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Old 01-21-2017, 04:18 PM   #3015 (permalink)
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oh yes! precharge resistor/capacitance/time. You need to figure out the RC time constant for your setup. Like this:

Let's say you are using 1000uF of bus capacitance, and you are using a precharge resistor that is 500 Ohms (just random values). Also, let's say you are using 400v for your battery bank.
Things to consider:

1. The precharge relay will have to be able to make 400v/500Ohm = 0.8 amps.

2. The time you need to wait is 5*R*C seconds. So,
5*500Ohm * 0.001Farad = 2.5 seconds. (0.001 Farad = 1000 microfarad)

3. The energy that your precharge resistor must be rated for is:
Energy = 0.5 * C * V * V = 0.5 * 0.001Farad * 400v * 400v = 80 Joules

That's a lot of energy for a resistor. You need a pulse handling resistor like the king of surge resistors, the Ohmite pulsEater (I love that name!):

That can handle way more than 80 Joules, so it would be very safe. If a resistor doesn't list its pulse rating in Joules, I would say you should probably not use it.
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