Talk:Adrians Prusa Notes
Hi, I hadn't seen your design when I made http://www.thingiverse.com/thing:10823
I did my slots differently, I wonder if there's much difference in the end?
Off-board heated bed mosfet over-heating and dying - details and solution
Hello Adrian, perhaps you might consider adding a note, if you think my problem described below is not uncommon:
I am using Sanguinololu v1.2 and initially, inspired by your notes, mounted a IRF2804 mosfet on a (hacked) Prusa heated bed. After just one session (at the beginning of the next, in fact) the mostfet smoked. I replaced the heated bed mosfet with another brand/model - a gift from another handy RepRap user during a RUG meet up we had in Bristol - RepRap makes the world a friendlier place, thank you :) Result: Identical failure symptoms/mode (the following morning).
I then removed the heated bed mosfet from the bed, attached a new one to a temporary, mid-sized TO-220 heatsink and began fault-finding.
It appears that with approx 300mm of cable or more, where the gate wire lays close to or is tied to the drain wire, the 10K gate pull down resistor on the Sanguinololu board looses it's effectiveness and the device can partially turn on - resulting in a high impedance Source-Drain, lots of heat, followed by smoke. Putting a second 10K Gate-Source resistor in parallel with the first (either on the device, or as I did on a choc-block for ease of testing the theory) solved the problem, to the point I don't believe I probably need a heatsink; It willl however be staying.