9 ms·

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

131 points by eaguyhn 8y ago

tlb 8y ago
Mercury rectifiers are beautiful. I once built a vacuum tube audio amplifier with directly heated triodes and mercury rectifiers, aiming for a symmetrical orange-blue-blue-orange glow. It turned out that mercury rectifiers make a lot of electrical noise when they switch, so there was an annoying 120 Hz buzzing noise. But it did look cool.
- dekhn 8y ago
  I recently built a "vacuum tube nightlight" (http://www.electronixandmore.com/projects/vrtubenightlight/index.html http://www.electronixandmore.com/projects/vrtubenightlight/i...) because I thought the plasma tubes give off such a nice glow. But they aren't very bright and it's hard to see the plasma; these rectifiers are way more attractive but I can't bring myself to work with them.
  - planteen 8y ago
    > these rectifiers are way more attractive but I can't bring myself to work with them. Cool project! Do you say you can't bring yourself to work with them for safety reasons (as in, I imagine mercury vapor is very dangerous if it escapes the tube) or is there another reason?
    - dekhn 8y ago
      just the mercury vapor being dangerous.
- antiphase 8y ago
  More likely that it generates a massive amount of wideband noise that happens to be modulated at 120Hz in this case.
deleted 8y ago
[deleted]
clebio 8y ago
> capacitors are still the bulkiest components in the MacBook charger, as you can see below. This seems like an odd statement, given the image directly next to that sentence (and all evidence from any electronics device I've ever taken apart). The transformer is the largest piece, followed possibly by the inductors. IANA electrician, though, so maybe there's some context that I'm missing (i.e. a transform isn't technically a single component)?
- danielvf 8y ago
  The transformer may be the biggest single piece in the MacBook charger, but the many capacitors together take up more volume than the transformer.
- kens 8y ago
  I meant that in total, the capacitors take up more space than the transformer. But the sentence is unclear, so I rewrote it.
blattimwind 8y ago
Using controlled rectifiers like this usually isn't regarded as a SMPS, though. (Notably because this technique does not allow to select the switching frequency, which is fixed by the grid in this circuit, so this circuit does not allow high-frequency switching, which is one of the main reasons SMPSs are much lighter than traditional power supplies -- the higher frequencies mean a much smaller core can be used while avoiding saturation). Using SCRs or triacs to pre-regulate the voltage of a linear regulator was a very common technique well into the late 80s for high power, precision power supplies. The power supply shown here works very much the same, except there being no linear post-regulation stage. Contrast this with the HV generation in devices using CRTs, which early on (~50s) started to use high-frequency converters, usually of the resonant kind.
stickfigure 8y ago
20 years ago I visited the Energetica museum in Amsterdam. One of the exhibits was a large cart/trailer that was used to charge submarines from the WWI era (!). It had two HUGE hand-blown glass mercury rectifiers in the front of it; you could fit a small person inside each, and there was a couple inches of mercury sitting at the bottom. I was shocked when a docent walked over and turned it on. The rectifiers lit the whole room up with a beautiful purple glow. Beautiful. Doing a quick web search it looks like the museum is no longer. Too bad - even though I couldn't read the plaques, it was still one of the most fascinating museum experiences I've ever had.
- ddingus 8y ago
  Word of the day: docent. I rarely see this in common expression. Nice choice. Old tech has great appeal. Scale is our size, and features forces, artifacts we can see, hear and feel easily. I can imagine that glow. Damn cool.
  - tingletech 8y ago
    what other word is used for the volunteers who interact with patrons at a museum?
    - testvox 8y ago
      > The title "docent" is not widely used outside the United States, with the terms "guide", "facilitator", or "educator" preferred. https://en.wikipedia.org/wiki/Museum_docent https://en.wikipedia.org/wiki/Museum_docent
      - ddingus 8y ago
        Nailed it. It is not all that widely used in the US either, in my experience anyway.
      - sannee 8y ago
        Interestingly, the same word is used at some european universities to mean roughly "associate professor".
lightlyused 8y ago
Wow, never thought to look at the PS when I played with one of these at the university ham club station back in the 80's. Now I wonder what other cool stuff I missed.
Animats 8y ago
Nice. Thyatrons were about the only component available for high-power control in the tube era. Today we have MOSFETs which approach ideal power switches, but it took a long time to get there. As someone pointed out, that's a switching voltage regulator, not a switching power supply. The transformers there are all upstream of the switching. I've restored five Teletype machines like the OP's Model 19 [1], so I've needed similar 120VDC 60mA power supplies. So I designed my own switching power supply.[2] This has a USB port for input, and a 120VDC 60mA output for directly driving the Teletype machine. It's powered entirely from the USB port. This seemed impossible to some people. There's only 5V at less than 500mA coming in, and 120VDC 60mA out. But it's not impossible, because the load is inductive and intermittent. The selector magnet in old Teletypes has a huge inductance, about 5.5 Henries. (Not mH, H.). The 120VDC is only needed for about the first 1ms of each bit time, to force current through that huge inductance. By 5ms or so, you only need about 6V. So you can charge up a capacitor to get the initial 120V, then let a sustain supply take over. My design is totally modern, built from surface mount components and in a small case. Here's the schematic.[3] There's an explanation in [2]. It's been amusing to see the reaction of the Teletype community. They like it, but most can't solder surface mount. One hobbyist is making these things for others. I put the design on Github as open source and made a few for myself, and I've sold some board kits. Not enough potential volume to have it manufactured. Informally, here's how a switching power supply works. Everywhere else in electronics, you try to get rid of spikes. In switching power supplies, you make and use big ones. You start with a source of DC power, and you hook that to the primary winding of a transformer, with a switch so you can turn the power on and off. You turn the switch on, and current flows into the transformer. The magnetics in the transformer charge up, storing energy. After a while (milliseconds) the magnetics will saturate, and can't store any more energy. You now have a short circuit, DC going through a low-resistance transformer. But you turn off the switch before that happens. (Switching power supplies are always milliseconds from burnout, which is why they burn up if the switching fails.) When you turn the switch off, you now have an open circuited inductor. The energy in that inductor has to go someplace. It comes out as a huge spike, in theory infinite voltage if the transformer resistance was zero, and in practice it can be a few hundred volts. It can't come out the primary, because the switch is open. So it comes out the transformer's secondary winding, where it's fed through a diode into a capacitor. There's the output. It's simple. An old-style auto ignition with a coil and breaker points works this way. The problems come in as you make it well-behaved. First, controlling the switch is complicated. You want to open the switch before the transformer hits saturation. Failure to do this will burn something out. So there's usually current sensing. Then you want to turn the switch back on when the output voltage from the inductor drops below the voltage in the output capacitor, because no more current will flow through the diode after that. That just makes it output power. Then you need output voltage sensing, which shortens the charging time to reduce output to maintain the desired voltage. You need protection to shut everything down if the switch gets stuck. (MOSFETs tend to fail in the ON state, and lack of good protection circuitry causes fires.) This thing works by making big spikes at a few hundred kilohertz. That makes it a radio transmitter. You need inductors and bypass caps to prevent it from blithering all over the RF spectrum. Or sending spiky noise to its output or input. The bypass caps and inductors need to be close to the source of the spikes, so PC board layout really matters. These things will not work on a breadboard. All this is why switching power supplies have so many small parts. Once you get it right, they work beautifully. Very high efficiency and low heat. [1] http://www.aetherltd.com http://www.aetherltd.com [2] https://github.com/John-Nagle/ttyloopdriver https://github.com/John-Nagle/ttyloopdriver [3] https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/images/schematic.png https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/m...
kazinator 8y ago
This is not a switched-mode power supply (SMPS), sorry. This is more like a vacuum analog of the SCR. Switching on and off based on phase angle isn't the same thing as the working principle in SMPS's. An SMPS, in a nutshell, uses pulses of current to "charge" an inductor, which continues to source current during the off periods when the current pulse is cut off and the magnetic field is collapsing. (Inductors oppose changes in current.) There is also non-inductive SMPS using capacitors only: the charge pump.
- kens 8y ago
  > An SMPS, in a nutshell, uses pulses of current to "charge" an inductor, which continues to source current during the off periods That's exactly what happens in this power supply. The tubes charge up a giant (grapefruit-sized, if grapefruit were cubes) inductor, which supplies the current when the thyratrons aren't. I don't see a good reason not to consider this a switching power supply. Phase angle is just PWM at a lower frequency.
  - CamperBob2 8y ago
    Whether this qualifies as a switching supply is an interesting question. I don't think I agree with Animats in that regard. Regulation is accomplished by switching a nonlinear element via a feedback loop, and that's good enough to qualify it as a "switched-mode" supply in my book. You certainly couldn't call it a linear regulator, because the thyratrons are either fully on or fully off at all times. From the input's point of view it will exhibit negative resistance, with a reduction in the current required as the input voltage rises. That makes the classification especially hard to refute. It's true that energy is being stored in the smoothing choke, but that alone doesn't make the difference since the choke could be used to accomplish the same thing in a traditional linear supply. The choke isn't inside the feedback loop, though, and that is a point in favor of the "It's not a switcher" camp. But it's the only one. In any event, charge-pump supplies don't need an inductor at all, and nobody questions whether they operate in linear or switched-mode. (Edit: looking at the schematic, the filter choke is indeed inside the feedback loop. Case closed, it's a switching supply.)
CamperBob2 8y ago
More pr0n for thyratron fans (Monarch 10EE lathe): https://youtu.be/k15pWPBNAUE?t=45s https://youtu.be/k15pWPBNAUE?t=45s https://www.garagejournal.com/forum/showthread.php?t=264474 https://www.garagejournal.com/forum/showthread.php?t=264474 These were made from 1939 through at least the 1990s, although the vacuum tubes went away in the mid-1980s. Amazingly long-lived product.
chiph 8y ago
The power supplies I worked with on the Model 28 were the typical linear design. They had huge resistors on them that after a while, the heat from them would lift the copper pads off the circuit card. So they'd be hanging off the card, only held on by their solder connection. If the inspectors took a dislike to us, they'd write us up for them. But mostly they knew this was a design "feature" and would overlook it. One of the cool things about that era of Teletype was the mechanical serial to parallel converter. This was a multi-lobe camshaft that as the pulses came in and the electromagnet selector engaged/released, would (if your timing was set right) select five bars (for your Baudot code) that controlled the position of the type box where all the letters and numbers were.
- Animats 8y ago
  Yes, the usual Teletype loop current supply is a 120VDC supply with a 10K 2W resistor, to get 60mA constant current. Efficiency is below 10%, with over 90% of the power going to heating the resistor. That's partly why I built a switching supply interface. Far less heat. Small box, no ventilation required.
  - chiph 8y ago
    I briefly considered taking one of the cards along to my high-reliability soldering class just to provoke a reaction from the instructor.. But figured it might bring down attention from On High into the state of our circuit cards and that wouldn't have been good. Besides, they lasted 5+ years like that before needing replacement. We pretty much only replaced them when the boards became brittle from the heat.
MisterTea 8y ago
Thyratrons were the early form of SCR, the silicon controlled rectifier. These aren't SMPS's in the sense we think of them today but they do switch the input at a controlled rate to create a controlled output so the nomenclature can be used in this sense. The proper term is: Phase-fired controller: https://en.wikipedia.org/wiki/Phase-fired_controller https://en.wikipedia.org/wiki/Phase-fired_controller You can see the idea is to run a timer that is phase locked to the incoming mains frequency. Now you can control when the switches turn on during a half cycle. If you turn the switch on at the beginning of a half cycle, you have full power. If you turn the switch on midway through the cycle, you get half power and so on. So your command signal asks the controller for more and more of the complete wave cycle until you get the full RMS value of the line voltage plus the load current (minus the losses in the rectifiers of course). The resultant output is a chopped up EMI laden mess but it does the job quite nicely after some filtering. They were also used in early motor drives to control the speed of a brushed DC motor from single or three phase AC source. I know some lathes from the 50's or 60's had thyratron motor drives in them. At my work we have an Electron beam welder which uses an SCR controller for the high voltage power supply. It's interesting: the controller is directly fed 480V three phase. From there in comes in through a breaker, a contactor, and two current sensing transformers.Like so (dammit variable width fonts...): A--~~--||--S--^^^^^^--/--SCR---\---) B--~~--||-----^^^^^^--|-BRIDGE-| )Inductor C--~~--||--S--^^^^^^--\----------/---) Key: ~~ fuse, || contactor, S current sensor, ^^^^^^ transformer primary The three phases then run to the power three supply transformers in series and then off to a three phase SCR bridge for a total of six SCR's. The output of the bridge has a huge 200 pound inductor across it. The idea is the bridge is phase fire controlled and the inductor is so high in value that the controller can slowly watch the current ramp when the SCR's are turned on and wait until the feedback from the power supply matches the command from the potentiometer and adjust the phase angle firing accordingly. It's creating a controlled short circuit using the series transformers as the load. It's a primitive solid state method of varying an AC voltage. Before the SCR system they used a motor generator with an op-amp PID loop watching the feedback and control pot who's output controlled a small phase fired SCR bridge that delivered a varying DC voltage to the generator field winding. You effectively had a motor generator who's output varied from 0-480V AC three phase. Today you'd have a small metal oil tank containing an entire SMPS which is smaller than the control cabinets for our old linear supplies.
6nf 8y ago
Also check out Mercury Arc Rectifiers, amazingly cool old tech: https://www.youtube.com/watch?v=yjMZ5qtyCUc https://www.youtube.com/watch?v=yjMZ5qtyCUc
Ricardus 8y ago
I've been following curiousmarc's channel for a couple years. I love his videos. This teletype restoration is amazing. The most recent video on his channel is of him and the guys going to a place that sells replacement parts... for teletypes. I mean this guy they visit literally has every part you would need to repair, and probably build teletypes from scratch. They had everything Marc needed to complete his restoration. Amazing!
growlist 8y ago
I had no inkling I needed a mercury thyratron until now! Amazing stuff, this is like something from Fallout brought to life, and the mercury makes it suitably hazardous. I'm guessing the mercury is integral to the hue? I saw something like this in a railway museum in Yorkshire or thereabout when I was a kid - I recall its label included 'rectifier' and I think it contained mercury, and was flashing or sparking in some way. I guess it must have been one of the mercury rectifiers as already mentioned, though I don't remember it glowing quite so beautifully. Certainly had me intrigued.
Aardwolf 8y ago
Reminds me a bit about this thing covered by photonicinduction: https://www.youtube.com/watch?v=QY6V2syGnZA https://www.youtube.com/watch?v=QY6V2syGnZA Fortunately he didn't "pop" that one "for science" unlike most other things he covers, it's too valuable :D
sephamorr 8y ago
There are a lot of comments here saying that this isn't a SMPS, then proceed to describe a flyback converter (and the difference vs this unit) as a justification. Just note that the field of power electronics is quite large, and as an example, some power converters don't even have an inductor or transformer (though most do). As I understood, the definition of a SMPS relates to switching between discrete states (i.e. on/off) as opposed to a linear supply, which dissipates power to regulate. Source: am a power electronics engineer.

131 points by eaguyhn 8y ago

tlb 8y ago

Mercury rectifiers are beautiful. I once built a vacuum tube audio amplifier with directly heated triodes and mercury rectifiers, aiming for a symmetrical orange-blue-blue-orange glow. It turned out that mercury rectifiers make a lot of electrical noise when they switch, so there was an annoying 120 Hz buzzing noise. But it did look cool.

dekhn 8y ago
I recently built a "vacuum tube nightlight" (http://www.electronixandmore.com/projects/vrtubenightlight/index.html http://www.electronixandmore.com/projects/vrtubenightlight/i...) because I thought the plasma tubes give off such a nice glow. But they aren't very bright and it's hard to see the plasma; these rectifiers are way more attractive but I can't bring myself to work with them.
- planteen 8y ago
  > these rectifiers are way more attractive but I can't bring myself to work with them. Cool project! Do you say you can't bring yourself to work with them for safety reasons (as in, I imagine mercury vapor is very dangerous if it escapes the tube) or is there another reason?
  - dekhn 8y ago
    just the mercury vapor being dangerous.
antiphase 8y ago
More likely that it generates a massive amount of wideband noise that happens to be modulated at 120Hz in this case.

deleted 8y ago

[deleted]

clebio 8y ago

> capacitors are still the bulkiest components in the MacBook charger, as you can see below. This seems like an odd statement, given the image directly next to that sentence (and all evidence from any electronics device I've ever taken apart). The transformer is the largest piece, followed possibly by the inductors. IANA electrician, though, so maybe there's some context that I'm missing (i.e. a transform isn't technically a single component)?

danielvf 8y ago
The transformer may be the biggest single piece in the MacBook charger, but the many capacitors together take up more volume than the transformer.
kens 8y ago
I meant that in total, the capacitors take up more space than the transformer. But the sentence is unclear, so I rewrote it.

blattimwind 8y ago

Using controlled rectifiers like this usually isn't regarded as a SMPS, though. (Notably because this technique does not allow to select the switching frequency, which is fixed by the grid in this circuit, so this circuit does not allow high-frequency switching, which is one of the main reasons SMPSs are much lighter than traditional power supplies -- the higher frequencies mean a much smaller core can be used while avoiding saturation). Using SCRs or triacs to pre-regulate the voltage of a linear regulator was a very common technique well into the late 80s for high power, precision power supplies. The power supply shown here works very much the same, except there being no linear post-regulation stage. Contrast this with the HV generation in devices using CRTs, which early on (~50s) started to use high-frequency converters, usually of the resonant kind.

stickfigure 8y ago

20 years ago I visited the Energetica museum in Amsterdam. One of the exhibits was a large cart/trailer that was used to charge submarines from the WWI era (!). It had two HUGE hand-blown glass mercury rectifiers in the front of it; you could fit a small person inside each, and there was a couple inches of mercury sitting at the bottom. I was shocked when a docent walked over and turned it on. The rectifiers lit the whole room up with a beautiful purple glow. Beautiful. Doing a quick web search it looks like the museum is no longer. Too bad - even though I couldn't read the plaques, it was still one of the most fascinating museum experiences I've ever had.

ddingus 8y ago
Word of the day: docent. I rarely see this in common expression. Nice choice. Old tech has great appeal. Scale is our size, and features forces, artifacts we can see, hear and feel easily. I can imagine that glow. Damn cool.
- tingletech 8y ago
  what other word is used for the volunteers who interact with patrons at a museum?
  - testvox 8y ago
    > The title "docent" is not widely used outside the United States, with the terms "guide", "facilitator", or "educator" preferred. https://en.wikipedia.org/wiki/Museum_docent https://en.wikipedia.org/wiki/Museum_docent
    - ddingus 8y ago
      Nailed it. It is not all that widely used in the US either, in my experience anyway.
    - sannee 8y ago
      Interestingly, the same word is used at some european universities to mean roughly "associate professor".

lightlyused 8y ago

Wow, never thought to look at the PS when I played with one of these at the university ham club station back in the 80's. Now I wonder what other cool stuff I missed.

Animats 8y ago

Nice. Thyatrons were about the only component available for high-power control in the tube era. Today we have MOSFETs which approach ideal power switches, but it took a long time to get there. As someone pointed out, that's a switching voltage regulator, not a switching power supply. The transformers there are all upstream of the switching. I've restored five Teletype machines like the OP's Model 19 [1], so I've needed similar 120VDC 60mA power supplies. So I designed my own switching power supply.[2] This has a USB port for input, and a 120VDC 60mA output for directly driving the Teletype machine. It's powered entirely from the USB port. This seemed impossible to some people. There's only 5V at less than 500mA coming in, and 120VDC 60mA out. But it's not impossible, because the load is inductive and intermittent. The selector magnet in old Teletypes has a huge inductance, about 5.5 Henries. (Not mH, H.). The 120VDC is only needed for about the first 1ms of each bit time, to force current through that huge inductance. By 5ms or so, you only need about 6V. So you can charge up a capacitor to get the initial 120V, then let a sustain supply take over. My design is totally modern, built from surface mount components and in a small case. Here's the schematic.[3] There's an explanation in [2]. It's been amusing to see the reaction of the Teletype community. They like it, but most can't solder surface mount. One hobbyist is making these things for others. I put the design on Github as open source and made a few for myself, and I've sold some board kits. Not enough potential volume to have it manufactured. Informally, here's how a switching power supply works. Everywhere else in electronics, you try to get rid of spikes. In switching power supplies, you make and use big ones. You start with a source of DC power, and you hook that to the primary winding of a transformer, with a switch so you can turn the power on and off. You turn the switch on, and current flows into the transformer. The magnetics in the transformer charge up, storing energy. After a while (milliseconds) the magnetics will saturate, and can't store any more energy. You now have a short circuit, DC going through a low-resistance transformer. But you turn off the switch before that happens. (Switching power supplies are always milliseconds from burnout, which is why they burn up if the switching fails.) When you turn the switch off, you now have an open circuited inductor. The energy in that inductor has to go someplace. It comes out as a huge spike, in theory infinite voltage if the transformer resistance was zero, and in practice it can be a few hundred volts. It can't come out the primary, because the switch is open. So it comes out the transformer's secondary winding, where it's fed through a diode into a capacitor. There's the output. It's simple. An old-style auto ignition with a coil and breaker points works this way. The problems come in as you make it well-behaved. First, controlling the switch is complicated. You want to open the switch before the transformer hits saturation. Failure to do this will burn something out. So there's usually current sensing. Then you want to turn the switch back on when the output voltage from the inductor drops below the voltage in the output capacitor, because no more current will flow through the diode after that. That just makes it output power. Then you need output voltage sensing, which shortens the charging time to reduce output to maintain the desired voltage. You need protection to shut everything down if the switch gets stuck. (MOSFETs tend to fail in the ON state, and lack of good protection circuitry causes fires.) This thing works by making big spikes at a few hundred kilohertz. That makes it a radio transmitter. You need inductors and bypass caps to prevent it from blithering all over the RF spectrum. Or sending spiky noise to its output or input. The bypass caps and inductors need to be close to the source of the spikes, so PC board layout really matters. These things will not work on a breadboard. All this is why switching power supplies have so many small parts. Once you get it right, they work beautifully. Very high efficiency and low heat. [1] http://www.aetherltd.com http://www.aetherltd.com [2] https://github.com/John-Nagle/ttyloopdriver https://github.com/John-Nagle/ttyloopdriver [3] https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/images/schematic.png https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/m...

kazinator 8y ago

This is not a switched-mode power supply (SMPS), sorry. This is more like a vacuum analog of the SCR. Switching on and off based on phase angle isn't the same thing as the working principle in SMPS's. An SMPS, in a nutshell, uses pulses of current to "charge" an inductor, which continues to source current during the off periods when the current pulse is cut off and the magnetic field is collapsing. (Inductors oppose changes in current.) There is also non-inductive SMPS using capacitors only: the charge pump.

kens 8y ago
> An SMPS, in a nutshell, uses pulses of current to "charge" an inductor, which continues to source current during the off periods That's exactly what happens in this power supply. The tubes charge up a giant (grapefruit-sized, if grapefruit were cubes) inductor, which supplies the current when the thyratrons aren't. I don't see a good reason not to consider this a switching power supply. Phase angle is just PWM at a lower frequency.
- CamperBob2 8y ago
  Whether this qualifies as a switching supply is an interesting question. I don't think I agree with Animats in that regard. Regulation is accomplished by switching a nonlinear element via a feedback loop, and that's good enough to qualify it as a "switched-mode" supply in my book. You certainly couldn't call it a linear regulator, because the thyratrons are either fully on or fully off at all times. From the input's point of view it will exhibit negative resistance, with a reduction in the current required as the input voltage rises. That makes the classification especially hard to refute. It's true that energy is being stored in the smoothing choke, but that alone doesn't make the difference since the choke could be used to accomplish the same thing in a traditional linear supply. The choke isn't inside the feedback loop, though, and that is a point in favor of the "It's not a switcher" camp. But it's the only one. In any event, charge-pump supplies don't need an inductor at all, and nobody questions whether they operate in linear or switched-mode. (Edit: looking at the schematic, the filter choke is indeed inside the feedback loop. Case closed, it's a switching supply.)

CamperBob2 8y ago

More pr0n for thyratron fans (Monarch 10EE lathe): https://youtu.be/k15pWPBNAUE?t=45s https://youtu.be/k15pWPBNAUE?t=45s https://www.garagejournal.com/forum/showthread.php?t=264474 https://www.garagejournal.com/forum/showthread.php?t=264474 These were made from 1939 through at least the 1990s, although the vacuum tubes went away in the mid-1980s. Amazingly long-lived product.

chiph 8y ago

The power supplies I worked with on the Model 28 were the typical linear design. They had huge resistors on them that after a while, the heat from them would lift the copper pads off the circuit card. So they'd be hanging off the card, only held on by their solder connection. If the inspectors took a dislike to us, they'd write us up for them. But mostly they knew this was a design "feature" and would overlook it. One of the cool things about that era of Teletype was the mechanical serial to parallel converter. This was a multi-lobe camshaft that as the pulses came in and the electromagnet selector engaged/released, would (if your timing was set right) select five bars (for your Baudot code) that controlled the position of the type box where all the letters and numbers were.

Animats 8y ago
Yes, the usual Teletype loop current supply is a 120VDC supply with a 10K 2W resistor, to get 60mA constant current. Efficiency is below 10%, with over 90% of the power going to heating the resistor. That's partly why I built a switching supply interface. Far less heat. Small box, no ventilation required.
- chiph 8y ago
  I briefly considered taking one of the cards along to my high-reliability soldering class just to provoke a reaction from the instructor.. But figured it might bring down attention from On High into the state of our circuit cards and that wouldn't have been good. Besides, they lasted 5+ years like that before needing replacement. We pretty much only replaced them when the boards became brittle from the heat.

MisterTea 8y ago

Thyratrons were the early form of SCR, the silicon controlled rectifier. These aren't SMPS's in the sense we think of them today but they do switch the input at a controlled rate to create a controlled output so the nomenclature can be used in this sense. The proper term is: Phase-fired controller: https://en.wikipedia.org/wiki/Phase-fired_controller https://en.wikipedia.org/wiki/Phase-fired_controller You can see the idea is to run a timer that is phase locked to the incoming mains frequency. Now you can control when the switches turn on during a half cycle. If you turn the switch on at the beginning of a half cycle, you have full power. If you turn the switch on midway through the cycle, you get half power and so on. So your command signal asks the controller for more and more of the complete wave cycle until you get the full RMS value of the line voltage plus the load current (minus the losses in the rectifiers of course). The resultant output is a chopped up EMI laden mess but it does the job quite nicely after some filtering. They were also used in early motor drives to control the speed of a brushed DC motor from single or three phase AC source. I know some lathes from the 50's or 60's had thyratron motor drives in them. At my work we have an Electron beam welder which uses an SCR controller for the high voltage power supply. It's interesting: the controller is directly fed 480V three phase. From there in comes in through a breaker, a contactor, and two current sensing transformers.Like so (dammit variable width fonts...): A--~~--||--S--^^^^^^--/--SCR---\---) B--~~--||-----^^^^^^--|-BRIDGE-| )Inductor C--~~--||--S--^^^^^^--\----------/---) Key: ~~ fuse, || contactor, S current sensor, ^^^^^^ transformer primary The three phases then run to the power three supply transformers in series and then off to a three phase SCR bridge for a total of six SCR's. The output of the bridge has a huge 200 pound inductor across it. The idea is the bridge is phase fire controlled and the inductor is so high in value that the controller can slowly watch the current ramp when the SCR's are turned on and wait until the feedback from the power supply matches the command from the potentiometer and adjust the phase angle firing accordingly. It's creating a controlled short circuit using the series transformers as the load. It's a primitive solid state method of varying an AC voltage. Before the SCR system they used a motor generator with an op-amp PID loop watching the feedback and control pot who's output controlled a small phase fired SCR bridge that delivered a varying DC voltage to the generator field winding. You effectively had a motor generator who's output varied from 0-480V AC three phase. Today you'd have a small metal oil tank containing an entire SMPS which is smaller than the control cabinets for our old linear supplies.

6nf 8y ago

Also check out Mercury Arc Rectifiers, amazingly cool old tech: https://www.youtube.com/watch?v=yjMZ5qtyCUc https://www.youtube.com/watch?v=yjMZ5qtyCUc

Ricardus 8y ago

I've been following curiousmarc's channel for a couple years. I love his videos. This teletype restoration is amazing. The most recent video on his channel is of him and the guys going to a place that sells replacement parts... for teletypes. I mean this guy they visit literally has every part you would need to repair, and probably build teletypes from scratch. They had everything Marc needed to complete his restoration. Amazing!

growlist 8y ago

I had no inkling I needed a mercury thyratron until now! Amazing stuff, this is like something from Fallout brought to life, and the mercury makes it suitably hazardous. I'm guessing the mercury is integral to the hue? I saw something like this in a railway museum in Yorkshire or thereabout when I was a kid - I recall its label included 'rectifier' and I think it contained mercury, and was flashing or sparking in some way. I guess it must have been one of the mercury rectifiers as already mentioned, though I don't remember it glowing quite so beautifully. Certainly had me intrigued.

Aardwolf 8y ago

Reminds me a bit about this thing covered by photonicinduction: https://www.youtube.com/watch?v=QY6V2syGnZA https://www.youtube.com/watch?v=QY6V2syGnZA Fortunately he didn't "pop" that one "for science" unlike most other things he covers, it's too valuable :D

sephamorr 8y ago

There are a lot of comments here saying that this isn't a SMPS, then proceed to describe a flyback converter (and the difference vs this unit) as a justification. Just note that the field of power electronics is quite large, and as an example, some power converters don't even have an inductor or transformer (though most do). As I understood, the definition of a SMPS relates to switching between discrete states (i.e. on/off) as opposed to a linear supply, which dissipates power to regulate. Source: am a power electronics engineer.