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. ↩
    

Sonicare toothbrush teardown: microcontroller, H bridge, and inductive charging

My Sonicare electric toothbrush recently quit working, so I took it apart and examined the interesting circuitry inside. There's much more complexity than I expected inside a toothbrush, especially in the mechanism that drives the brush head at 31,000 strokes per minute. Internally, the brush appears to be designed for quality rather than ease of manufacturing. Unfortunately, moisture can get in, causing reliability problems.

The toothbrush is a Sonicare Flexcare Platinum with more features than you'd expect in a toothbrush: three brushing modes, three intensities and a couple timers, along with 10 LEDs to indicate its status. A pressure sensor in the toothbrush changes the vibration if you apply too much pressure while brushing. The toothbrush uses wireless inductive charging so it charges when set on the base. (This toothbrush may seem overly complicated, but it's nothing compared to the new model that includes Bluetooth.)[1]

Disassembling the Sonicare toothbrush. At the left is the induction coil used for charging.

The first step was to remove the toothbrush base, allowing the toothbrush mechanism to be removed from the case. The toothbrush head mounts on the right; it needed to be removed to disassemble the toothbrush. At the left is the charging coil used to wirelessly charge the toothbrush.

The photos below show the top and bottom of the toothbrush internals. I expected to find a simple, low-cost mechanism, so I was surprised at how much complexity there was inside. The vibration mechanism (right) is built from multiple metal and plastic parts screwed together, requiring more expensive assembly than I expected. The circuit board is literally gold-plated and has a lot of components, even if it doesn't quite reach Apple's level of complexity. Overall, the toothbrush's internal design is high quality (except, of course, for the fact that it quit working, as did an earlier one).

Inside the Sonicare toothbrush, top and bottom composite view. The charging coil is at the left. The battery (red) is in the lower left. The coil that vibrates the brush is in the center and the brushing mechanism is at the right.

The brush contains several key components, as can be seen above. In the center is the large red coil that causes the toothbrush to vibrate. On the right is the vibration mechanism, which has a powerful magnet that is moved by the coil. The brush head snaps on at the right. The battery (red, left) takes up about a third of the toothbrush. The long, thin circuit board (green) has the circuitry to operate the toothbrush. A white spacer sits on top of the circuit board, with holes for the LEDs and buttons.

The photo below shows the brush mechanism partially disassembled and separated from the electronics. The toothbrush still powers on in this state, as you can see from the illuminated LEDs. Note the flexible brown ribbon cable between the center of the brush mechanism and the electronics board. This connects the pressure sensor on the brush mechanism to the electronics board.

The brush mechanism (left) separated from the electronics (right). Note the illuminated LEDs. Alto note the flexible brown ribbon connecting the pressure sensor to the electronics board.

The diagram below shows the main components on the circuit board. The buttons are the most visible feature. The gold circles at the left are used to program the microcontroller. The MOSFET transistor switch the coil on and off to produce vibrations. Ten LEDs are scattered across the board. At the right, the diode bridge is part of the charging circuit.

The circuit board for the Sonicare toothbrush is crammed with tiny parts. The gold circles on the left are used to program the microcontroller chip. The tiny gold circles scattered across the board are test points for testing the board during manufacturing.

The circuit board is covered with tiny gold circles. These are test points, allowing test connections to most parts of the board. For instance, each LED and each button has a test point that can be used to test the component. During testing, spring loaded pogo pins on the test circuit make contact with these test points on the toothbrush board. The number of test points (about 56) looks like overkill to me.

The diagram below shows the components on the back of the circuit board. The toothbrush is controlled by a mid-range 8-bit microcontroller, the PIC16F1516.[2] This chip contains the code for all the toothbrush functions: reading the buttons, lighting the LEDs, controlling the coil, and managing charging. There are too many LEDs (10) for the chip to control individually, so eight of the LEDs are controlled by a separate LED driver chip.[3]

The back of the Sonicare circuit board contains the PIC16F1516 microcontroller chip. The sensor is probably a Hall-effect magnetic field sensor.

The microcontroller is an off-the-shelf part, not a custom chip, so it needs to be programmed with the right software. This is done during manufacturing through the large gold circles and triangle near the end of the toothbrush.[4] The resonator provides the clock signal for the microcontroller's timing.[5]

The driver mechanism and the H bridge circuit

The toothbrush head is driven by an electromagnetic coil that moves a magnet. The coil has two halves, wired in opposite directions, so the sides will have opposite magnetic fields. The coil is pulsed one way to rotate the magnet one direction, and then pulsed the opposite way to rotate the magnet the other direction. The result is the high-speed brushing vibration.

The diagram below shows the driver mechanism disassembled. The coil constantly switches polarity so the north pole will switch from the top to bottom (the yellow and blue poles of the coil). The magnet has poles on the front and back edges (perpendicular to the coils), so it will attempt to rotate back and forth to line up with the coil, along the long axis of the toothbrush. The mechanism limits the rotation to a few degrees, resulting in a rotational vibration back and forth rather than spinning like a motor. This rotational vibration is transmitted to the toothbrush head by the torsion bar causing the head and bristles to vibrate. More details on the driver mechanism are here.

Sonicare toothbrush driver mechanism. As the polarity of the coil switches, the magnet rotates back and forth slightly. The torsion bar transmits the rotation to the shaft, which causes the toothbrush head to vibrate around its axis.

The figure below shows the voltage across the coil. Every 2 milliseconds, there is a 4 volt pulse across the coil, followed by a negative 4 volt pulse. The pulses generate the reversing magnetic field that drives the magnet and causes the toothbrush to vibrate. If you count the positive and negative pulses as separate brush strokes, you get the advertised 31,000 brush strokes per minute. (Although counting an up-down cycle as a single stroke rather than two would make more sense to me.)

Voltage across the actuator coil in a Sonicare toothbrush. An H bridge drives the coil with +/- 4 volt pulse every 2 milliseconds.

You might think that driving a coil in two directions would use two switches, but instead it uses four, in a common circuit called an H bridge, as shown below. If switches 1 and 4 are closed, current flows in the forward direction. If switches 2 and 3 are closed, current flows in the reverse direction. In the toothbrush, transistors are used for the switches, and are turned on and off by the microcontroller.[6] An H bridge is often used to control motors that need to go forwards and reverse, for example in a hoverboard.

An H bridge circuit is used to drive the vibration coil. This allows the coil to be off or energized in either direction. Four switches (MOSFET transistors) are used in the H bridge.

Pressure sensor

One of the features of this toothbrush is a pressure sensor. If you press too hard while brushing, the vibrations start pulsing and the LEDs flash. The sensor itself is a tiny mystery chip (below) mounted on the drive assembly, and connected to the electronics board with a thin flexible cable. The cable is labeled with Vdd (1), Data (2), Clock (3), and Ground (4), so the sensor is probably sending a stream of bits using an I2C protocol. My suspicion is the sensor is a Hall effect magnetic field sensor that detects a change in the magnetic field if pressure is preventing the magnet from vibrating. The chip doesn't seem to be in a position to measure actual pressure, which is why I suspect it's measuring the magnetic field instead.

The pressure sensor on the toothbrush is connected to the electronics via a flexible cable. The sensor is probably a Hall effect magnetic sensor using the I2C protocol.

Charging

To charge the toothbrush, it is set on a stand and charges inductively without physically being plugged in. A coil in the stand is magnetically coupled to a coil in the toothbrush, transmitting the power wirelessly. You can see the coil at the bottom of the toothbrush. When set on the stand, the coil picks up about 12 volts, which is used to charge the battery. The power is transmitted at high frequency (80kHz) for efficiency.

The coil is connected to a diode bridge that converts the power to DC. It then goes through a transistor circuit that regulates the charging, as directed by the microcontroller. The battery in the toothbrush is a Sanyo Li-ion rechargeable battery, which is said to be 3.7V but I measured 4.0V.[7]

Voltage across the charging coil in a Sonicare toothbrush oscillates about about 80kHz.

The toothbrush is designed to conserve battery by using very little power when not in use. The microcontroller has a low power standby mode when it is waiting for a button press. When the toothbrush is activated, a transistor energizes the LEDs and the LED driver chip, while another circuit powers up the pressure sensor. This prevents these components from draining the battery while the toothbrush is not in use.

Conclusion

Overall, I was surprised by how much electronics was inside the toothbrush, as well as the complexity of the drive mechanism. It was designed with quality in mind, not low-cost production. Unfortunately, the brush has reliability issues—this was the second one to fail on me. The problem appears to be water seeping in around the shaft, eventually damaging the internals.

Some other Sonicare teardowns are here, here and here. I would have expected different models to be based on similar electronics that just changed the LEDs, buttons and software. Surprisingly the different teardowns show a variety of microcontrollers, circuitry, and drive coils. Some models even move the magnets from the toothbrush unit to the brush head.

Unfortunately after disassembling my toothbrush I was unable to fix its problem. But at least I got an interesting teardown out of it!

To find out about my latest teardowns, follow kenshirriff on Twitter.

Notes and references

[1] It's ironic for a toothbrush to include Bluetooth technology because Bluetooth is named after Harald Bluetooth, a tenth century Danish king who was called Bluetooth because he had a bad, discolored tooth. The Bluetooth logo itself is formed by combining two runes from the king's name.

[2] The PIC microcontroller runs at 16 megahertz. It has 8K of flash memory for the program, as well as 512 bytes of RAM (the RAM on microcontrollers is usually very small) and 128 bytes of flash memory for data. It includes analog-to-digital conversion, which I think is used to monitor the charging voltage. The toothbrush's 8-bit microcontroller is less powerful than the 16-bit microcontroller inside a Macbook power supply.

[3] The LEDs are controlled by a 75HC595A serial to 8-bit output chip. The benefit of this chip is that the microcontroller would use 8 pins to control 8 LEDs, while the microcontroller only uses 3 pins to communicate with the serial chip, freeing up 5 pins for other tasks.

[4] Programming of the chip is done using the ISCP protocol. This uses the programming contacts labeled Vdd, Vpp, Tx, and Ground, as well as the triangle contact, which provides the ISCP data. For some reason, the Tx and Rx circles are also connected to the chips's UART serial pins, allowing serial communication with the microcontroller. I'm not sure why one would want to communicate with the chip outside programming. Maybe there's serial communication with the microcontroller as part of testing. Or maybe the NSA can download information on your brushing habits :-)

[5] The resonator is a 3-pin unit with built-in load capacitors, similar to a quartz crystal oscillator. I suspect it's a CERALOCK®, or something similar.

[6] The H bridge uses a 6866S 20V dual N-channel MOSFET on the low side and a 6963SD 20V dual P-channel MOSFET on the high side.

[7] The charger circuit is puzzlingly simple. The voltage from the diode bridge goes through a microcontroller-controlled transistor (Q5) and then to the battery (through a tiny fuse), without the filtering, voltage regulator or battery voltage monitoring I'd expect. The microcontroller is connected to the AC side of the diode bridge, and presumably is monitoring the input voltage waveform.

Lacking safety features, cheap MacBook chargers create big sparks

You might wonder if it's worth spending $79 for a genuine MacBook charger when you can get a charger on eBay for under $15. You shouldn't get a cheap charger because they are often dangerous and lack safety features. In addition, they produce poor-quality power that isn't good for your laptop and may charge more slowly. I've written before about the safety problems with cheap chargers, but they say a picture is worth a thousand words, so here is why you shouldn't buy a cheap knockoff charger:

A knockoff MacBook charger emits large sparks if short-circuited. Genuine Apple chargers have safety features to protect against this.

If the connector comes in contact with something metal (a paperclip in this instance), it shorts out, creating a big spark. (Don't try this at home.) The genuine Apple charger (below) has safety features that protect against a short circuit. Shorting the connector on a genuine charger has no effect.

A genuine Apple MacBook charger has safety features that protect it from short circuits.

It's really hard to tell a genuine charger from a knockoff from the outside, since the knockoffs look just like the real thing. If you carefully read the text on this charger, you'll notice that "Apple" is missing. However, many knockoff chargers duplicate the text from a real charger, so often you can't tell if it is genuine or not just by looking. Big sparks, however, are a clear sign.

A cheap MacBook charger from eBay. Unlike most cheap chargers, this one doesn't claim to be an Apple charger, but just a "Replacement AC Adapter".

Why does a fake charger produce sparks, while a genuine one doesn't? The fake charger constantly outputs 20 volts, so if any metal shorts the connector, it produces a big spark with all its 85 watts of power. On the other hand, the genuine charger doesn't power up until it has been securely connected to the laptop for a full second. Until it is properly connected (details), it outputs a tiny amount of power (0.6 volts at 100µA) that can't produce a spark. To manage this, the genuine charger includes a powerful microcontroller (more powerful than the microprocessor in the original Macintosh by some measures). Since this processor increases the cost of the charger, knockoff chargers omit it, even though this makes the charger more dangerous.

As the photos below show, the cheap charger (left) omits as much as possible. On the other hand, the genuine Apple charger (right) is crammed full of components. Many of these components filter the power to provide higher-quality power to your laptop. The Apple charger also includes power factor correction, making the charger more efficient.

The cheap MacBook charger (left) omits most of the components found in a genuine Apple charger (right). The genuine charger includes more filtering, power factor correction (left), and a powerful microcontroller (board in upper right).

I've written in detail before about how chargers work, but I'll give a quick explanation here. The AC power comes in the red wires at the top and is converted to high-voltage DC (170V or 340V, depending on if you're in the US or Europe). A transistor (black component on left) chops the power into high-frequency pulses. The pulses create a changing magnetic field in the flyback transformer (large blue box), generating a high-current, low-voltage output. The output is converted to DC by diodes (black component, upper right), and filtered by capacitors (cylinders), to produce the 20 volt output (wires at bottom). A control IC (see photo below) controls the system to regulate the voltage. This may seem like an excessively complicated way to generate 20 volts, but switching power supplies like this are very compact, lightweight and efficient compared to simpler power supplies.

Shorting a cheap charger with a paperclip creates impressive sparks.

Looking at the underside of the cheap charger board shows it has very few components, while the genuine Apple charger's board is covered with tiny components. The two chargers are worlds apart as far as complexity, and this complexity is what provides more efficiency, more safety, and better quality power in the Apple charger.

The cheap MacBook charger (left) uses very simple circuits compared to the genuine Apple charger (right), which is crammed full of components.

Conclusion

While buying a cheap charger saves a lot of money, these chargers omit many safety features and can be hazardous to you and your computer. Don't buy a cheap knockoff charger; if you don't want to pay for a genuine Apple charger, at least buy a charger from a name-brand manufacturer.

Maybe you think these safety issues don't matter because you don't poke your charger with a paperclip. But if you have any metal objects on your desk, a random contact could yield a surprisingly large spark.

I've written a bunch of articles before about chargers, so if this article seems familiar, you're probably thinking of an earlier article, such as: Counterfeit MacBook charger teardown, Magsafe charger teardown, iPhone charger teardown or iPad charger teardown.

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Notes

If you're interested in the components inside the cheap charger, I have some details. The PWM control IC is a SiFirst 1560, a basic control IC for a flyback converter. The IC datasheet has the approximate schematic for the charger. The switching transistor is a 2N601 2 amp, 600 volt N-channel MOSFET. The voltage reference is an AZ431, similar to the ubiquitous TL431. The optoisolator is an 817C. The output diode is a MBRF20100C 10 amp Schottky diode pair. The electrolytic capacitors are from HKLCON.

A cheap charger emits large sparks if you short the connector with a paperclip. Safety features in a genuine charger protect against shorts.

555 timer teardown: inside the world's most popular IC

This article is translated into Vietnamese at: Bên trong chíp định thời 555.

If you've played around with electronic circuits, you probably know[1] the 555 timer integrated circuit, said to be the world's best-selling integrated circuit with billions sold. Designed by analog IC wizard Hans Camenzind[2] in 1970, the 555 has been called one of the greatest chips of all time with whole books devoted to 555 timer circuits.

Given the popularity of the 555 timer, I thought it would be interesting to find out what's inside the 555 timer and how it works. While the 555 timer is usually sold as a black plastic IC, it is also available in a metal can, which can be cut open with a hacksaw[3] revealing the tiny die inside.

Inside the 555 timer. The tiny die in the package is connected to the 8 pins by wires.

A brief explanation of the 555 timer

The 555 timer has hundreds of applications, operating as anything from a timer or latch to a voltage-controlled oscillator or modulator. The diagram below illustrates how the 555 timer operates as a simple oscillator. Inside the 555 chip, three resistors form a divider generating references voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation. In more detail, the capacitor will slowly charge (A) through the external resistors until its voltage hits the 2/3 reference. At that point (B), the upper (threshold) comparator switches the flip flop off and the output off. This turns on the discharge transistor, slowly discharging the capacitor (C). When the voltage on the capacitor hits the 1/3 reference (D), the lower (trigger) comparator turns on, setting the flip flop and the output, and the cycle repeats. The values of the resistors and capacitor control the timing, from microseconds to hours.[4]

Diagram showing how the 555 timer can operate as an oscillator.

To summarize, the key components of the 555 timer are the comparators to detect the upper and lower voltage limits, the three-resistor divider to set these limits, and the flip flop to keep track of whether the circuit is charging or discharging. The 555 timer has two other pins (reset and control voltage) that I haven't covered above; they can be used for more complex circuits.

The structure of the integrated circuit

The photo below shows the silicon die of the 555 through a microscope. On top of the silicon, a thin layer of metal connects different parts of the chip. This metal is clearly visible in the photo as yellowish-white traces and regions. Under the metal, a thin, glassy silicon dioxide layer provides insulation between the metal and the silicon, except where contact holes in the silicon dioxide allow the metal to connect to the silicon. At the edge of the chip, thin wires connect the metal pads to the chip's external pins.

Die photo of the 555 timer.

The different types of silicon on the chip are harder to see. Regions of the chip are treated (doped) with impurities to change the electrical properties of the silicon. N-type silicon has an excess of electrons (negative), while P-type silicon lacks electrons (positive). In the photo, these regions show up as a slightly different color surrounded by a thin black border. These regions are the building blocks of the chip, forming transistors and resistors.

NPN transistors inside the IC

Transistors are the key components in a chip. The 555 timer uses NPN and PNP bipolar transistors. If you've studied electronics, you've probably seen a diagram of an NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor, The transistor is illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make an NPN transistor. It turns out that transistors on a chip look nothing like this, and the base often isn't even in the middle!

Schematic symbol for an NPN transistor, along with an oversimplified diagram of its internal structure.

The photo below shows one of the transistors in the 555 as it appears on the chip. The slightly different tints in the silicon indicate regions that has been doped to form N and P regions. The whitish-yellow areas are the metal layer of the chip on top of the silicon - these form the wires connecting to the collector, emitter, and base. You can spot an emitter on the chip by its "bullseye" structure, while the base rectangle surrounds the emitter.

An NPN transistor in the 555 timer chip. The collector (C), emitter (E) and base (B) are labeled, along with N and P doped silicon.

Underneath the photo is a cross-section drawing illustrating how the transistor is constructed. There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross section below the 'E', you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C).[5] The transistor is surrounded by a P+ ring that isolates it from neighboring components.

PNP transistors inside the IC

You might expect PNP transistors to be similar to NPN transistors, just swapping the roles of N and P silicon. But for a variety of reasons, PNP transistors have an entirely different construction. They consist of a small circular emitter (P), surrounded by a ring shaped base (N), which is surrounded by the collector (P). This forms a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors.

The diagram below shows one of the PNP transistors in the 555, along with a cross-section showing the silicon structure. Note that although the metal contact for the base is on the edge of the transistor, it is electrically connected through the N and N+ regions to its active ring in between the collector and emitter. A metal line is routed between the collector and base, but is not part of the transistor.

A PNP transistor in the 555 timer chip. Connections for the collector (C), emitter (E) and base (B) are labeled, along with N and P doped silicon. The base forms a ring around the emitter, and the collector forms a ring around the base.

The output transistors in the 555 are much larger than the other transistors and have a different structure in order to produce the high-current output. The photo below shows one of the output transistors. Note the multiple interlocking "fingers" of the emitter and base, surrounded by the large collector.

A large, high-current NPN output transistor in the 555 timer chip. The collector (C), base (B) and emitter (E) are labeled.

How resistors are implemented in silicon

Resistors are a key component of analog chips. Unfortunately, resistors in ICs are large and inaccurate; the resistances can vary by 50% from chip to chip. Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant.

A resistor inside the 555 timer. The resistor is a strip of P silicon between two metal contacts.

The photo above shows a 1KΩ resistor in the 555, formed from a strip of P silicon (visible as an outline). Note that the resistor connects two metal wires and another metal wire crosses it. The resistor below is an L-shaped 100KΩ pinch resistor. A layer of N silicon on top of the pinch resistor makes the conductive region much thinner (i.e. pinches it), forming a much higher but less accurate resistance.

A pinch resistor inside the 555 timer. The resistor is a strip of P silicon between two metal contacts. An N layer on top pinches the resistor and increases the resistance.

IC component: The current mirror

There are some subcircuits that are very common in analog ICs, but may seem mysterious at first. The current mirror is one of these. If you've looked at analog IC block diagrams, you may have seen the symbols below, indicating a current source, and wondered what a current source is and why you'd use one. The idea is you start with one known current and then you can "clone" multiple copies of the current with a simple transistor circuit, the current mirror.

Schematic symbols for a current source.

The following circuit shows how a current mirror is implemented with two identical transistors.[6] A reference current passes through the transistor on the left. (In this case, the current is set by the resistor.) Since both transistors have the same emitter voltage and base voltage, they source the same current, so the current on the right matches the reference current on the left.

Current mirror circuit. The current on the right copies the current on the left.

A common use of a current mirror is to replace resistors. As explained earlier, resistors inside ICs are both inconveniently large and inaccurate. It saves space to use a current mirror instead of a resistor whenever possible. Also, the currents produced by a current mirror are nearly identical, unlike the currents produced by two resistors.

Three transistors form a current mirror in the 555 timer chip. They all share the same base and two transistors share emitters.

The three transistors above form a current mirror with two outputs. Note the three transistors share the base connection, tied to the collector on the right, and the emitters on the right are tied together. The transistor on the left is a Widlar current source, a modified mirror that produces a smaller current. On the schematic, the two transistors on the right are drawn as a single two-collector transistor, Q19.

IC component: The differential pair

The second important circuit to understand is the differential pair, the most common two-transistor subcircuit used in analog ICs.[7] You may have wondered how a comparator compares two voltages, or an op amp subtracts two voltages. This is the job of the differential pair.

Schematic of a simple differential pair circuit. The current sink sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally between the two branches. Otherwise, the branch with the higher input voltage gets most of the current.

The schematic above shows a simple differential pair. The current sink at the bottom provides a fixed current I, which is split between the two input transistors. If the input voltages are equal, the current will be split equally into the two branches (I1 and I2). If one of the input voltages is a bit higher than the other, the corresponding transistor will conduct more current, so one branch gets more current and the other branch gets less. A small input difference is enough to direct most of the current into the "winning" branch, flipping the comparator on or off.

In the 555, the threshold comparator uses NPN transistors, while the trigger comparator uses PNP transistors. This allows the threshold comparator to work near the supply voltage and the trigger comparator to work near ground. The 555's comparators also use two transistors on each input (Darlington pair) to buffer the inputs.

The 555 schematic interactive explorer

The 555 die photo and schematic[8] below are interactive. Click on a component in the die or schematic, and a brief explanation of the component will be displayed. (For a thorough discussion of how the 555 timer works, see 555 Principles of Operation.)

For a quick overview, the large output transistors and discharge transistor are the most obvious features on the die. The threshold comparator consists of Q1 through Q8. The trigger comparator consists of Q10 through Q13, along with current mirror Q9. Q16 and Q17 form the flip flop. The three 5KΩ resistors forming the voltage divider are in the middle of the chip.[9] Urban legend says that the 555 is named after these three 5K resistors, but according to its designer 555 is just an arbitrary number in the 500 chip series

Click the die or schematic for details...

How I photographed the 555 die

Integrated circuit usually come in a black epoxy package which require inconveniently dangerous concentrated acid to open. Instead, I bought a 555 in a metal can (below). To examine the die, I used a metallurgical microscope. Unlike a standard microscope, the metallurgical microscope shines light down through the lens allowing it to work with opaque objects (such as chips). I stitched the photos together with Hugin (details).

The 555 timer in an eight-pin metal can package. (Banana for scale)

The failed improved 555

Given the popularity of the 555, it's surprising that it has several rookie design flaws; unbalanced comparators, large operating currents, an asymmetric output waveform, and temperature sensitivity.[10]

In 1997, Camenzind redesigned the 555 to create a much better chip that could run at much lower voltages. The improved chip was sold by Zetex as the ZSCT1555, but unfortunately was a flop. The continuing success of the original 555 and the failure of the improved successor can be viewed as an example of the worse is better principle.

Conclusion

I hope you've found this look inside the 555 timer chip interesting. Next time you're building a 555 project, you'll know exactly what's inside the chip. If you enjoyed this article, I've also reverse-engineered the 741 op amp and 7805 voltage regulator. Thanks to Eric Schlaepfer[11] for helpful comments.

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Notes and references

[1] The 555 timer is iconic enough to appear on mugs, bags, caps and t-shirts.

The 555 timer is popular enough to appear on t-shirts. Courtesy of EEVblog.

[2] The book Designing Analog Chips written by the 555's inventor Hans Camenzind is really interesting, and I recommend it if you want to know how analog chips work. Chapter 11 has an extensive discussion of the 555's history and operation. Page 11-3 claims the 555 has been the best-selling IC every year, although I don't know if that is still true. The free PDF is here or get the book.

[3] You can cut an IC can open with a plain hacksaw, but a jeweler's saw gives a much cleaner cut. I got a jeweler's saw on eBay for $14, and used the #2 blade. Make sure you cut near the top of the IC so you don't hit the die as I did.

[4] The brilliant part of the 555 timer is that the oscillation frequency depends only on the external resistors and capacitor and is insensitive to the supply voltage. If the supply voltage drops, the 1/3 and 2/3 references drop too, so you might expect the oscillations to be faster. But the lower voltage charges the capacitor more slowly, canceling this out and keeping the frequency constant.

This voltage insensitivity is so tricky that the chip's designer didn't figure it out until near the end of the 555's design, but it made a big difference. The original design was more complex and required nine pins, which is a terrible size for an IC since there are no packages between 8 and 14 pins. The final, simpler 555 design worked with 8 pins, making the chip's packaging much cheaper. (See page 11-3 of Designing Analog Chips for the full story.)

[5] You might have wondered why there is a distinction between the collector and emitter of a transistor, when the typical diagram of a transistor is symmetrical. As you can see from the die photo, the collector and emitter are very different in a real transistor. In addition to the very large size difference, the silicon doping is different. The result is a transistor will have poor gain if the collector and emitter are swapped.

[6] For more information about current mirrors, check wikipedia, any analog IC book, or chapter 3 of Designing Analog Chips.

[7] Differential pairs are also called long-tailed pairs. According to Analysis and Design of Analog Integrated Circuits differential pairs are "perhaps the most widely used two-transistor subcircuits in monolithic analog circuits." (p214) For more information about differential pairs, see wikipedia, any analog IC book, or chapter 4 of Designing Analog Chips.

[8] The 555 schematic used in this article is from the Philips datasheet.

[9] Note that the three resistors for the voltage divider are parallel and next to each other. This helps ensure they have the same resistance even if there are electrical variations across the silicon.

[10] I'm not criticizing the 555; Hans Camenzind points out the design flaws and attributes them to "the early period of IC design (and the inexperience of a rookie designer)"; see Designing Analog Chips, page 11-4. The design of a 555 replacement is discussed in detail in "Redesigning the old 555", IEEE Spectrum, September 1997. That article makes it clear how much much faster IC design is now than in 1970. It took months to create the layout of the 555 chip by hand and manually verify it for correctness. The new chip took two days to layout and 20 minutes to verify.

[11] Evil Mad Scientist sells a very cool discrete 555 timer kit, duplicating the 555 circuit on a larger scale with individual transistors and resistors — it actually works as a 555 replacement. Their 555 footstool is also worth a look.

Large-size 555 timer created by Evil Mad Scientist Lab.