DIYbanter - View Single Post - Is the wattage linear on a wirewound adjustible resistor?

MOP CAP[_2_]

On 2017-02-26 19:51:07 +0000, said:

While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).

The 25 watt rating is for the entire resistor. If you tried to put
25watts into only 1/3 of the R it would overload both the R wire as
well as the ceramic core. I have never seen a formula for this
calculation.
If you put only 25/3 watts in the 8 ohms you might also damage the
resistor due to uneven thermal expansion. No definate answer, but go
easy. CP