Heatbed stuck on

Hello all!
So, I'm new to the world of 3D printing (and electronics in general...). I've completed the mechanical build for a Prusa, with a Sanguinololu board.

I got it going, and at first the heatbed seemed to behave fine. However, it now seems to be "stuck on" - in that whatever I tell it to do, and whatever the temperature of the bed, it continues to heat up. I've taken a voltmeter to the board, and found that on the MOSFET the voltage between what I think is pin 1 and the other two switches from zero to ~5V when I turn the bed on and off in the software (between what I think are pins 2 and 3 is always zero). Whatever I do, there is always ~12V across the output to the heatbed.

I don't really know what behavior to expect from a MOSFET; is it behaving correctly? Does anyone know what the problem might be?

Thank you.

Well for the benefit of anyone finding this thread at a later date, I worked out the mosfet was damaged by measuring the resistance across each pin and comparing it to that of a "healthy" mosfet (ie one of the other ones on the board). I found all the pins were connected, whereas on a healthy mosfet you'll find there is an infinite resistance between some of the pins. I bought a new mosfet, and after soldering it in place of the ruined one the board now works again

Edited 1 time(s). Last edit at 02/05/2013 02:27PM by little-matt.