Pulse power unit for teletype from the 1940s (with luminous mercury thyratrons!)

 
3r3-31. Recently, we began the process of restoring the Teletype Model 1? a naval communications system from the 1940s to 3r-32r.[1]
. [3] . 3r3633.
 
 
Pulse power unit for teletype from the 1940s (with luminous mercury thyratrons!)
 
3r33470. Partially disassembled teletype Model 19 [/i]
 
 
3r?500. Current loop
 
Teletypes communicate with each other through a 60mA current loop: the presence of current in the circuit gives the value "marker" (teletype, respectively, perforates punched tape), and if the current is interrupted, then we get a value called "space". Each character is transmitted in seven bits: the start bit, 5 data bits and the stop bit. If you have ever used serial devices on your PC, then you should know that teletypes introduced the notion of start and stop bits. A bodrate was named after the inventor of 5-bit coding - 3r382. Emile Bodo
. The REC-30 power supply delivered 900 mA at 120V DC, enough to power 15 teletypes.
 
 
Perhaps you are wondering why teletypes simply did not use voltage levels instead of this strange current loop? The main reason is that when sending signals by wire to another city, it is very difficult to know what the final voltage will be at that end, due to the voltage drop along the way. But if you send 60mA, the receiver will receive the same 60mA (if there is no short circuit, of course) 3r3888.[4]
. Wdwd , 3r3156. CC BY ???r3r3641. . 3r33471.
 
 
Unlike the diodes in the diagram above, the thyratron tubes in the power supply can be switched on and off, thus giving you the ability to control the output voltage. The basic idea is to include a thyratron in a certain fixed phase of an alternating current cycle, as in the animation below. If the thyratron is on a full cycle, then we get the full voltage, if the half cycle is on, then half the voltage, and if only for a small fraction of the cycle, then the output will be a very small voltage 3r3163.[8]
. 3r3165.
This technique is called 3r3167. phase control
, because the device is turned on only at a certain phase angle (for example, between 0 ° and 180 ° for an AC sinusoid). A very similar method is used in the usual dimmer lighting , except that they use semiconductor triacs instead of thyratron tubes [9] 3r33175.
.
 
 
3r3181.
 
3r33470. Phase regulation scheme. The upper part of the animation shows what part of the pulse is used, and the lower one shows the moment in which the thyratron is on. Image belongs to 3r3185. Zureks
, 3r3187. CC BY-SA ???r3r3641. . 3r33471.
 
 
The thyratron tubes of the power supply unit resemble radio tubes, but unlike them, they contain argon and mercury vapors inside a glass bulb (whereas vacuum tubes are maintained in radio tubes). Thyratron tubes consist of three components: the filament (cathode), the anode and the grid. Incandescent filament, similar to those used in conventional light bulbs, heats up and emits electrons. The anode, mounted on top of the tube, captures these electrons, thus allowing current to flow from the cathode to the anode. The reference electrode (grid) located between the anode and the cathode serves the purpose of blocking the flow of electrons. When electrons flow to the anode, the mercury vapor ionizes, thus opening the thyratron and producing a side effect in the form of a blue glow, which you can observe in the photo (but in conventional radio tubes, although there is a stream of electrons, there is nothing to ionize). Ionized mercury creates a highly conductive path between the cathode and the anode, allowing a fairly strong current to flow (1.5A). Once the mercury is ionized, the grid no longer controls the thyratron, and it remains open until the voltage between the anode and cathode drops to zero. At this point, the ionization subsides and the tube turns off until it is again in the open state.
 
 
3r3198.
 
3r33470. REC-30 power pack for teletype. The blue luminescence of the thyratron tubes is visible; the orange luminescence of the neon lamp used as a source of reference voltage. The timer and relay are noticeable from the top left [/i]
 
 
The voltage on the grid is controlled by a thyratron. Negative voltage reflects negatively charged electrons, thus preventing electrons from flowing between the cathode and the anode. But when the voltage at the anode becomes strong enough, the electrons overcome the repulsion of the grid, and the thyratron opens. The important point is that the higher the negative voltage on the grid, the stronger the repulsion occurs and the higher voltage is required to open the thyratron. Thus, the grid voltage controls the phase of the alternating current cycle in which the thyratron opens.
 
 
The power supply control circuit stabilizes the output voltage by changing the voltage on the grid, controlling the timings of the thyratron [10] corresponding note 3r3641.
. The main detail that is important to notice is how the peaks of the blue-green and yellow curves shift to the left with increasing output voltage, and this means that the tiratrons are triggered earlier.
 
 

 
3r33470. By changing the phase, the output voltage is regulated from 130V to 170V. Yellow and blue-green are the voltages on the thyratrons. Pink - signal control grid. Blue - inverted output voltage. 3r33471.
 
 
The image below shows the schematic of the REC-30 power supply unit (larger — 3r-3236. Here, 3r-?641.). The AC input circuit is green. In it, the autotransformer stabilizes the input voltage to 230V and supplies it to the control transformer. Installed thyratron tubes have an interesting feature - they must be preheated before use to ensure that the mercury is in a gaseous state. Warming up is performed by using bimetallic 3r3643. timer for 20 seconds
[13]
RoHS )
 
 
 
External control
 
Bimetallic timer and relay
 
16-bit MPS430 microcontroller
 
 
 
Switching elements
 
Thyratron tubes 3r3402. 323 3r3641.
 
N-channel power 11A MOSFET'y
 
 
 
The reference voltage source is
 
Neon gas discharge lamp GE NE-42
 
Bandgap TSM103 /A
 
 
 
Switching control
 
Pentode 3r33432. 6F6
 
Resonance controller L6599 3r3641.
 
 
 
Switching frequency
 
120Hz 3r3452.  
approximately 500 kHz
 
 
 
 
I measured the quality of the REC-30 output signal (in the image below). The power supply produces a much higher quality signal than I expected - a pulsation of only 200 mV (waves on the blue horizontal line), which is very close to the level of devices from Apple. However, on the oscillogram one can also see narrow bursts (vertical lines) of about 8 volts, which occur when switching thyratrons. These bursts are quite large compared to the power supply from Apple, but still much less than in
cheap 3r3641 chargers. .
 
 

 
3r33470. The output signal of the power supply REC-30. You can see a slight ripple and surges when switching power. 3r33471.
 
 
3r?500. Conclusion 3r3015.
 
The REC-30 power supply delivers over 100 watts of DC power for a teletype. Released in the 1940s, the REC-30 was an extremely early impulse power supply, using mercury thyratron tubes for greater efficiency. It was monstrously large for a 100W power supply: the weight was more than 45 kilograms. A comparable modern power unit is smaller and lighter by more than 100 times. Despite its age, the power supply worked flawlessly, as you can see in Mark's video. In addition, the work process itself looks very beautiful - the blue glow from the tiratrons and the orange from the large neon lamp.
 
 

 
 
Thanks to Carl Claunch and Marc Verdiell for their work with this power supply!
 
 
3r?500. Notes 3r3015.
 
3r3504.
1. The first mention of the introduction of teletypes for the Navy was in the journal 3r3506. BuShips Electron
from September 1945 The development of a radioteletype (RTTY), in which frequency manipulation (FSK) is commonly used, allowed the use of teletypes for the needs of the Navy. At first, the fleet used radioteletayps only to communicate the coast stations with each other, and only then began to use them on ships. The key advantage of the teletype was speed: it was four times faster than sending a message by radio by an operator manually. In addition, messages on punched tape could be automatically copied and forwarded. Also, a teletype could be integrated with cryptographic equipment, such as 3r3508. SIGTOT
based on the cryptosystem of disposable notebooks. More about teletypes of World War II can read 3r33510. here
. 3r33512. ↑
 
 
3r33518.
2. In the 1870s, Emile Bodo invented a 5-bit code named after him. Another 5-bit code was created by Donald Murray in 1901 and was standardized as ITA-2 (CCITT-2). Both coding schemes look haphazard - the characters appear scattered in random order. However, the original Bodo code was also the Gray code, and Murray's code was optimized to make fewer perforations for the most common symbols, which allowed reducing the wear of mechanisms. 5-bit codes were relevant to ASCII standardization in the 1960s, in which the alphabetic and binary order of the characters are the same. 3r33520. ↑
 
 
3r33526.
3. More detailed information on how teletype works - 3r33528. here
. In addition, there is an even more extensive document - 3r33530. Fundamentals of Telegraphy (Teletypewriter)
, Army Technical Manual TM 11-65? 1954. Drawings on the REC-30 can be downloaded hence r3r3641. , and documentation - 3r33434. here . 3r33536. ↑
 
 
3r33542.
4. Note that, in contrast to a system based on voltage measurement, the components of the current loop, as the name implies, must form a topological loop in order for current to flow through them. If you exclude any device from the chain, then the loop will break if there is no loop closing mechanism. As a result, the teletype communication system contains many sockets that close when the component is disconnected so that the current loop continues to function. 3r3544. ↑
 
 
3r33550.
5. The main reason that the REC-30 is so large and heavy, compared to modern switching power supplies, is that the pulse frequency is only 60 Hz, while modern power supplies operate at a frequency of tens of kilohertz. Since 3r???. EMF transformer
proportional to the frequency of its operation, the high-frequency transformers can be much smaller in size than the low-frequency ones (3r35454. more 3r3641.). 3r33556. ↑
 
 
3r? 3562.
6. The REC-30 can operate with a wide range of input voltages (9? 10? 11? 12? 19? 21? 23? 250 volts AC) and currents of various frequencies (2? 4? 50 and 60 Hz). Modern pulse power supplies automatically adjust to the input voltage, but the REC-30 requires connecting the contact to the appropriate terminal of the autotransformer to change the input voltage. You may find the frequency at 25 Hz very strange for the input current of the power supply, but many regions of the United States used 25 Hz power in the 1900s. In particular, Niagara Falls generated an electric current of 25 Hz due to the design features of the turbines. In 191? more than 2/3 of the energy output in New York was at a frequency of 25 Hz, and in Buffalo it was only in 1952 that they began to use a 60 Hz current in large volumes than 25 Hz. Because of the popularity of 25 Hz current, many IBM punchers of the early 1900s were able to operate at 25 Hertz (3–3–3564. More, 3–3–?641.). 3r3566. ↑
 
 
3r? 3572.
7. Isolating input AC from output DC is a key safety element in most power supplies, including chargers, computer power supplies, and the considered REC-30. This decoupling prevents strong electric shock in contact with the output contacts. For the REC-3? the anode transformer plays a critical role as an insulator. Note that the autotransformer does not provide any insulation protection, since it has only one main winding and to touch its output is the same as touching the AC input. The rest of the circuit is neatly designed in such a way that there is no direct path between the input and the output: the control system is entirely on the secondary side, the filaments of the thyratrons are fed from the winding, isolated from the autotransformer, and the relays provide insulation to the timer. In addition, the 120V output is made push-pull instead of grounding one of the contacts: this means that you need to grab 2 contacts at once to get an electric shock. 3r? 3574. ↑
 
 
3r? 3580.
8. Modern pulse power supplies use pulse-width modulated (PWM) circuits to switch power at a frequency of thousands of times per second. This allows them to have a much smaller size and a smoother output than the power supplies, which switch only once in one AC cycle. But at the same time, they need a much more complex management system. 3r33582. ↑
 
 
3r33588.
9. The current solid-state equivalent of thyratrons is 3r33590. silicon rectifier
which is also called SCR or thyristor (a combination of the words "thyratron" and "transistor"). SCR has four semiconductor layers (compared to a 2-layer diode and a 3-layer transistor). Just like a thyratron, the SCR is in the off state until current is applied to the control electrode. The SCR remains on and acts as a diode until the voltage drops to 0 (strictly speaking, until the flowing current becomes less than the holding current). A triac is a semiconductor element that is very similar to SCR, except for the fact that it transmits current in both directions, which makes it more convenient in alternating current circuits. 3r? 3592. ↑
 
 
3r3-3598.
10. Initially, I thought that, with increasing load, the thyratrons would be open for longer periods of time in order to provide more current. However, after connecting the oscilloscope and studying the behavior of the thyratrons under different loads, I did not notice any phase shift. It turned out that this is the expected behavior: the transformer produces in general a constant voltage, regardless of the load. Thus, the timings of the thyratrons remain constant with changes in the load, and the transformer simply produces more current. 3r3600. In this video 3r3641. You may notice how the luminescence of the thyratrons changes with increasing current intensity. 3r3602. ↑
 
 
3r3608.
11. Under a light load, the power supply may even occasionally skip the AC cycle completely, instead of switching the thyratrons in the middle of it. Visually, this can be observed as the flickering of tiratrons, instead of a constant glow. Not sure if it's a bug or a feature. 3r3610. ↑
 
 
3r31616.
12. On the oscillogram, the yellow and blue-green lines indicate the voltage on two thyratrons. The flat part of the lines (at this moment the difference in voltages is near zero) means that at this moment the thyratron is turned on. Thyratron tubes are asymmetrical, and therefore the one to which the yellow signal is fixed usually turns on later (visually, one thyratron glows brighter than the other). The pink line is the voltage of the control grid. Note that it increases in order to increase the output voltage, and this increase causes the thyratrons to operate earlier. The vertical splash of the pink line is just noise due to the triggering of thyratrons. The blue line below is the output voltage (inverted: the line goes down as the voltage rises).
 
 
It's a mystery to me whyyes, at least one thyratron works - constantly either the yellow or the blue-green line is at zero. I would expect to see a gap between zero voltage on one thyratron and the moment of opening of the second. I suspect that large inductors induce a negative charge on the cathode, thus, even when the anode itself is negative, the potential difference between the cathode and the anode is still positive. 3r3622. ↑
 
 
3r3628.
13. A 20-second delay before energizing the tubes is achieved by a timer and a relay. The timer uses a bimetal plate with a preheater. When you turn on the power supply, the cathode receives power immediately 3r3r6363.  
to warm up the tubes. At the same time, the heater inside the timer heats the bimetallic plate and at some point the plate bends enough to close the contacts and power the tubes. At the same time, the relay is activated and in turn also closes the contacts. 3r33232. ↑
 
 
3r3638.
14. The cathode-related circuit is a bit tricky, since the filaments of the thyratrons are used both as heating tubes and directly as cathodes. They are fed 2.5V from the autotransformer. In addition, since the filaments in the thyratrons are also cathodes, they themselves produce an output voltage and are connected to the high side of the output signal. In order to ensure the fulfillment of both tasks, the split winding of the autotransformer imposes a voltage of 2.5V on the filament, but at the same time passes the output voltage directly. Both thyratrons use a total of 35W on the filament only, so that, as you can see, the preheat spends a lot of energy and generates a lot of heat, and thus, in some way, negates the benefits of a switching power supply. 3r3640. ↑
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