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Old 20th September 2004, 06:18 AM   #1
Prune is offline Prune  Canada
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Question Overvoltaging resistors

Most small resistors nowadays are only rated to 250 V, beyond which the insulation starts conducting and changes the value.

My question is, in a position where exact value doesn't matter, will there be a problem if I run them at 400 V? In other words, besides the issue mentioned above, will the insulation degrade over time, perhaps failing eventually?
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Old 20th September 2004, 07:13 AM   #2
johnf is offline johnf  Germany
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Any chance you could use two half resistance-value resistors in series?
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Old 20th September 2004, 08:34 AM   #3
Prune is offline Prune  Canada
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Yes, but that means removing 32 and putting in 64 resistors. Otherwise I wouldn't have asked and just done it.
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Old 20th September 2004, 08:38 AM   #4
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Why have you in the first place used 250 V resistors if you have 400 Volts? You have two options as I see it:

1 Replace to high voltage types.

2 Insert resistors in series with the others.

3 Don't bother and take a chance (I wouldn't take a chance because off the very small cost to replace those resistors. It costs me 30-50 cents for all resistors.)
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Old 20th September 2004, 08:43 AM   #5
Prune is offline Prune  Canada
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LOL, it's not cost, it's time. Soldering 64 resistors will take me two hours.
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Old 20th September 2004, 09:06 AM   #6
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If they are Yaego or Xicon, they are extremely conservatively rated. I run 1/4 and 1/2 watt, 250V Yaego's at 400V for years in tube projects and have yet to have one fail.
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Old 20th September 2004, 01:04 PM   #7
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Hi,

Quote:
If they are Yaego or Xicon, they are extremely conservatively rated. I run 1/4 and 1/2 watt, 250V Yaego's at 400V for years in tube projects and have yet to have one fail.
Lucky you...

However it would be wise, very wise even, to do the resoldering now iso waiting for a fire....

Two hours of your life is peanuts compared to no life at all....

Cheers,
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Old 20th September 2004, 01:29 PM   #8
johnnyx is offline johnnyx  United Kingdom
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I would recommend using resistors well within their voltage ratings. I have had failures due to excessive voltage, not immediately, but over time, and in one instance a 1.8M startup resistor became o/c. The other resistors and leakage around the area was of a similar magnitude, with the result that it was very difficult to spot while probing with a dmm.

It will work - for a while, but in the interest of safety and reliability, spend a couple of hours changing them.

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Old 20th September 2004, 03:39 PM   #9
Prune is offline Prune  Canada
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Thanks.
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Old 20th September 2004, 03:58 PM   #10
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Default Re: Overvoltaging resistors

Quote:
Originally posted by Prune
Most small resistors nowadays are only rated to 250 V, beyond which the insulation starts conducting and changes the value.

My question is, in a position where exact value doesn't matter, will there be a problem if I run them at 400 V? In other words, besides the issue mentioned above, will the insulation degrade over time, perhaps failing eventually?

1/2 Watt Carbon composition resistors can be used at 400 V -- you see them all the time in de-commissioned military equipment -- the problem is that they have to be de-rated as the resistance changes with voltage above. The resistance will also change as they absorb moisture -- and release it when they are heated.

There is a derating chart on page 373 of Horowitz and Hill.
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