Power supply shuts down when heated bed MK2a is connected.

I am using a 450W power supply. The resistance on the PCB measures at around 1.1 Ohms both at the point of connection with the PCB and at the ends of the wires that connect with the power supply / RAMPS. I have measured 12V output on the appropriate RAMPS connector. I have tried powering on with a DVD drive connected to put a 5V load on the PSU. The power supply refuses to come on whenever it is connected to the PCB, either through RAMPS or directly, even when the RAMPS is disconnected. 12V/1.1Ohms should put me at around 11 amps which should be well within the range of this PSU's limits.

Disclaimer: my limited understanding of electronics makes it likely that I have made a rookie mistake in one or more of my observations.

I'd greatly appreciate some help in diagnosing this!

Does the power supply work ok when connected to the hot-end?
Also, try disconnecting the heat bed wiring to D8 and check for any shorting between the wires (which will include the pcb heat-bed tracks) and ground- not between the 2 wires themselves.

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Waitaki 3D Printer

Yes, the hot end heats up and gets hot enough that the thermistor reads 230 degrees and it can melt ABS. No problem with that so far. I'm not sure I understand what you are suggesting that I try; am I right in thinking that the heated bed should work fine when connected to +12V and ground directly to the power supply? I mean it wouldn't be able to regulate the temperature, but it should at least power on and begin heating, right?

Yes, that's correct.
I have heard of some Chinese heat bed imports (which I use) that are shorting to ground through the corner mounting holes

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Waitaki 3D Printer

Interesting; I checked though and mine do not short out. I just checked my multimeter though on a 10W 4.7 Ohm flameproof resistor (that I got to load the 5V rail on the PSU) and it read 5.6 Ohms; maybe my multimeter is not accurate enough and the PCB resistance is actually substantially lower than I measure. On the other hand, maybe it's the resistors that are a little off...

Just realized my multimeter reads 1.1 Ohms when I short out the probes... I guess this means I have a defective multimeter? That would also explain why I measured the resistor at about 1 Ohm higher than its label.

Edited 1 time(s). Last edit at 04/02/2014 12:43AM by ghobbs.

Does your meter have a "zero" button like most do, which compensates for the varying resistances between test leads and dirty banana plugs?????

No; this is my multimeter. Does this mean my heatbed is shorting out? Should I return the multimeter?

If your meter is definitely reading 1.1 ohms shorted and heat beds are usually around the 1.1 ohm mark then the bed does look suspicious. Your meter is still very usable - It might even be the leads themselves.
Also, remove the heat bed and check the mounting holes are clearly isolated from the tracks.

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Waitaki 3D Printer

Hmm... I removed the PCB heatbed and it still reads the same resistance as before; I also inspected the mounting holes visually (they're close but not obviously problematic) and by testing with a multimeter I didn't find a short between the electrical connections on the heated bed and any of the mounting screws. I had a resistor and LED installed before but cut them off to try and eliminate potential problems.

Here are some pictures of my board.

It looks like there is a thin sheet of metal embedded near the bottom of the board - maybe my wires are connecting with this somehow, causing a short?

Edited 1 time(s). Last edit at 04/04/2014 06:09PM by ghobbs.

My Mk2A is similar to yours on the front (led side) but on the back of yours there are another pair of elongated tracks where you have soldered your wires to. Are those connections not connected electrically to the whole back plane - i.e shorting the wires?
Maybe try just soldering the wires to the front (track side) of the board but drill out the through hole plating first

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Waitaki 3D Printer

waitaki: That did the trick! I did what you suggested and the resistance went up by about 1 Ohm, and now my bed seems to be working perfectly! Thanks for all the help!