For the past two weeks I've been trying to get my heated bed to function properly. I've been trying different combinations of series and parallel for my 4 2Ω 10W resistors powered by a 12v PSU rail. I found one combination that worked, but it could only get my bed up to about 80°C. I need about 100-110 for ABS to stick well. After adding some pink fiberglass insulation under the bed my temps shot right up to 110-120.
Unfortunately, that was the only time the heated bed worked, the temperatures must have been too high and the sensor reading delayed. The next day I tried to fire up the heated bed and I could only get 40°C. After fiddling with it for a while I removed the bed to find this:
Heated bed burned up when I added insulation. The wires fried, but also the two resistors on the right are toast.
It turns out that a lot of the wiring had burnt up. After rewiring the whole thing I still only got 40°C of heat on the bed. I took an ohmeter to the circuits and discovered that the two right-most resistors in the above pic did not function anymore. Well, that was an entire day of wasted effort. The next day I had to go shopping for a solution to my heated bed. More to come on that...