Im using a resistor (68Ohm) to discharge a capacitor bank rated at around 400V. This is the graph of the current and the power in the resistor, as you can see there is a 2KW (4.5A) peak. But it's just in around 100ms, I wont repeat the pulse in 3 seconds or more. What value should I use for the power rating of the resistor ? Thanks.
RMS in 2 seconds: ~20W
RMS in 3 seconds: ~13.5W
Best Answer
Calculate the average power. A simple average is used for power, not RMS. It looks like your average power and resistance requirement will make a wire wound vitreous enameled resistor a likely choice. The Ohmite Manual of Engineering Information indicates that for individual pulses in the millisecond range, calculations need to be performed based on the assumption that all of the heat goes to raise the temperature of the wire. Apparently heat is transferred to the resistor core and the air more slowly. I have seen pulses of power cause the resistance wire to break without the outer surface temperature of the resistor rising much above room temperature.
http://www.ohmite.com/techdata/engineering-info.pdf