across the two thyratrons. The flat part (where the voltage difference is approximately zero) is where the thyratron is firing. The two thyratron tubes are not totally symmetrical for some reason, with the yellow one usually firing later. (This is visible while watching the thyratrons, as one glows more than the other.) The pink line is the grid control voltage. Note that it increases to make the output voltage increase, causing the thyratrons to fire earlier. A vertical spike is visible in the pink line; this is noise as the thyratron fires. The blue line at the bottom is the output voltage (inverted); the line goes down as the voltage increases.
    One puzzle is that there is always one thyratron firing; either the yellow or cyan line is always at 0. I would expect a gap between the zero point of the plate voltages and when the other thyratron fires. My suspicion is the large inductors are pulling the filament negative so even when the plate is negative, there is still a positive voltage between the filament and the plate. ↩
    
13. The filament circuit for the power supply is a bit tricky since the thyratron filaments are used both to heat the tubes and as the cathode. The filaments are provided with 2.5V by the autotransformer. In addition, the filaments act as the cathodes in the thyratrons, so they produce the output voltage and are connected to the high side of the output. To perform these two tasks, the split winding of the autotransformer superimposes the 2.5V filament voltage but passes the output voltage straight through. The two thyratrons use a total of 35 watts just for the filaments, so you can see that filament heating wastes a lot of energy and gives off a lot of heat, somewhat negating the advantages of a switching power supply. ↩
    
14. The introduction of Teletypes for Navy use was described in BuShips Electron, Sept 1945. The development of radio-connected Teletypes (RTTY), typically using frequency-shift keying, allowed the adoption of Teletypes for Navy use. The Navy first used radio Teletypes for communication between shore stations, and then moved to shipboard use. The biggest advantage of a Teletype was it was at least four times as fast as a radio operator transmitting by hand. In addition, paper tape allowed messages to be automatically copied and relayed. Teletypes could also be integrated with cryptographic equipment such as SIGTOT which used a one-time pad. More on Teletypes in World War II here. ↩
    

Inside the die of Intel's 8087 coprocessor chip, root of modern floating point

Looking inside the Intel 8087, an early floating point chip, I noticed an interesting feature on the die: the substrate bias generation circuit. In this article I explain how this circuit is implemented, using analog and digital circuitry to create a negative voltage.

Intel introduced the 8087 chip in 1980 to improve floating-point performance on 8086/8088 computers such as the original IBM PC. Since early microprocessors were designed to operate on integers, arithmetic on floating point numbers was slow, and transcendental operations such as trig or logarithms were even worse. But the 8087 co-processor greatly improved floating point speed, up to 100 times faster. The 8087's architecture became part of later Intel processors, and the 8087's instructions are still a part of today's x86 desktop computers.1

I opened up an 8087 chip and took die photos with a microscope yielding the composite photo below. The die of the 8087 is fairly complex, with 40,000 transistors (according to Intel) or 45,000 transistors (according to Wikipedia). The photo shows the metal layer of the chip, the connections on top of the chip. The thickest white lines provide power and ground connections to all parts of the chip. Hidden underneath the metal are the polysilicon and silicon that form the chip's transistors. (Click the photo for a large image.)

Die photo of the Intel 8087 floating point coprocessor chip.

The bottom half of the chip holds the 80 bit wide arithmetic circuitry: an adder, shifters, mathematical constant storage and registers. The large rectangle in the middle of the chip is the microcode that controls the chip. At the top is control logic and bus circuitry that interfaced with the 8086 processor. (I'll discuss the inner workings of the 8087 in more detail in later blog posts.)

The black lines around the outside of the die photo are the tiny bond wires connecting the pads on the die to the 40 pins of the chip. By studying the 8087 datasheet, it's not too hard to figure out which pad on the die corresponds to each pin of the chip; the chip's 40 pins (numbered counterclockwise) are wired in order to 40 pads on the chip. The diagram below zooms in on the center right part of the die, labeling some of the pads. (Note that the ground and +5V power (Vcc) pads have multiple wires in parallel to carry more current.) However, one puzzle appeared—an extra pad and wire located between pads 40 and 1, not associated with any of the chip's pins.

Each pad on the die of the 8087 FPU chip is wired to one of the 40 pins of the chip. But there is one extra wire between pins 1 and 40. It is connected to the chips's substrate.

Looking at the bond wires on the chip (below) revealed that the mystery pad wasn't connected to one of the pins but to a tiny cubical block to the right of the die. Since the cube is on the same metallic base as the die, it connects to the die's underlying silicon, the substrate. I did some reverse-engineering and determined that this is part of the 8087's substrate bias circuit, which uses this connection to put a negative voltage on the substrate. The rest of this blog post explains how this circuit works.

The die of the 8087 FPU chip, showing the bond wires from the die to the package.

What is substrate bias?

High-density integrated circuits in the 1970s were usually built from NMOS transistors. The diagram below shows the structure of an NMOS transistor. The integrated circuit starts with a silicon substrate, and transistors are built on this. Regions of the silicon are doped with impurities to create diffusion regions with desired properties. The transistor can be viewed as a switch, allowing current to flow between two diffusion regions called the source and drain. The transistor is controlled by the gate, made of a special type of silicon called polysilicon. A high signal voltage on the gate lets current flow between the source and drain, while a low signal voltage blocks current flow. An insulating oxide layer separates the gate from the silicon underneath; this insulating layer will be important later. These tiny transistors can be combined to form logic gates, the components of microprocessors and other digital chips.

Structure of a MOSFET as implemented in an integrated circuit.

For high-performance integrated circuits, it was beneficial to apply a negative "bias" voltage to the substrate. 2 To obtain this substrate bias voltage, many chips in the 1970s had an external pin that was connected to -5V.3 However, engineers didn't like chips that required an inconvenient extra voltage. Even worse, chips of that era often required a third voltage,4 so systems required three power supplies to support these chips. In addition, the number of pins on ICs was limited (typically just 18 pins for memory chips), so using up two pins for extra voltages was unfortunate. Part of the solution, developed around the end of the 1970s, was for chips to generate the negative bias voltage internally. The result was chips that used a single convenient +5V supply, making engineers happier.

Inside the 8087's substrate bias circuit

You might wonder how a chip can turn a positive voltage into a negative voltage. The answer is a circuit called the charge pump, which uses capacitors to generate the desired voltage.

The 8087's bias generator has two charge pumps working in alternation. The schematics below show the operation of one of the charge pumps. The charge pump is driven by an oscillating signal (Q) and its inverse (Q). In the first step, the upper transistor is switched on, causing the capacitor to charge to 5 volts with respect to ground. The second step is where the magic happens. The lower transistor turns on, connecting the high side of the capacitor to ground. Since the capacitor is still charged to 5 volts, the low side of the capacitor must now be at -5 volts, producing the desired negative voltage at the output. When the oscillator flips again, the upper transistor is turned on and the cycle repeats.5 The charge pump gets its name because it pumps charge from the output to ground. If you view the diodes as check valves, the charge pump is analogous to a manual water pump.

Schematic of the charge pump used in the Intel 8087 to provide negative substrate bias.

To reverse engineer the charge pump circuitry, I examined the die with a microscope. The metal layer obscures the transistors underneath, making it difficult to see the circuitry. But by dissolving the metal layer with hydrochloric acid, I exposed the polysilicon and silicon layers, revealing the transistors and capacitors, as seen below. (The colorful regions are simply interference patterns due to some oxide that wasn't fully removed.) The die photo below shows the two charge pumps: one to the left of the pad, and one below. Each charge pump matches the schematic above, with two diodes, a large capacitor, and two drive transistors.

The substrate bias circuit of the 8087. The metal layer has been removed in this die photo.

The capacitors are the most visible feature of the substrate bias circuitry. Although microscopic, they are huge by chip standards. The area used by the capacitors is about the same as 72 bits of register storage, over 400 transistors. Each capacitor consists of polysilicon over a silicon region, separated by insulating oxide; the polysilicon and silicon form the plates of the capacitor. In the photo, the capacitors are studded with squares; these squares are contacts between the polysilicon or silicon and the metal layer on top. (The metal layer is not visible as it was removed.)

The four drive transistors are much larger than regular transistors since they must handle high current. The red lines are the polysilicon wires forming the gates. The green lines are contacts to the metal layer, connecting the transistors to +5V or ground. The diodes next to the pad are formed from transistors by connecting the gate and drain together (details).

The charge pumps are driven by the ring oscillator at the bottom of the above image. This ring oscillator consists of five inverters in a loop as shown below. Because the number of inverters is odd, the system is unstable and will oscillate. For instance, if the input to the first inverter is 0, the output from the fifth inverter will be 1. This will flip the first inverter, and the "flip" will travel through the loop causing oscillation. To slow down the oscillation rate, two resistor-capacitor networks are inserted into the ring. Since the capacitors will take some time to charge and discharge, the oscillations will be slowed, giving the charge pump time to operate.

The ring oscillator circuit in the 8087's charge pump.

Before explaining the ring oscillator, I'll show how a standard NMOS inverter is implemented in silicon. The diagram below shows an inverter, its schematic, and how it appears on the die. The inverter uses a transistor and a pull-up resistor (which is really a transistor). If the input is low, the transistor is off and the pull-up resistor pulls the output to +5V. If the input is high, the transistor is on, pulling the output to ground. Thus, the circuit inverts the input.

How an inverter is implemented with NMOS logic, and how it appears on the chip die.

In the die photo above, the inverter's physical layout matches the schematic. The large beige regions are doped silicon. The thinner yellow areas bordered with purple are polysilicon. The input is a polysilicon wire. Where it crosses the doped silicon it forms the gate of a transistor between ground (below the input) and the output (above the input). The pull-up resistor is implemented with a transistor that has the gate and drain tied together; the indicated contact forms this connection between the transistor's polysilicon gate and its silicon drain. The polysilicon also forms the output wire. Thus, an inverter is implemented on the chip with two transistors.

The ring oscillator in the 8087 FPU chip, as seen on the die.

The photo above shows how the ring oscillator appears on the die. The five inverters are outlined. Each inverter has a different orientation to optimize the layout, but careful examination shows the same transistor and pull-up structure explained above. The resistors and capacitors for the R-C delays are also indicated. The resistors are simply transistors with a long distance between source and drain, reducing the current flow. These capacitors are constructed like the charge pump capacitors, but are much smaller; the silicon on the bottom and the polysilicon on top form the capacitor plates, separated by the thin insulating oxide layer.

Conclusions

The substrate bias generator on the 8087 chip is an interesting combination of digital circuitry (a ring oscillator formed from inverters) and an analog charge pump. Substrate bias generator circuits were introduced in the late 1970s, helping memory chips and microprocessors to operate from a single +5V supply, much more convenient than requiring three different voltages. The substrate bias generator produces a negative voltage from the positive supply voltage by using a charge pump.

While the bias generator may seem like an obscure part of 1970s computer history, bias generation is still part of modern integrated circuits but has become much more complex, with multiple carefully regulated biases in multiple power domains. There is even a standard (IEEE 1801 power format) that allows IC design tools to generate the necessary circuitry.6

Likewise, even though Intel's 8087 floating point unit chip was introduced 38 years ago, it still has a large impact today. It spawned the IEEE 754 floating point standard used for most modern floating point arithmetic, and the 8087's instructions remain a part of the x86 processors used in most computers.

I'll end with one more 8087 die photo; this one shows the polysilicon and silicon after stripping off the metal. You may recognize the substrate bias generator circuit at the center right. (Click for a large image.)

Die photo of the Intel 8087 floating point unit. The metal layer has been stripped off with acid, revealing the polysilicon and silicon underneath.

I announce my latest blog posts on Twitter, so follow me at @kenshirriff for future 8087 articles. I also have an RSS feed. Thanks to Ed Spittles and Eric Smith for comments.

Notes and references

1. The 8087 introduced a bunch of new instructions to the 8086, such as FADD (floating add), FDIV (floating divide) and FPTAN (tangent). These instructions were implemented using the 8086's ESC "escape" instruction, which was designed to let the 8086 processor interact with a coprocessor.

   The 8087 led to the IEEE 754 floating point standard in 1985; this defines the floating point used by most computers today. For more information on how the 8087 works, see The Intel 8087 numeric data processor by John Palmer or The 8087 Primer. ↩

2. Putting a negative bias voltage on the substrate had several benefits. It decreased parasitic capacitance making the chip faster, made the transistor threshold voltage more predictable, and reduced leakage current. ↩

3. Early DRAM memory chips and microprocessor chips often required three supplies: +5V (Vcc), +12V (Vdd) and -5V (Vbb) bias voltage. In the late 1970s, improvements in chip technology allowed a single supply to be used instead. For example, Mostek's MK4116 (a 16 kilobit DRAM from 1977) required three voltages while the improved MK4516 (1981) operated on a single +5V supply, simplifying hardware designs. (Amusingly, some of these chips still kept the Vbb and Vcc pins for backwards compatibility but left them unconnected.) Intel's memory chips followed a similar path, with the 2116 DRAM (16K, 1977) using three voltages and the improved 2118 (1979) using a single voltage. Similarly, the famous Intel 8080 microprocessor (1974) used enhancement-mode transistors and required three voltages. An improved version, the 8085 (1976), used depletion-mode transistors and was powered by a single +5V supply. The Motorola 6800 microprocessor (1974) used a different approach for a single supply; although the 6800 was built from the older enhancement-load transistors it avoided the +12 supply by implementing an on-chip voltage doubler, a charge pump that increased the voltage. ↩

4. The third (+12V) supply in old chips is unrelated to the substrate bias. This supply was used because early MOS integrated circuits used enhancement-mode transistors as pull-up loads in gates. These transistors couldn't pull signals all the way up to the +5V level, so chips added an an even higher +12V supply. In the mid 1970's, new technology (ion implantation) allowed the creation of depletion-load transistors, which functioned much better as pull-up loads and eliminated the need for the +12V supply. For details, see Wikipedia, StackExchange and VLSI design techniques for analog and digital circuits page 539. ↩

5. I've simplified the charge pump discussion slightly. Due to voltage drops in the transistors, the substrate voltage will probably be around -3V, not -5V. (If a chip requires a larger voltage drop, charge pump stages can be cascaded.) For the pump direction, I'm referring to current flow. If you think of it as pumping electrons, the negative electrons are being pumped the opposite direction, into the substrate. ↩

6. Bias generators are now available as IP blocks that can be licensed and be plugged into a chip design. For more information on bias in modern chips, see Body bias, Multi bias domain implementation, or this presentation. ↩