Melzi - problem with extruder heater temperature

Hi!

I started repraping not so long ago and I already run into troubles: After I plug power to Melzi board extruder heater starts heating. Temperature sensor works ok - I can see proper reading in Proterface. I measure output for extruder and it's 12V constantly, no metter if termerature reached it's target set in Proterface. It's 12V even if don't turn on extructer heating. Everything except that works perfectly.

What could be the problem for such behaviour? Is it possible to fix it or I should get ready for new control board?

PS. I don't know much about electrnics.

Thanks for any suggestions!
Artur

One side of the load/extruder, is tied to V+, e.g. 12V. As a result V+ appears at that point always, as long as psu is turned on.

The mosfet is a low side switch, meaning mosfetsinterrupts the gnd/low side of the load. For example, when fet is off, the load would start working if it could find an alternate path to gnd.

If you want to check if its working properly, measure the voltage between the 2 leads of resistor, to each other. When fet is off, both of them will be at 12v, and their difference =0v between. When fet is on, one side is pulled low, so all supply voltage appears across resistor, giving difference between resistor leads =12v.