1. ## Bleeder Resistors

Does anyone know if there is a formula for sizing a bleeder resistor on a power supply? I'm building a 67vdc 20a supply.
Thanks
Bert

2. Do you really want a formula?....pick 1k, 1 watt resistor if you want to perform it experimentally. V = V(i) e-t/RC is the formula.....

4. 050807-1524 EST USA

bbergami:

The discharge rate for a capacitor C shunted with a resistance R is based on the equation ViperTX gave you. This corresponds to a 63% drop in voltage for each time period = R * C. This is called the RC time constant. Time = seconds for R in ohms, and C in farads.

Suppose you want the output voltage to drop to 5% of its initial value in 5 seconds, then calculate ( - T/RC ) = ln ( 0.05 ) = -2.996. Basically 3 time constants. 5% of 67 V = 3.35 V. If your filter capacitor is 50,000 mfd, then the bleeder resistance for 5 sec is T = 3 * R * 0.05, and thus R = 5/0.15 = 33.3 ohms. The power dissipation when the power supply is on is 134 watts. You do not want to do this. A 1 k resistor will take 150 seconds or 2.5 minutes to do the same job and dissipates 4.5 watts.

A 5 to 20 ohm resistor switched by a relay when power is turned off is a better idea. A 10 watt wirewound power resistor should work for this. This will give you fast discharge but not consume power when the power supply is on. Note: an Ohmite power wirewound will tolerate a 10 times overload for a longer time than this discharge time.

Note: if your 20 amp load is kept on after the power supply is turned off, then at least for some period of time it is equivalent to a 3.35 ohm load, and serves as a good bleeder.

.

• Presuming that you have a Cooling Fan in your PSU, I suggest using an appropriate Voltage Regulator, (7812, 7824 etc), across the Capacitor which then feeds your Fan. When the power is switched off, this will bleed the capacitor and will also run the Fan for a few more seconds. This gives you an audio feedback that your Capacitor is bled and a little extra cooling after switch off.

• 050808-0504 EST USA

Imagineering:

Your idea is useful, but the 7812 (up thru 7818) has an absolute maximum input rating of 35 V.

If the load current is sufficently constant, then you might gamble with an input series dropping resistor. But not a good way.

An alternative is a 12 V Zener connected to the base of an emitter follower. The transistor can be sized for both power dissipation and voltage rating as required. The junction point of the transistor base and Zener cathode requires a resistor from the collector to supply sufficient base current to drive the transistor with the given load.

This will provide a constant current load on the capacitors during bleeder discharge until the available current to drive the base is too low.

The ultimate question on the sizing of a bleeder is based on how fast you want to discharge the capacitor bank.

.