Electronic – What should the minimum voltage rating of replacement power resistors be


I'm looking for four power resistors of the same type to replace R87, R88, R89, and R90 in the POWER AMP section of the amplifier of Klipsch SW-12 subwoofer shown below:

R87, R88, R89, R90

What should the minimum voltage rating of the following new resistors be?

Choice 1:

5W 1.5K ohm

Choice 2:

5W 1K ohm

Choice 3:

7W 1k ohm

Choice 4:

7W 750 ohm

Choices 1-3 all have their voltage rating listed (350V – 500V), but I can't find the voltage rating for Choice 4:




If I can't find the voltage rating for Choice 4, should I drop it from my list of choices for the replacement resistors?

[A picture showing R88 and R90 with four other white resistors: Added 2/13/17]

There's a similar-looking row of six white resistors on the other side of the amp, too (bottom in the picture).

six white resistors

[Added 2/13/17]

I received a reply from a US distributor of the Choice 4 resistor:

Received: Mon, Feb 13, 2017 9:33 am
Subject: MCPRM07WJP751B00


The max voltage rating is 500v. I don’t have a data sheet to send but the product manager was able to contact the mfg to get the voltage.

Have a nice day.

. . . . . . . . . . .

[Added 2/14/17]

This is the resistor's listing that prompted me to ask a question about voltage rating for resistors:


I hadn't been able to find it until this morning.

Best Answer

I wouldn't worry about this too much. The implicit minimum voltage rating of a resistor is

  V = sqrt(W Ω)

where V is the voltage, W the power in Watts, and Ω the resistance in Ohms. If it were anything less, then the resistor couldn't dissipate the rated power.

The lowest of your choices is the 5 W 1 kΩ resistor, which must be able to handle at least 71 V to dissipate its rated 5 W.

This circuit has ±81 V supplies, with four of these resistors in series from one supply to the other. Assume worst case that the center point can be driven to either rail, so 162 V across two resistors in series. It looks like the resistors will divide that fairly evenly, so a bit over 80 V across any one of them worst case.

Any resistor you find that can dissipate 5 W or more is going to be physically big enough to handle well over 100 V. You're not going to be able to find a resistor that meets the power and resistance requirements that can't handle whatever voltage this circuit can possibly throw at it.