[TerraMex]

Hey.

There's actually two ways of doing this.

1) connect a relay to the PWRON (or Power Good) wire with a ground wire (GND).
It switches the PSU on as it has a small current flowing through one of it's logical circuit. That's what the shunt does.

I used a simple 5V DC- 5 V DC relay. PSU 1 feeds 5V to the relay and switches it from OFF to ON and simply powers on the PSU 2 with the above method.

The relay doesnt need a heatsink, it operates with very low currents. You can (and should IMO), use, however, a diode (1N4001 will do) for backcurrent protection in parallel with the psu1 feed.

2) Connect permanently the PWRON and GND. Use a 12v DC -240v AC relay for the mains (110V there i think, just replace the 240v) , same ideia, different approach.

This relay doesnt need a heatsink too, however it should be rated for up to 3 amps continuous use, as its secondary will feed psu2.

*3) go the extra mile and do both, bit redundant imo.

I used 2) thou. I switched to 1) when i did the "modular" upgrade but havent tested it yet (should work anyway).
Any electronics store should have those relays. Get the 5V DC, as it doesnt need more, else, get a 12v-12v DC relay. Also works fine.
I am, however, assuming your psu works as mine does, as the PWRON signal goes. You could just get a DMM and check (recheck, and check again).
PWRON is always maked on the PSU PCB, but it's usually the green wire on the 24 pin molex.