Glowing mercury thyratrons: inside a 1940s Teletype switching power supply
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.<p>As someone pointed out, that&#x27;s a switching voltage regulator, not a switching power supply. The transformers there are all upstream of the switching.<p>I&#x27;ve restored five Teletype machines like the OP&#x27;s Model 19 [1], so I&#x27;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&#x27;s powered entirely from the USB port.<p>This seemed impossible to some people. There&#x27;s only 5V at less than 500mA coming in, and 120VDC 60mA out. But it&#x27;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.<p>My design is totally modern, built from surface mount components and in a small case. 
Here&#x27;s the schematic.[3]  There&#x27;s an explanation in [2].<p>It&#x27;s been amusing to see the reaction of
the Teletype community. They like it, but most can&#x27;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&#x27;ve sold some board kits.  Not enough potential volume to have it manufactured.<p>Informally, here&#x27;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&#x27;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.)<p>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&#x27;t come out the primary, because the switch is open. So it comes out the transformer&#x27;s secondary winding, where it&#x27;s fed through a diode into a capacitor. There&#x27;s the output.<p>It&#x27;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&#x27;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.<p>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.)<p>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.<p>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.<p>[1] <a href="http:&#x2F;&#x2F;www.aetherltd.com" rel="nofollow">http:&#x2F;&#x2F;www.aetherltd.com</a>
[2] <a href="https:&#x2F;&#x2F;github.com&#x2F;John-Nagle&#x2F;ttyloopdriver" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;John-Nagle&#x2F;ttyloopdriver</a>
[3] <a href="https:&#x2F;&#x2F;raw.githubusercontent.com&#x2F;John-Nagle&#x2F;ttyloopdriver&#x2F;master&#x2F;board&#x2F;images&#x2F;schematic.png" rel="nofollow">https:&#x2F;&#x2F;raw.githubusercontent.com&#x2F;John-Nagle&#x2F;ttyloopdriver&#x2F;m...</a>