Is there a rule of thumb for Ohms and Wattage of "Bleeder Resistors" for Power Supply Capacitors? Thanks in advance for your time spent in responding to my question!! Robert
That depends on your power supply voltage. As a general rule, you'll probably want it to bleed down the power supply in under 30 seconds after turn off.
If it leaves a second degree burn on your thumb, it's too hot over the long term. See next line.
If it's smoking hot, it's way too hot. Increase resistance and wattage. No sense in excess, wasted energy and heat.
Ohm's Law for Power: Watts = Voltage (squared) / Resistance
Look up Ohm's Wheel for the whole set of equations, and make a printout of the graphic for future reference.
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It never bleeds to ZERO. 37% after one RC. How "bled" is "bled"?
As a bad rule of thumb, less than 5% of your total load isn't enough bleed to be worth the effort, over 10% of total load is likely to be distressing in heat and next-size-up power transformer. But for the LARGE caps which are cheap today, this usually means lethal voltages for *minutes* not seconds.
If the bleed resistor runs HOT, that increases the chance it will burn open, of course when you least expect it and are foolishly trusting it to bleed.
Estimate how long it will take you to access the capacitor terminals, or some other high voltage point, after switching off. Divide this time by 3 or 4. This is the maximum time constant. Use this to calculate the maximum bleeder resistor. Then calculate the wattage, allowing some extra for being under a hot chassis.
In addition, check the voltage rating of the resistor. Some resistors may be as low as 150 or 200 volts. You may have to connect more than one resistor in series.
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