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I am continuing work on designing a vacuum tube tester and curve tracer. I have described part of the challenge in this question: Designing an analog high voltage multiplexer

The component I'm studying at this point is meant to solve the problem of connecting the signals produced by the tester to the pins of the vacuum tube under test. To recap we have 6 signals: plate supply (0-800V/500mA), screen supply (0-800V/100mA), grid supply (-100-+20V/1mA), heater supply (up to 12.6V/2A approx up to 10W), GND, open. These need to be routed to and one of the pins on the tube socket, which in my case go up to 9 pins. For a bunch reasons the intention is to have these signals be oscillating in their range at "easy" audio frequencies, say between a few hundred Hz and maybe 10 kHz.

In the previous question the purpose was to design a little multiplexer board controlled by a microcontroller to switch any of the 6 signals to one pin, and then build 9 of these boards, one per pin. I thought there might be a way to build using solid state devices, but turns out might be impossible to find a suitable part that is low cost enough (although maybe something like Panasonic's AQH3223, a light-controller TRIAC, might be a possible way to go), so the choice is oriented to latching relays to keep power consumption reasonable. Maxim makes a neat little driver MAX4821 that can conveniently be used to drive 4 relays.

However nice, this solution is quite expensive (45 relays are needed, and it looks like this could be about $300 of parts just for the relays), so I was looking into how to achieve a similar result in some other way, and it occurred to me to use 2 pole, 6 way rotary switches (one per pin), such as this one: Lorlin CK1459, which is rated for 250V AC or DC and 350mA.

The purpose of using a 2 pole switch is to use one pole to route the signal, and the other for sensing, so that the microcontroller can make sure the connections are properly configured before sending high voltages through to the sockets.

schematic

simulate this circuit – Schematic created using CircuitLab

I am writing because I would like to understand better the voltage and current ratings of these switches. If I understand correctly what I read the current rating is limited by the mating surfaces realizing the contact having a small resistance, heating up due to the current flow and potentially welding the contact shut in place. This makes sense to me, although it seems like it would be a relatively slow process if the rating was exceeded for a brief period of time (I'm guessing a 350mA contact would withstand 500mA just fine if it was for 1ms every 100ms for a total of 3 seconds, because heat would take some time build up on the part and the rest of the metal in the switch would have time to sink the heat away).

I understand much less the voltage rating though: I can see the issue that as the contacts get closer, eventually an arc might form, and that this happens more easily at higher potential differences. But if (as I intend to do) the switch never changes voltage with a high tension applied (because the microcontroller oversees that, and the assumption is that the operator is not malicious to the machine), it seems we're left with a small potential difference on the connected contact (the contact is rated for 20mOhm contact resistance, so at 500mA we're talking 10mV of potential difference), and maybe a larger one between adjacent contacts. Would these be built close enough to one another to create problems here? The datasheet indicates a 'Proof Voltage' of 1500V, I don't know what that means.

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  • \$\begingroup\$ The rated voltage is between any pins on the switch. Having 800V on one pin and 12V on another exceeds this by a long way. And "Proof Voltage" is really only part of the test procedure, not normal operating conditions. \$\endgroup\$ – Finbarr Aug 30 '19 at 11:11
  • \$\begingroup\$ Sincee you have a MCU controlling the system, I suggest you ramp-down the high-voltage power supply before changing any relay settings. Then ramp-up the high voltage, and proceed to make your next tube tests. \$\endgroup\$ – analogsystemsrf Aug 30 '19 at 14:35
  • \$\begingroup\$ @analogsystemsrf: yes that's exactly the idea. The switch is not intended to change state with high voltages present, it's only meant to stay put when high voltages are around or move when nothing is going on \$\endgroup\$ – Luca Fascione Aug 30 '19 at 20:00
  • \$\begingroup\$ @Finbarr: yes I understand that's what they say, but actually happens when the rated potential difference is exceeded? How long does it take? How does it depend on availability of currents from the supply or frequencies at hand? \$\endgroup\$ – Luca Fascione Aug 30 '19 at 20:01
  • \$\begingroup\$ 250V is the maximum that it's guaranteed to work safely at. You don't use it beyond that. It's that simple. \$\endgroup\$ – Finbarr Aug 30 '19 at 21:51

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