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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

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    \$\begingroup\$ How many joules? CV^2/2 \$\endgroup\$ – Spehro Pefhany Aug 8 '15 at 2:02
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    \$\begingroup\$ C = 8883uF | V = ~400V Max | E = ~710J Max \$\endgroup\$ – user291252 Aug 8 '15 at 2:14
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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

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  • \$\begingroup\$ Some inspiration might be drawn from the short-circuit capacity calculations for power cables. That calculation is also based on all heat goes into heating up the conductor. (I think the word is 'adiabatic'.) \$\endgroup\$ – Li-aung Yip Aug 9 '15 at 20:18
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Look for a resistor with a pulse rating in the 700J+ range- from TE's chassis mount range, it looks like a 250W resistor. Obviously it won't need much of a heatsink for <20W. The 300W one is actually worse- don't take anything for granted- this is pretty severe service.

There are also some interesting parts like these.

Ceramic composition parts are compact and have good pulse capability but I don't see any that can handle your rep rate.

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