Confirming PSU spec for steppers.

[Jonathan]

This bridge has a forward voltage drop of 1.1v [...] each diode will dissipate 1.1v * 12A = 13.2W so you'll have 26W of heat to dissipate.

The mean current will be significantly less than 12A, so a heatsink with a much higher thermal resistance could be used. On my stepper PSU, which also uses a 500VA 50V transformer with 3 motors, I used one of these (or this one) and no heatsink is required as I've never noticed the rectifier get 'very warm' let alone 150°C. Still, you can find good heatsinks for free in lots of things, or since this evidently doesn't need much you could just attach it to a reasonable size sheet of aluminium.

Agree with the calculation for the bleed resistor, but surely since the stepper drivers will be switched on until the voltage has dropped to about 20V, resistors aren't critical since the energy from the capacitors will dissipate into the motors? If you unplugged the stepper drivers before turning off the power supply then the resistor is needed, but that would be a strange thing to do since it risks breaking the drivers.

Thank you, that's very, very useful. I was looking at caps that cost about £3 each though, is there an important part of the spec that I didn't take into account?

The ones you have linked to will be fine, I'd get 4. The ones irving linked to are better quality (lifetime etc), but cheap capacitors are fine for this application.