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Heatbed won't turn off

Posted by villainous 
Heatbed won't turn off
August 26, 2014 06:17PM
Judging from the results of my Googling, it seems like a lot of people have had the problem where their heatbed won't switch *on*, but is anyone having the reverse problem? I'm trying to calibrate mine right now and get it set up for its first print, but the heatbed gets very hot and I'd like to be able to switch it off. Setting the temperature in Pronterface does nothing. I have a RepRapPro/eMaker Huxley.

I tried placing the bed MOSFET signal lead on Vcc and Ground but to no avail.
Re: Heatbed won't turn off
August 27, 2014 01:39AM
Since your MOSFET appears to doing nothing, there should be an issue with the circuitry on the heated bed.
If you mount the components yourself, chances are that you shortcircuited something when soldering.
Look attentively on the board and see if something looks odd. Use any image of the PCB to compare with.
It's less probable, but still possible that the MOSFET is gone and it always conduct.
Re: Heatbed won't turn off
August 27, 2014 02:00AM
The heated bed PCB came from RepRapPro as is. Maybe i fried the MOSFET during soldering but I doubt it. I notice on the Melzi controller there's connectors for the heated bed; can these be used in lieu of the MOSFET since it appears the MOSFET is always open no matter what?
Re: Heatbed won't turn off
August 27, 2014 08:45AM
Well, firstly you have to find out what's wrong with the HB. If you don't, any modification may propagate the fault elsewhere. It's really hard to fry a power MOSFET, but could happen. Since the HB warms up, it means the MOSFET is in short-circuit, permanently providing permanent ground to the PCB resistor (+19V is permanently supplied).
It should be possible to use the circuitry on the Melzi, but you have to change the wiring and the jumper connection on the HB. Also, make sure the wires providing 19V to Melzi support both HE and HB (in original configuration, power is provided separately to HB (directly) and HE (via Melzi).
To change:
- desolder the jumper wire from the pads and solder it on the middle pad and the unused pad
- disconnect the ground wire from the screw terminal block on the HB
- identify the "active" position of the HB terminal block on Melzi (one should be +19V permanent, and the other is "active"", coming from the on-board MOSFET)
- route a wire from the "active position" on Melzi to the unused position on the terminal block on the HB (+19V should be in the middle, the other was used by the original ground configuration)
Check thoroughly for shorts! You are the only one responsible for the results smiling smiley
Re: Heatbed won't turn off
August 27, 2014 09:30AM
Thanks for your replies!


What does the jumper wire on the HB actually select? I have it soldered right now per the Huxley instructions.

What is the "active" HB terminal and what is it for? Right now only two terminals in a 3-terminal block are connected on my HB connector (the power connector coming from the main supply, not the ribbon cable).
Re: Heatbed won't turn off
August 27, 2014 11:08AM
Hey all,

Turns out my heatbed PCB was faulty and I'm getting another one sent to me. Thanks for your help!
Re: Heatbed won't turn off
August 28, 2014 12:15AM
Just to answer your questions smiling smiley
- The jumper switch from the on-board electronics to one residing on the controller.
- I said "active" because is controlled by the MOSFET. But it refers to the terminal block on Melzi, the one that is dedicated to controlling the HB.
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