I made a dual +/- 12 V power supply for synthesizer circuits using a schematic by Moritz Klein (image below). The input of the supply is from a 12 VAC transformer.

After soldering the circuit on a prototype board (pad-per-hole) and testing it with my oscilloscope and multimeter, everything seemed to be working fine. I got a little too confident and tried to find out just how much it could do, and connected a 12 VDC motor between positive and ground, then negative and ground. Then I figured that if I connect +12 V and -12 V across a 24 V motor, it would power it just fine.

The LM7912 negative voltage regulator then stopped working, outputting -14.5 V and producing a lot of heat. It is also possible that I shorted the output of the power supply. I would like to know how I killed it, so that I can avoid repeating the mistake after replacing the voltage regulator.

My question is specifically: was it the high demand of the motor that killed the regulator, or connecting +12 V and -12 V? I have seen op-amps powered by the +12 V and -12 V of this specific power supply, would that be different from connecting a normal resistive load to the supply?

Circuit diagram:

Power supply by Moritz Klein

  • 3
    \$\begingroup\$ You haven't shared any details of your motor, so answering if "was the high demand of the motor what killed the regulator" is not possible. \$\endgroup\$
    – brhans
    Jan 10 at 19:59
  • \$\begingroup\$ @brhans I'm not sure of the details, they were both salvaged brushless motors. Sorry about my lack of vocabulary, I'm an amateur when it comes to electronics. I should clarify my question: could any high current draw kill the regulators, they are heat-sinked well and have thermal protection as far as I know. \$\endgroup\$ Jan 10 at 20:06
  • 3
    \$\begingroup\$ Why no anti-parallel diode across the 7912? Such a diode may have "saved it". Would be an interesting experiment to replace the 7912, add a diode across it, and try the motor again. \$\endgroup\$
    – rdtsc
    Jan 10 at 20:40
  • \$\begingroup\$ Did the 7912 output 14V, or -14V? Because, with no load, -14V would be normal. Did you measure before shorting it? \$\endgroup\$
    – Justme
    Jan 10 at 20:45
  • \$\begingroup\$ @rdtsc The 7912 has an anti-parallel diode built in, according to the circuit designer. \$\endgroup\$ Jan 10 at 20:49

1 Answer 1


Moritz Klein does a great job with his tutorials. I've built this same circuit for my modular synth explorations.

The LM7812/7912 regulator is rated at 1 to 1.5 A. They usually have over-temperature and short-circuit protection, but not always. Most other components in the circuit should be OK if you subject it to high current or short-circuit conditions. The regulators, even with some built-in protections, can still fail due to excessive heat. Replace them and test again, ideally with a known load. (For example, you could use a 24 ohm power resistor (like this one) rated for 25 watts or more to draw 1 A at 24 V.)

It sounds like you already are aware that the regulators need good heatsinking, but make sure that the heatsinks are actually big enough to keep them in their operational temperature range. If you test the supply with a constant load like I suggested above, you should only do it for a short time unless you know that the heat dissipation is adequate for longer durations.

A motor, even a relatively small one, may draw more current than 1 A, at least in startup/stall.


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