How to calculate power supply bleeder resistor

How to calculate a bleeder resistor on a power supply. Assuming
80VDC and 10,000 UF. This resistor is a safety device to slowly
discharge the capacitors in a power sypply to a non lethal leval when
the power supply is turned off

The explanation and formulas for time constants are at this URL.

http://www.sweethaven.com/acee/forms/frm1002.htm



Also need to know I = E/R and P = I X E. Where

E = Volts
I = Current in amps
R = Resistance in ohms
P = Power in watts



1) Pick a time you want the dishcharge to happen in. 5 minutes
sounds reasonable to me.

2) Pick a voltage you want to discharge to in 5 minuits. lets say
about 5 volts.

3) Pick a RC time constant that will give you aproximitly that
voltage. A time constant of 3 will give you 4 volts. Formula .05 X
Vmax = discharge voltage. Because .05 X 80VDC = 4VDC (formula from
URL above).

4) Calculate the resistance. were R = T/C.

T = Time in seconds
R = Resistance in ohms
C = Capasitance in FARADS

For time we will take the desired 5 minute time and convert it to
seconds. Or 5 minuits = 300 seconds. Next we know we will wait 3
time constants for the discharge we want, so divide 300 seconds by 3
for 100 seconds.

R = T (100 seconds) / C (10,000UF = .01 Farads). So R = 10,000 ohms.

5) Next calculate the power in watts the resistor will disapate. P =
I X E so we need to find the current to use the power formula. I =
E/R
I = E (80VDC) / R (10,000 ohms). I = .008 amps

P = I (.008 amps) X E (80 Volts). P = .8 watts

When picking a resistor wattage rating as a general rule never run a
resistor more than 50% of its rating. So .8 watts X 2 = 1.6 watt
rating. 1.6 watt resistors are not made so go to 2 watt.

6) You are done, your bleeder resistor is 10K ohm (10,000 = 10K) and
2 watt.


Wally K.