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Old 20th May 2013, 08:57 PM   #1
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Default Relays for tube voltages

I'm looking for relays to switch voltages at different stages in my DIY tube poweramp. Switching mains is not a problem, but I can't find any relay that can switch voltages over 400 VDC. It needs to be of the PCB mountable type. Most PCB relays will only do 30VDC.
Or am I reading it wrong. They all say they will do e.g. 30 VDC @ 5A. Does that mean they will do 150VDC @ 1A??
So, what relays are you guys using for high DC voltage switching? Or does any type do, as long as I use high current types at low currents??
Thanks.
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Old 20th May 2013, 09:00 PM   #2
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Why do you want to switch the HV?

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Old 20th May 2013, 09:02 PM   #3
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The main purpose would be to to bypass the inrush current limiting resistor for the high voltage power supply. But I'm curious in general. I might want to turn off separate tube stages completely in the future.
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Old 20th May 2013, 09:07 PM   #4
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Quote:
Originally Posted by funk1980 View Post
They all say they will do e.g. 30 VDC @ 5A. Does that mean they will do 150VDC @ 1A??
Not at all. The voltage and current ratings are maximums, not a total wattage. At 1A you still switch 30Vdc. And, beware: AC and DC ratings for relays are not the same. DC is very difficult to switch at high voltages. A relay that is rated for 125VAC may only be rated for 30Vdc.

High voltage DC relays are usually expensive vacuum relays. Personally, unless mechanical isolation is needed, I'd use a MOSFET or IGBT.
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Old 20th May 2013, 09:08 PM   #5
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Originally Posted by funk1980 View Post
The main purpose would be to to bypass the inrush current limiting resistor for the high voltage power supply. But I'm curious in general. I might want to turn off separate tube stages completely in the future.
Place the inrush limiting resistor on the primary side and use a standard relay to bypass it.
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Old 20th May 2013, 09:08 PM   #6
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I'd prefer to switch the primary side of the HV transformer and keep the HV line free from any switching stuff that might arc or fail.

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Old 20th May 2013, 09:10 PM   #7
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To turn off an individual stage, you might just change the bias of that stage and keep the HV applied.

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Old 20th May 2013, 09:12 PM   #8
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Moving it to the primary side might be an option. But with a 1000uF @ 530V capacitor bank plus 7 amps of heaters, that would make a big power resistor. Or am I wrong? Any tips on values and ratings?
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Old 20th May 2013, 09:17 PM   #9
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Moving it to the primary side might be an option. But with a 1000uF @ 530V capacitor bank plus 7 amps of heaters, that would make a big power resistor. Or am I wrong? Any tips on values and ratings?
In that case I'd use a separate filament transformer.
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Old 20th May 2013, 09:18 PM   #10
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What are you going to power with 1000F @ 530V? Aiming for obscenely low ripple or is it a really big power stage?

I'd use different transformers for the HV and the heaters. So you can switch them independently and still keep the switching stuff on the primary side.

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