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Old 20th June 2011, 02:09 AM   #1
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Default Preventing arcing on standby switch?

I thought about adding a parallel Ressistor and Capacitor over a standby switch to prevent arcing and burning of contacts.

I have good film cap: 22000pF (0.022uF) 1500V (good old Ei Niš) (I have couple of them)
in series with 1k 1W ressistor
Should ressistor be greater or not?
Thanks in advance

Edit:
There is 190V-0V-190V before silicone reactifing, so about 265V is at switch.

Last edited by infernal; 20th June 2011 at 02:12 AM.
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Old 20th June 2011, 02:51 AM   #2
kevinkr is offline kevinkr  United States
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I'd use a much smaller resistor, in some cases for DC no resistor is used, but this is dependent of the cap's ESR limiting current to a value safe for the cap during switch opening and closing. I'd start with about 10 - 22 ohms.
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Old 20th June 2011, 08:59 AM   #3
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Are you using the switch to connect the CT to ground?

If so, then the switch would see an AC current. If you are using solid state diodes then you probably have a high level of filter capacitance directly fed by the rectifier, and so the turn-on current pulses will be relatively high, and if that aligns with switch contact bounce then arcing from transformer leakage inductance would be quite significant.

An RC snubber would allow a current path to continue when the switch bounces open, rather than forcing an arc at the contacts. A resistance of 10-100 would be fine. The most benefit is when the RC is physically small and close to the switch.

Another technique is to reduce the magnitude of the charging current at turn-on - such as by using a NTC resistor sized for the job.

Another technique is to add a bypass resistance across the switch to allow a bleed charging current to raise the HT to say 50-100V. This would reduce the initial charging current level, and also run the tubes with some anode voltage rather than just heater only.

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Old 20th June 2011, 09:29 AM   #4
mdm102 is offline mdm102  Russian Federation
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high voltage ceramic disc capacitors rated at 0.01-0.1uf and placed in || to the contacts are often used to prevent arcing
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Old 20th June 2011, 06:27 PM   #5
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Thanks for the info. I'll add 22Ohm ressistor 2W. I think it will do the good job.
I will solder RC onto the switch directly (so no wires and enough close to switch)
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Old 21st June 2011, 01:19 AM   #6
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Ideally the resistor would (briefly) carry the same current as the switch... so for breaking 200 mA at 300V (for example), it would be 1500 Ohms or so. Switch has to carry double the current at turn-on, but nothing's free...
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Old 21st June 2011, 02:13 AM   #7
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In anger, the switch would be carrying in-rush current of way more than 200mA - which I think is when the arcing will be problematic due to contact bounce.

Breaking at turn-off could also be significantly higher if it occurs during the peak of a diode conduction waveform.
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