Heated bed shorted out, board won't power from 12v anymore

  • Last week I replaced the Melzi board in my Monoprice Maker Select with a DuetWifi after the power terminals on the Melzi board burnt out (happens a lot with Maker Selects and the Wanhao Duplicator i3's they're a rebrand of). The new board has been amazing for the last week, and much quieter than the stock Melzi board. Today when I was printing the print abruptly stopped and when I toggled the power to the DuetWifi a bit later the board didn't light up. After poking around a bit I realized that the heated bed has a short.

    The board still turns on and appears to work fine when powered over USB (can connect through the browser, temps are reading correctly, etc). When I power it from the 12v psu and probe V_IN and ground with a voltmeter it only reads around 2v (the psu does put out 12v when not connected to the duet). I hoped that it was the bed mosfet that had blown because I measured about 3M ohms between drain and source on the bed mosfet while the two extruder mosfets showed no continuity between D & S but that doesn't appear to be the case because I desoldered the bed mosfet and the board still won't turn on. I don't believe it is the fuse as it measures ~3ohms, and I don't think it's the A4403 buck converter because when I move the steppers the V_IN LED lights up and if I move them fast enough the 3.3v & 5v leds light up which from looking at the schematic should only happen if V_IN is being successfully regulated down to 5v by the A4403 (USB and 5V_IN were not connected for this).

    After looking through the schematic and pcb layout I'm running out of ideas to test. Has anyone run into anything similar in the past, or does someone more familiar with the schematics have any ideas on other components to check?

  • administrators

    Unless something on the Duet is getting very hot, I suspect that the PSU has been damaged and can't handle even a light load. But also check that the screws on the VIN terminal block are tight and the wires are not burned.

  • Well, I feel pretty dumb now. I had inserted and removed the wires going to VIN a bunch of times as I was testing different things, and made sure the screws were tight every time. But I never checked on the psu end… the spade connector going to the psu ground was a bit crusty, so I cleaned it up and connected it to another one of the ground connectors and what do you know, it turned on just fine. I was able to move all the steppers, heat up the hotend, and extrude some filament, but I had to leave for work before I could stress the psu anymore.

    I guess I ruined the bed mosfet for nothing. Oh well, I think I'll be upgrading to a 110V silicone heater and SSR relay instead anyways, I should just be able to solder a wire to the bed mosfet gate pad and use that (and ground) to control an SSR, correct? Moral of the story, don't be dumb and assume the PSU is working just because it says 12v when you probe it's screw terminals!

  • administrators

    Yes you can control the SSR like that.

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