Before the Thyristor, was the Thyratron!

  • Simomax
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4 months 1 week ago - 4 months 1 week ago #6030 by Simomax
I was repairing an old Russian DP-5V counter yesterday and noticed an unusual orange flashing glow coming from the probe PCB that I have never seen before, so decided to investigate. It is caused by two thyratrons on the board, flashing away with the clicks of the counter. They are similar to modern thyristors in their function. These thyratrons are filled with neon gas, hence their orange glow. This is from wikipedia:

Conductivity of a thyratron remains low as long as the control grid is negative relative to the cathode because the grid repels electrons emitted by the cathode. Space charge limited electron current flows from the cathode through the control grid toward the anode if the grid is made positive relative to the cathode. Sufficiently high space charge limited current initiates Townsend discharge between anode and cathode. The resulting plasma provides high conductivity between anode and cathode and is not limited by space charge. Conductivity remains high until the current between anode and cathode drops to a small value for a sufficiently long time that the gas ceases to be ionized. This recovery process takes 25 to 75 μs and limits thyratron repetition rates to a few kHz.

I made a short video of them in action and there is also another video from someone showing some much larger, more powerful thyratrons. I'm starting to understand how old tubes, both vacuum and gas filled, actually work. I couldn't quite get it how they are able to pass substantial currents, but all came clear when reading about how they create a plasma inside which is used to conduct the current. Interesting stuff.

Last edit: 4 months 1 week ago by Simomax.

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