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
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.
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.