Calculating Temperature Rise of a Resistor

(cindy) #1

Long story short, I have a load bank using resistors. To get the right vale resistors, there are various combinations of 1-4 resistors in combinations of up to two parallel pairs in series.

In particular, one load option is deisgn for 4 volts at 160mA (25 ohms) It actually consists of a 10 ohm and a 15 ohm resistors in series.

Voltage measurement across the resistors reads 3.74V, with 1.48V across the 10ohm and 2.22V across the 15ohm.

The design power calculations are 10ohm is 0.256W 15ohm is 0.384W

I used 1/2 Watt resistors for both. I can only touch them long enough to tell I can’t touch them. I know touch is a poor determination of actual temperature.

How do I determine an estimate for the temperature rise of each resistor given the numbers above? (unfortunately I don’t have a IR temp gun) Before this gets off into the discussion on how they are mounted, etc. The are spaced away from the board. I was not silly enough to have them lay on the board as they are normally installed. There is, quite literally, 1/2" of lead on both ends between theresistor and the board.

At what point should I be concerned? Normally I don’t like my electronics to get hotter than I can touch, but this is a load bank. It is supposed to get warm. Warm, not expecting hot. In terms of wattage, the 10ohm is at 50% and the 15ohm at 76%. Off my cuff, I would think that’s plenty, but these are hot.