Quote Originally Posted by AvionicsMaster1 View Post
Rambling a bit here but just some thoughts.

Well, your location says UK so I'm assuming 220 VAC 50 HZ. The datasheet only specs a 60 Hz curve but I doubt that is the source of your heat.

If I understand you correctly, your powering this rectifier by 12VAC input to the rectifier. I'm also assuming the output has twice the input frequency so it should be 100 Hz before capacitor smoothing. If the bridge rectifier is good you should have a doubling of the frequency. You could check that to make sure the rectifier is good.

I'd think anything metal would be a good enough heatsink for your current load. A small chunk of metal with a little transfer compound on it should work. I'm curious if that will do it.
I'm not sure why you mention frequency. It has absolutely nothing to do with the problem. If the circuit (no schematic shared) is a full wave bridge rectifier, then at any point in time 2 of the 4 diodes will be conducting.(except for the very small time the AC is within 0.9V of 0V).
From the data sheet, at 5A the forward volt drop will be 0.9V.
2 elements are conducting, so 5A x (0.9 * 2) = 9 Watts being dissipated in a 1/2 cubic inch of material = very toasty. Hence the giant hole in the middle of the device for a #8 screw to bolt it tightly to a heat sink. How big of a heat sink? Work out the thermal impedance to ambient air. How do you do that? Click on the link in my previous post.