# pre-charge resistor

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### Erwin

Aug 18, 2015, 12:55:27 AM8/18/15
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hi. where i can buy pre-charge resistor with rating 4ohm and 2.5kwatt for my evc402 controller?

### Gabriel Maria

Aug 18, 2015, 1:37:18 AM8/18/15
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2.5 kilowatts? The most you would ever need would be 50 Watts. I am not an expert but the most we ever had on our evc402 was 10 Watts for good measure. The Watt rating of a resistor is the max energy it can dissipate continuously.

### Erwin

Aug 18, 2015, 1:59:34 AM8/18/15
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its 50watt enough? because the manual state for start the controller we must limit the current at most 30A. i have supply with 100V so i need resistor 2.5-3Kwatt resistor based on P=IV. correct me if im wrong..

### Gabriel Maria

Aug 18, 2015, 5:31:28 PM8/18/15
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All correct. except you will not be passing 30A through the resistor for very long. The current will very quickly come to zero. looking at my EVC402, there seams to be 0.012F of capacitance. see if you can use this equation to shed some insight on your situation.  http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/capchg.gif

If however there was a short, or if the precharge was still engaged when the motor is driven, the resistor would receive 3KW of power and melt the resistor, it might be a good idea to use a slow blow fuse in the pre-charge circuit.

after you find how long it takes to charge your capacitors with your 4ohm resistor, you might want to choose say a 10ohm resistor because you don't need the precharge to complete in a few micro seconds.

I hope I helped.

### Gabriel Maria

Aug 18, 2015, 5:49:36 PM8/18/15
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I should also say that the Watt rating on a resistor is for continuous power. for example a 100W resistor can receive 100J of energy every second, forever. However if we put 10,000W into the resistor for 0.000001 seconds, you could say that we only put in 0.01J, only for one second.     W*s=J.     W=J/s.