Heated bed won't heat

For some reason i can't make my melzi board turn on the heat bed output.

It's not the minimum temp function as it is set to 15c and the termister reads 25c.

My voltmeter reads 0 on the heatbed screw terminal, when it's supposed to heat.

I hope someone have an idea what it might be.

Behind the plug is a 3 legged device called a MOSFET, this device is turned on and off by a digital signal to the gate pin. With the gate enabled the device makes a connection from the ground of the Headed bed to the power ground turning it on.

For details see [reprap.org]

Things that can go wrong:

Firmware, your HOTBED pin is defined incorrectly. If your firmware has a Melzi setting and you haven’t played with pins.h This is unlikely.
Hotend thermistor. If you running marlin and don’t have a hotend thermistor it wont let you turn on the hotbed.
Dead pin on your 1284p cpu. This is a possibility.
Dead MOSFET, this by far the most common issue.
Or a bad connection somewhere between the 1284p pin and bed connector socket. This is unlikely.
Bad heated build platform.

Get yourself a multimeter, you going to need it to test anything.

First I would check the resistance of the headted build platform. If its a standard 200x200 PCB HBP is should be about 1.2 ohms (some cheap multimeter cant read this low).

If that’s good then Unplug you bed, and try and turn it on in software. Since this board has no indicator leds, check for output voltage on the hotbed plug?anything? yes? its a heated bed issue, probably a short.

If no voltage check for 5 volt on the MOSFET gate pin, if its there, you have a dead MOSFET, if there is no voltage on the gate then it a 1284p or interconnect or firmware.

It used to work back when i used a 12v 18amp power supply, but it broke. I changed to an old 400watt atx power supply. I use 2 pairs of
plus and negative 12v from the molex connectors, i don't know if i will be able to draw enough current through the molex connectors.

I allready measured the bed output screw terminals with the heated bed turned on in pronterface. But it diden't read anything. A thermister is connected and it reads about 24c degress in idle.

I will try to measure if the mosfet gets 12v and report back. Thank you.

Edited 1 time(s). Last edit at 09/30/2013 03:42PM by killerfruit36.

I removed the crappy screw terminal that was presoldered from [reprap.me] and soldered the wires directly and now it works.