After some months of good quality prints, since yesterday my heatbed does not heat anymore.

I've a BQ Prusa i3 Hephestos with this heatbed, controlled by this mechanical relay and powered by this power source. I know that a mechanical relay is not the best choice for a 3D printer, but I only had one of those when I've built the bed and setting bang bang control for the plate gave me some satisfactory prints.

Since if I connect directly the heatbed to the power source, the latter stops working (I think because of its shortage protection), I'm almost sure that in some way I've managed to fry my heatbed. Before buying a new one, though, I'd like to understand how I've broken the current heatbed without doing anything but printing, to avoid similar situations in the future.

I'm afraid I've forgotten some component whose functionality is to protect the heatbed (maybe a diode or something similar, I'm sorry but I have never studied electronics).

I've attached a very simple wiring diagram showing the current configuration.

Wiring diagram

  • 1
    $\begingroup$ If you fried the bed, the resistance would be infinite (no continuity), else it would read 1 to 3 Ohms depending on the option (24 V/12 V). $\endgroup$
    – 0scar
    Oct 4, 2020 at 9:22
  • $\begingroup$ @0scar a short is either NC or 0 $\endgroup$
    – Trish
    Oct 5, 2020 at 17:07
  • $\begingroup$ @Trish Technically, the resistivity of air is 1.3*10^16 to 3.3* 10^16 Ohm, of circuit board 10^17 Ohm, so that counts for nearly infinite to me. :-) Hence the "no continuity" in brackets. $\endgroup$
    – 0scar
    Oct 5, 2020 at 21:58
  • $\begingroup$ but there's also the short of no resistance ;) $\endgroup$
    – Trish
    Oct 5, 2020 at 23:28

2 Answers 2


I see four points of error:

  • The heatbed could be faulty.
  • The lines could be faulty.
  • The trigger signal could be faulty.
  • The power supply could be faulty.

So, how to check what it might be? We need to measure, but we need to measure in a setup of 12/24 V circuits while they are live. While not necessarily considered "dangerous", take extra precautions! Remove the parts you will need to access from the machine, ground the power supply, and use your utmost care! Make sure the heatbed is not connected to the Power supply, so we can measure it in the safest way.

Ruling out the Heatbed

The first step can be done without power: the resistance of the heatbed should be non-0, non-open line. It's best to measure directly at the input pins to the bed. If it shows close to 0 (some milli-Ohms), open line (OL), or no continuity (NC), it has a short or is burned out and needs to be replaced.

Ruling out the Power Supply

Once we have reached a safe setup, take a voltmeter and measure the voltage (potential difference) on the pins you reserve for the heatbed. If it is 12 V with some tolerance, it's good, otherwise you might need to get a new power supply or have a professional fix yours.

Ruling out the trigger signal

Next, move your measuring to the output of the board. Now order your board to heat up the bed. Make no illusions, we will trigger TRP, as the bed is disconnected. This will need us to reset the board between tests. You should see a 5 V trigger signal. If not, your board or firmware might be faulty and require different investigations.

Ruing out the lines and switch

Next step is to swap to Resistance mode on the multimeter and measure the resistance of the switch, best with the whole power line. Under "heating", the setup should show a close to 0 Ohm resistance, if not it should show an infinite or open line.

What now?!

If it is neither of the 4, reassemble on the test stand, reconnect the bed to the power supply and investigate all connections between the elements we just checked. Now order a heating procedure - as all components should be ok, it has to be one of the interfaces between the parts.

  • $\begingroup$ Thanks a lor for the very precise answer. During the week a friend of mine will lend me a better multimeter than I have in order to make all the necessary mesures. Thanks again and I'll let you know what I understand from the measurements. $\endgroup$
    – user23804
    Oct 4, 2020 at 13:07
  • $\begingroup$ Thanks a lot! It turned out to be a broken cable. After I saw all the measurements were good, I've noticed with the continuity tool that wether it sounded or not depended on how I kept the cable in my hands. I've changed the cable and now it seems to work. Thanks again $\endgroup$
    – user23804
    Oct 11, 2020 at 13:35

Assuming you are using the correct voltage hot bed, what you have set up should be just fine.

On the other hand, if it’s a 12 volt bed with a 24 volt power supply, you could have easily burned it out.

If the voltages were matched properly, you may have just gotten unlucky.

  • $\begingroup$ Tomorrow morning I'll double check, but the power supply has 12V in output (from specs and the sticker attached to it) and I've soldiered together pins 2 and 3 as printed on the heatbed. Moreover if the voltage were wrong, shouldn't it have been fried some months ago? I've used it without any problem for nearly an year. Thanks a lot for the quick reply $\endgroup$
    – user23804
    Oct 4, 2020 at 1:02

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