relays for PSU to hotbed and extruder

Hi Guys,
I have been given a brand new high end PSU (TDK NV-350) which I want to use as the power source for the hot bed and extruder, this unit has two seperate 12V 16A modules built in, so I'm thinking of adding a couple of heavy duty 12V 80A relays driven from the main board to link the new PSU to the hot bed and extruder heater via fuses, are there likely to be any problems ?

If you are using RAMPS and connecting it to heaters and fans connector, I would put some freewheeling diode to protect the transistor. When FET stops conducting, the coil current goes down. Coil in relay is inductor and inductor doesn't want that. It releases its stored energy as higher voltage and might burn your FET. Now putting diode between coil's ends this energy is feed back to coil where it eventually will be heat.

Putting the diode is simple. Put it wrong way and it is short circuit and put it right way it works as freewheeling diode. Right way to put it is GND ---|>--- 12V

Also you probably need to tell firmware how to control your new heater.

Thanks for reminding me about the Diodes ! I've got most of the 1N4 series in my bits box(s)

I think a mechanical relay's clicking is going to create a lot of noise. Your extruder heater is only pulling between 2 to 4A and the FETS will handle it just fine. Depending on your bed heater the FETS/Connectors may or may not be fine. I'd use the FET to drive a good SSR if your worried about the bed heater but up to about 8A (96W) it should be OK also.

By good I mean one like this [www.auberins.com]