On Apr 17, 2:35 pm, Highland Ham
wrote:
Uncle Peter wrote:
Does reducing the resistance affect the wattage rating
of a power rheostat? I'd imagine it does, but I'm not
sure how it would be derated..

============================
The power rating of a rheostat is based upon its total resistance R.
allowing for a max current I with P(max) equals I*I*R

If the adjustable resistance is reduced to say 1/5 of the total
resistance that reduced resistance can accept the same current I .
This means the power rating has been reduced by a factor 5 ,in other
words the power rating is proportional to the set resistance.

Frank GM0CSZ / KN6WH

That's logical.
But just to nit-pick! :-)
Got one of those ceramic tubular adjustable slide resistors. Rated at
300 watts IIRC.
If I put the slider half way along I will have half the resistance
wire in the circuit and dissipating heat at full current rating;
correct?
But still the same thermal mass of ceramic and metal. So could one
perhaps argue that it might be 'slightly' (very slightly perhaps) a
little more than half the power rating. Probably nothing to have any
effect whatsoever in practice because one would not run a component at
maximum rating at any setting anyway?