Automatic Discharge Circuit

The best way is to discharge through a high wattage resistors.

It will lengthen the discharge time and limit the current.

How about a couple of NC contacts on a relay which is energised when the power is switch on and can discharge the caps through resistors when it is switched off?

Al

Why not use a transistor or even a zener diode? Appropriately biased by the incoming supply, turn off the discharge path across the capacitors. When the supply fails, turn the discharge path on. Or is that too simple?

Just connect the resistor in series with your relay to absorb the surge of current and you solve the problem. RC product gives you the time to discharge and U/R gives you initial (maximum) discharge current (to protect your relay). For instance if your resistor R = 1 kohm and capacitor voltage 100 V the maximum current is 100 mA and RC = 47 msec. Maximum instanteneous power developed at the resistor is 100 V*100 mA = 10 W. Therefore with 10 W 1 kohm resistor and relay that can switch current 100 mA your circuit will discharge your capacitor in 47 msec to about 30 V and shotly after to (almost) 0 V.

GA

Plus do you care if you use a little extra power. A ½ Watt 100Ω resistor in parallel with the caps across a 5V supply will discharge them in about 10 seconds and will only consume an extra ¼ Watt.

A common (crude & cheap) solution is to put a high wattage, high resistance resistor across the each capacitor so they discharge in a reasonable time.

For 100V a 2W, 5k resistor across each 47mF cap will discharge to a safe level in a few minutes.