RepRap Heated Build Platform

I have been trying to get a RepRap working, on and off, for a few months. It was bought second hand, supposedly complete except for a power supply. Obligatory tat item: I have a spare ATX PSU, which ran the main RAMPS electronics and extruder fine, but cut out when I tried to power up the heated bed. This is apparently a known problem; the solution is to put a dummy load on the 5V rail. I tried 4 off 3.3ohm 10W resistors in parallel, it helped, but it still cut out whenthe heater cut in. The resistors got toasty though. Which made me think:

Instead of wasting the power going through the ballast resistors, use that to provide a base heater load and then have a lower power - which can be handled - on the 12V supply to give control. It needed nichrome wire in 18SWG and 20SWG. The original build platform was a piece of unetched circuit board, which I re-used, sticking the wires onto the non-copper side as shown in the diagram using Kapton tape. I think the copper should give better heat distribution, but I have no proof of that. I put Kapton over the copper and pinned it all down with bulldog clips, and it works just fine. The shorter (green) track is 18SWG, attached to the 5V supply. The longer   (red) track is 20SWG, and is connected to the D8 terminal on the RAMPS board.