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?
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?
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.)
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.)
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.
It will work - for a while, but in the interest of safety and reliability, spend a couple of hours changing them.
Prune said:
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.
Prune said:
I guess being in a jail cell you are out of touch.
I'm not sure if your avatar was supposed to be a UPC code made to look like bars on a jail cell but jail cell was what the image speaks to me at first glance. If you are in a jail cell you probably aren't too busy doing other stuff that you can't spend two hours soldering resistors.
rcavictim said:
I guess being in a jail cell you are out of touch.
I'm not sure if your avatar was supposed to be a UPC code made to look like bars on a jail cell but jail cell was what the image speaks to me at first glance. If you are in a jail cell you probably aren't too busy doing other stuff that you can't spend two hours soldering resistors.
actually, he probably has an RFID chip implanted behind his ear.
So yeah, AFAIK:
1/4W = <300V
1/2W = up to 500V
1W = 600V
2W = 1kV I guess
Etc.
You get into 10kV ranges with long teens-watt types.
IIRC, the calculation goes where 330k on a 1/2W resistor is the maximum; any higher resistance is voltage limited (too much resistance to draw rated power), any lower is wattage-limited (draws power such that ratings are reached before voltage max). A rule of thumb around here should extend to all types.
Hmm, on second thought, no (unless you square or exponential-ize something). A quick calc shows a 10W 330k resistor at all of 2kV...
I have literally never had a 1/2W resistor fail on me!! That doesn't count, of course, the occasions when the resistor was obviously overstressed (like... putting 20V across a 100 ohm resistor). I speak of RadioShank 5% carbon films. I have several in my collection which are slightly darkened around the midsection but still measure well within tolerance.
Tim
1/4W = <300V
1/2W = up to 500V
1W = 600V
2W = 1kV I guess
Etc.
You get into 10kV ranges with long teens-watt types.
IIRC, the calculation goes where 330k on a 1/2W resistor is the maximum; any higher resistance is voltage limited (too much resistance to draw rated power), any lower is wattage-limited (draws power such that ratings are reached before voltage max). A rule of thumb around here should extend to all types.
Hmm, on second thought, no (unless you square or exponential-ize something). A quick calc shows a 10W 330k resistor at all of 2kV...
I have literally never had a 1/2W resistor fail on me!! That doesn't count, of course, the occasions when the resistor was obviously overstressed (like... putting 20V across a 100 ohm resistor). I speak of RadioShank 5% carbon films. I have several in my collection which are slightly darkened around the midsection but still measure well within tolerance.
Tim
I'm using 1/4 W 20M as equalizing resistors in the multi-mini-capacitor bank. I had one across each capacitor which gets charged about 400 V. I guess I'll replace each with two 1/4 W 10M.
I did fry a few ballast resistors due to overheating though, and they melted holes in my carpet. Besides resistors, my jumper cables keep exploding like fuses -- the gauge is too small. Last time I buy wire from this guy.
I did fry a few ballast resistors due to overheating though, and they melted holes in my carpet. Besides resistors, my jumper cables keep exploding like fuses -- the gauge is too small. Last time I buy wire from this guy.
Prune,
20 megohms is WAY too high a value to use across a power supply capacitor for equalizing. It may as well not be there. For electrolytics it is customary to use about 250 K ohms for 400 volts. I would not exxceed 500K for 500 volts in power supply cap equalization. That gives you 1000 ohms per volt. That will draw only 1 mA of equalizing current (which is not a lot compared to the leakage current of the electrolytics) so dropping 500 volts will dissipate exactly 1/2 watt of power. A 1 watt or 2 watt resistor would be fine.
20 megohms is WAY too high a value to use across a power supply capacitor for equalizing. It may as well not be there. For electrolytics it is customary to use about 250 K ohms for 400 volts. I would not exxceed 500K for 500 volts in power supply cap equalization. That gives you 1000 ohms per volt. That will draw only 1 mA of equalizing current (which is not a lot compared to the leakage current of the electrolytics) so dropping 500 volts will dissipate exactly 1/2 watt of power. A 1 watt or 2 watt resistor would be fine.
Well, then I must have messed up my math here.
The capacitors are 330 uF 450 V Cornell Dubilier 380LX series. The datasheet gives leakage current as "<= 3root_sign(CV) uA at 5 min." The root_sign stands for the root sign which I cannot copy. I'm assuming that the three is a multiplier, not 3rd root. Now, I don't know what the "at 5 min." means, so maybe that's where my mistake is, but here's my calculation (each is run at under 400 V): 3 * sqrt(330E-6 F * 400) = 3 * sqrt(0.132) = 3 * 0.363318042 = 1.08995413 uA.
I've been told to use equalizing resistors that will pass about ten times the leakage current. So, 10 * 1.08995413E-6 A = 1.08995413E-5 A. 400 V / 1.08995413E-6A = 36.6987921E6 Ohms. Thus 20E6 Ohms should be just fine.
I hope someone comments soon so I know whether I'm mistaken and need to buy different resistors.
The capacitors are 330 uF 450 V Cornell Dubilier 380LX series. The datasheet gives leakage current as "<= 3root_sign(CV) uA at 5 min." The root_sign stands for the root sign which I cannot copy. I'm assuming that the three is a multiplier, not 3rd root. Now, I don't know what the "at 5 min." means, so maybe that's where my mistake is, but here's my calculation (each is run at under 400 V): 3 * sqrt(330E-6 F * 400) = 3 * sqrt(0.132) = 3 * 0.363318042 = 1.08995413 uA.
I've been told to use equalizing resistors that will pass about ten times the leakage current. So, 10 * 1.08995413E-6 A = 1.08995413E-5 A. 400 V / 1.08995413E-6A = 36.6987921E6 Ohms. Thus 20E6 Ohms should be just fine.
I hope someone comments soon so I know whether I'm mistaken and need to buy different resistors.
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