The almost finished Green RepRap Mendel
And a few closeup
Rear view - with better view of the Heater and Temperature control board
The design of the Heater and Temperature control board has two on/off (PWM) channels for the Heated Bed and two for the extruder heater. I plan to add a fan and light on/off (PWM) board with 4 channels. I use MOSFET HUF75337 as it has several built-in protection features - and can easily handle more than 20Amp (specifications say 42Amp continues and 75Amp bursts), so I expect the PCB wires to "blow" way before the MOSFET.
The PC power supply hook up.
There are two separate 12V rails, one for the Heated bed, and one for the rest. Designed with the intent to make sure that less than 10Amp is drawn on each wire/socket pin (preferably much less per wire/socket pin). Currently the heated bed can draw 2x7Amp (rail 1 on two wires at max 15Amp), the extruder heater 2x2½Amp, all the steppers run fine on a handy 2Amp test power supply that used to be an external USB drive power supply (rail 2 on three wires at max 8Amp). So far no wires or power connectors should overheat.