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

mike[_22_]

On 2/26/2017 11:51 AM, wrote:
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).

If you tie the ends together, you get a 12.5 ohm resistor.
I'm too lazy to do the math, but as you dial it off center,
the resistance goes down until you get to 8-ohms. You have two
parallel resistors that total 8 ohms.
Limit the current in the smaller part to 1 amp.
Calculate the additional current in the bigger part.
Sum 'em up, calculate the max power.
Sure, you can write a formula for that, but I'm way too lazy.