Hi,

There are 2 series capacitors ( 70,000UF, 65Vdc each) and a big resistor put across 2 caps. My question is how do we choose the value of a big resistor? What is the purpose of this resistor, discharge?
The input voltage is 38Vdc.
Thanks

KN

 

Those are bleeder resistors to provide a discharge path for the charge on those big capacitors. The value is quite variable, depending on how fast you want the caps to discharge. But there is no point in wasting energy as heat, so something around 47K is probably good.

 

Well, how quickly do you want to discharge those capacitors? Remember 5 x RC (seconds) will discharge 98% of the voltage charged. Use a 20 watt resistor as some heating will occur. Avoid shorting the terminals with a screwdriver (Big Spark!) as this stresses the capacitor un-necessarily.

Regards, DPW