I have a simple 12V 10 A power supply with just a transformer and a rectifier. After doing some research and simulations, I've added 3 10 mF capacitors in parallel to smooth out the output.
My problem is that after turning the supply off, capacitors remain charged for quite some time. I can get small sparks after shorting the output even 5 minutes after turning the supply off. Right now I only have a single LED connected to the capacitors and it takes more than 10 minutes for it to turn off completely after powering the supply down and the capacitors still aren't fully discharged when it turns off.
The most obvious way to solve the problem would be to put a resistor and a switch on the output and connect the resistor to the capacitors after turning off the supply by hand, but I'm hoping to get something a bit smarter and a bit safer.
Another point is that I want to use the supply's original case which has very little free volume now that I've added the capacitors, so just putting a ceramic 11 W resistor could be a problem because there would be very little free space around it for safe cooling.
Answer
Appropriate bleeder resistors are the usual solution. They aren't usually switched, although they can be.
The value depends on the time you require to discharge the capacitors. The formula is
$$ V_{t} = V_{0} \, e^{ -t / RC } $$
where \$V_{t}\$ is the voltage at time t and \$V_{0}\$ is the initial voltage at time 0. It's an exponential function, so I'd just assume 1/10 of the initial voltage.
It isn't a power function, as someone edited it!
You should find that the power taken by the bleeder resistors is negligible compared to the 120W capability of the supply.
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