
Glowing mercury thyratrons: inside a 1940s Teletype switching power supply - eaguyhn
http://www.righto.com/2018/09/glowing-mercury-thyratrons-inside-1940s.html
======
Animats
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/m...](https://raw.githubusercontent.com/John-
Nagle/ttyloopdriver/master/board/images/schematic.png)

~~~
jacquesm
That reminds me of a funny story. The windmill I built worked well in medium
and high winds, but did nothing but spin idly when the wind was low. Still,
that's (a little) energy that you could capture in theory but it simply did
not have enough voltage to get over the battery terminal voltage which makes
it impossible to charge the battery.

But a windmill is mostly coils and magnets and when you short it it actually
_will_ charge the battery, briefly robbing the blades of some momentum. So, if
you periodically short the coils using a bunch of powerfets and an oscillator
you can charge a windmill in low wind conditions that would otherwise do
nothing.

So far so good. Built it, tested it, worked like a charm. And then one day I
decided to temporarily decouple the windmill from the switchboard, but I had
forgotten about that little booster. _FOOM_ , instant fire on the booster
board after I pulled the switch. The FET circuitry and the oscillator had
happily continued to work on the power provided by the windmill, and had
allowed the end stage of the circuit to reach a very large multiple of the
voltage that it normally dealt with because the battery kept the voltage
pegged to a maximum of about 48V!

Needless to say that led to a somewhat more robust V2...

~~~
sephamorr
What you've built here is a rudimentary boost converter, using the motor
inductance as your inductor, and your battery as your output capacitance. The
next step up here is to build a controllable device that can implement maximum
power point tracking; a buck-boost power topology might be what you're looking
for here. Note that MPPT on a wind turbine is more difficult than, say solar,
because the rotor speed is another state variable to consider. A naive/greedy
algorithm might apply maximum torque to the rotor to extract maximum power for
a moment, but this strategy will try to stall the rotor. As power is torque *
angular velocity, both terms require consideration.

------
stickfigure
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.

~~~
raverbashing
> The rectifiers lit the whole room up with a beautiful purple glow.
> Beautiful.

I wonder what other EM ranges it emits. Glowing purple makes me think of UV
but maybe also some X-Rays?

~~~
VLM
Tons of UV, that's how legacy fluorescent lamps work, a nice intense UV flux
excites the phosphors inside the tube. The UV can't escape the glass but the
white light from the phosphors certainly can. Germicidal lamps use UV-
transparent quartz.

As for x-rays the typical voltage drop across a long tube is like 100 volts,
the whole point of a ballast is matching the normal hundred or so volt drop to
whatever your local line voltage is using at a constant controlled current.
Anyway the penetrating power of an xray is linked to the voltage drop, so it
would be difficult even theoretically to generate xrays stronger than a
hundred volts or so. So given that it takes 50 KV to 100 KV for a dental xray
source to beam completely thru a head, you're looking at like a thousandth the
penetrating power. Very hand wavy estimate that 100 volt xray source will
likely not penetrate the glass and almost certainly not penetrate clothing or
the surface layer of dead skin and skin oils.

The risk assessment for being nearby a heavy cloud of vaporized hot mercury
would seem to imply the main risks would be biochemical in nature not physics
in nature. Actually the most realistic risk is an economic death penalty if
that leaks and requires hazmat cleanup.

------
MisterTea
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.

------
tlb
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
I recently built a "vacuum tube nightlight"
([http://www.electronixandmore.com/projects/vrtubenightlight/i...](http://www.electronixandmore.com/projects/vrtubenightlight/index.html))
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
> 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
just the mercury vapor being dangerous.

------
blattimwind
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.

------
CamperBob2
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.

------
kazinator
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
> 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
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.)

------
Aardwolf
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

------
chiph
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
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
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.

------
growlist
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.

------
sephamorr
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.

------
6nf
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
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!

------
clebio
> 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
The transformer may be the biggest single piece in the MacBook charger, but
the many capacitors together take up more volume than the transformer.

------
lightlyused
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.

