Do resistors really have voltage ratings?

[J M Fahey]

As Tom said, there are definite voltage limits for resistors, which can't be exceeded because a spark will arc from terminal to terminal or over the surface of the resistor, or even over the paint.
They are different to dissipation problems.
here's a Panasonic datasheet , for 0.5W and 0.25W carbon film resistors (the most popular type) which being a serious Company shows everything (good ):
http://users.ece.utexas.edu/~valvano...mresistors.pdf
where they state, among other things:

Carbon Film Resistors
ERDS1 (0.5 W)
ERDS2 (0.25 W)

Limiting Element Voltage
(Maximum RCWV)(1) where: RCWV: Rated Continuous Working Voltage

ERDS1 300V
ERDS2 250V

(1) Rated Voltage=Sqrt (Power Rating * Resistance Value) or Limiting Element Voltage (max. RCWV), whichever is less.
(RCWV: Rated Continuous working Voltage).

As you see, voltage may be limited by dissipation or 300/250V (0.5W/0.25W size) "maximumlimit voltage, whichever is less.

So, say, a 100 ohms resistor will be limited to the voltage which makes it dissipate rated power, but , say, a 4M7 resistor will be limited to 300 or 250V, depending on size and not dissipation.

Then why make , say, a 100k plate resistor in 1/2 , 1 or 2W if it will never dissipate the power it's rated for?

Because by being built in a "larger frame" it will also have a larger voltage rating, based exclusively on size (I guess distance between terminals is the main parameter, obviously distance is larger on a higher rated body.

FWIW I posted here the design of my cheap voltage probe attenuator , meant to see plate waveforms on a software based PC scope, I needed a 2M resistor to drop voltage to 100 or 200mV RMS which is what a PC soundcard input can handle, and I did NOT use a 2M resistor of any rating but 4 x 470k resistors in series, inside a piece of crystal PVC tube, filled with Epoxy.

Works like a charm.

Remember tube plates can have routinely 1000V peak waveforms and overdriven into speakers way over that, *easily* 1400V peaks.

No single standard resistor can handle that safely.

[km6xz]

As Juan states, resistors really do have and need max voltage ratings, for the same reason wire does. Every type of resistor has different HV tolerance due to construction and materials. When mounted on a pc board additional voltage limits come into place such as the dielectric breakdown voltage of the case, paint or wire which if exceeded becomes an arcing risk. For a while, when AMPEG started building their SVT's in Vietnam they had a rash of blown fuses and burned boards. They were using the same parts and build methods as when built in the US which did not have that problem. It was arcing on output sub pc board, with screen resistors pushed right down onto the PC board instead of a 1/4in above the board as the US mounting method, that turned out to culprit. The resistor max voltage was exceeded in all the models but only in the VN version was the reason for the max spec a factor in problems. I found that in a number of new amps and contacted Ampeg and found they were just getting swamped with new amp failures. They factory got notice to lift those resistors in all the yet to be installed boards.
Any dirt on the shell of a resistor lowers its max safe voltage, as does any carbonized paint. If a resistor looks scorched it is a potential problem not only from the fact that it is dissipating too much power but because its max voltage rating needs to be lowered.
Most hookup wire used in amps is rated at 600volts, so lead dress is important in portions of circuits subject to flyback currents which can easily far exceed the highest voltage in the amp.
Techs who learned on solid state or later eras probably don't think about these less commonly discussed parameters but us old farts who started before transistors became readily available factor max voltage ratings into every new design, because we all experienced problems when it was not factored in.

[vintagekiki]

Load Table resistor

Table display the maximum allowed current and the maximum permitted voltage drop on the resistance.

The first vertical column of the table are the most used resistance values, and other columns give the maximum current and maximum voltage drop on the resistance in the permitted load resistance of 0.25; 0.5; 1; 2; 3; 5 and 10 W

Example:
For resistance 300 Ohm / 2 W (column 1) in the same horizontal row in column V (2W) findings are maximum values for current (I = 24 mA) and voltage (U = 24 V), which that this resistance can support.

[J M Fahey]

That table is fine as a quick reminder, but is based mainly on dissipation, and allows voltage up to scary 1000V .
But 0.25W resistors can't take more than 250V and 0.5W ones no more than 300V

Don't know about 1W, 2W or higher resistors but doubt any reaches 1000V .
Of course, we should dig original datasheets and check there, at least same dissipation other brand might be a clue.

That said, no doubt that table can be trusted up to 250/300V , whichever applies

As far as old fart memories go: I remember 50s and 60s ARRL/Radio Amateur Handbook transmitter projects, many of which used 1000/1200/1500V supplies, and to drop that to sensible values needed by oscillators and such or plain metering +V , they used strings of at least 4 resistors, and in one case, 6 of them.

I never actually built any of them, my personal limit is around 450/500V , period.
I'd rather parallel 20 6L6 than use a couple 807 or 813 ,although way back then military surplus ones were available for peanuts.

[Boogie]

(1) Rated Voltage=Sqrt (Power Rating * Resistance Value) or Limiting Element Voltage (max. RCWV), whichever is less.
(RCWV: Rated Continuous working Voltage).

I think I get it, though it still seems somewhat dodgey I guess it makes sense from an accounting point of view. So, just to make sure I understand this, in similar fashion to the above quote, it would be also necessary to say

Rated Power=Power Rating or ((Limiting Element Voltage (max. RCWV))) squared, divided by the resistance value), whichever is less.
(RCWV: Rated Continuous working Voltage).

The 6V6 tube example above (with different bias currents) seems different because the tube is actually capable of operating at 9.4 watts if biased a certain way (with high enough current), however the 1/2 watt 10M resistor is in no way capable of ever operating at 1/2 W no matter how it's connected.

I wish my Grandfather was still alive. He was a tube guy from way back. When I was about eleven "I" constructed a tube radio completely from scratch, without even referring to a schematic, using an old chassis from another radio, and with him there explaining every step of what to do and how to connect everything and why.... I wish I could remember more of it.

Realistically, what would be the highest voltages likely to be seen between the plate and high voltage supply of a 12AX7 wired in the usual way as a preamp? The studio .22 seems less of a concern since the HV isn't very high, it says 285V on the schematic.

And would there be any issue with whatever the voltage limit is of "fairly standard" 5W 100K resistors bought from ebay?

I always used to wonder how cheap multimeters could have voltage ranges that went up to a 1000V range but be non-approved to measure the 240V Australian mains (I think the reason is because the mains voltage can have spikes higher than 1000V?), i guess this is the same kind of thing.

[Boogie]

I guess it does make more sense to sell a large selection of resistors, with different resistance values, identical physical dimensions and construction, as "1/4 watt" (or any particular value), and table them in catalogues etc by their resistance values, rather than to require two columns of data for each individual resistor, a resistance and a power rating that's different for each resistor value.

[Jazz P Bass]

Quote:"I was also thinking about reducing resistor noise, specifically whether its a good idea to swap wire wound for carbon 100k plate resistors. I've got some 5W ones which is more than the original carbon ones (1 or 2 W I think)."

A word of caution is called for here.

If you go way overboard on the wattage rating of a resistor that the circuit calls for, you run the risk of doing more damage if something goes wrong.

Some resistors you 'want' to go open in a failure.

[Boogie]

Would that be the case here? (plate resistor in a 12AX7 preamp).... What would fail first usually, and if the resistor was really big, what would fail first?

[Boogie]

I just found this "Using a higher wattage is fine, and is probably advisable with a valve amp. Assuming by 'plate' you mean the anode loads of the triodes?, these are probably the biggest cause of failure in valve amps - replacing them with good quality higher wattage resistors should make the amp a LOT more reliable, It 'might' help your hiss problem as well? as it's quite likely the existing ones are already higher than they should be?. Whenever I replace a faulty anode load I always use a higher wattage one, and always replace them all - not just the faulty ones. "

Resistors -- using higher-than-required watt rating - diyAudio

[pdf64]

...
The 6V6 tube example above (with different bias currents) seems different because the tube is actually capable of operating at 9.4 watts if biased a certain way (with high enough current), however the 1/2 watt 10M resistor is in no way capable of ever operating at 1/2 W no matter how it's connected.
...
Realistically, what would be the highest voltages likely to be seen between the plate and high voltage supply of a 12AX7 wired in the usual way as a preamp? The studio .22 seems less of a concern since the HV isn't very high, it says 285V on the schematic.

And would there be any issue with whatever the voltage limit is of "fairly standard" 5W 100K resistors bought from ebay?

I always used to wonder how cheap multimeters could have voltage ranges that went up to a 1000V range but be non-approved to measure the 240V Australian mains

To get approved to be connected to the line supply, stuff usually has to be designed to fail safe, ie a Y rated cap approved for connection between line and ground fails open circuit, not short.
Plus that approval process costs money which would increase the price.

The only way to know the voltage rating of a particular part is to check it. If the seller can't supply that info then best to go elsewhere.
The voltage rating of resistors is not related to their power rating, it needs checking.
5 watt wirewounds may only to 100V.
Stuff on ebay could be anything from anywhere; if you want genuine parts with valid specifications, info and a QA manufacturing process, I suggest buying from an established parts supplier.
I suggest a voltage rating of 350V for pre-amp anode / plate resistors.
It's a good idea to note your location in your member info, then others have got that useful context.

[Boogie]

Wow - that still seems so dodgey to me. According to my calculations, 100V would be 0.1 watt at 100k ohms, but it's sold as a 5 watt resistor..... I used to think the whole point of wirewounds was for high power, to dissipate the heat (like a mini radiator), but even a well built 1/4 watt carbon (or any) type of resistor might do better than that?

The exact same ebay ad is now gone (since I bought them over a year ago) but they are pretty much this . They say "5W100KJ" on them and no other markings. I don't know what the J means -- no, cross that out, it means 5% - just looked it up.

10X 5W Watt 100K OHM 5 Wirewound Ceramic Cement Resistor | eBay

[Boogie]

What would be the point of a "5 watt" wirewound that could only actually handle 0.1 watts?

[pdf64]

What would be the point of a "5 watt" wirewound that could only actually handle 0.1 watts?

For 100k none; but the main requirement for higher power resistors is at much lower values. That range of resistors probably didn't go above a few k.
And I was just going from memory, it may have been 200V; whatever, too low for the voltage across them on power-up when used as 1k B+ droppers.
Of course there are high power resistors with a higher voltage rating; my point was that you can't assume it.
Maybe your ebay resistors were rated at 1kV but without a credible spec it's just assumption.

[Jazz P Bass]

A 5 Watt wire wound resistor on the preamp tube plates is just plain silly.

[Mick Bailey]

A 5 Watt wire wound resistor on the preamp tube plates is just plain silly.

It may make for a pretty effective microphone, though.