Bleeder resistor: switch resistance on when capacitor is powered off?

Possible Duplicate:
How to discharge smoothing capacitors?

I have an aluminum capacitor (~600-800uF) smoothing a DC current (~60-70V, ~50-70mA). I have a resistor in parallel to drain the capacitor when powered off. It is important that this capacitor is drained quickly (under 3 sec) when powered off and also that it provides maximum power when powered on.

I've learned that using a lower resistance drains the capacitor quicker but sacrifices running power as wasted heat. On the other hand, a higher resistance provides optimal power and minimal heat but simply drains the capacitor too slowly.

Is there a simple (cheap and small) way to switch a resistor on only when the capacitor needs to be drained? This way maximum power can be achieved and the bleed time can be precisely controlled.

Thanks!