I built a simple swamp cooler that consists of a 12v DC pump and a fan connected in parallel to a power source.

When I repeatedly connect and disconnect the power source, eventually the fan dies. I lost multiple fans this way already. I'm not sure if it happens on the connect or disconnect. When the first fan died I thought that water might have killed it, but it happened twice more with a different, waterproof fan. Once there was a burning smell.

I thought that maybe an inductor in the pump produces a voltage spike when the power gets disconnected, burning out the fan in the process, and that the addition of a flyback diode might prevent that.

When I measure the voltage across the pump in isolation while disconnecting the power, I see the voltage spike shown in the screenshot below. I expected a negative spike but I see a positive one, and it seems a flyback diode won't help with that.

  • What could be the reason for this spike?
  • Could it be responsible for burning out a 12v fan (or should the fan be able to tolerate this duration and/or level of spike?)
  • how could I fix it?

oscilloscope screenshot showing voltage spike oscilloscope screenshot showing voltage spike

Update: One commenter recommended a 10000uF capacitor but the largest I had around was a 470uF one rated at 50V. I tried that, and it fixed the problem, no more voltage spikes.

Adding capacitor enter image description here

Out of interest I also checked if the fan produced any spikes that might damage the pump. It did produce some small negative spikes on power down but the magnitude seemed harmless. I quickly checked if a flyback diode would help and it did, but I ended up not adding that to keep it simple.

Fan back emf without diode Fan back emf with diode


1 Answer 1


You are right, a pump is a motor, a motor stores energy in its windings, and in its movement's energy. When disconnecting power, this energy has to go somewhere, hence the voltage spike (which can and does burn a 12 V fan, at nearly four times its rating).

The spike is positive because your pump's driver circuit behaves like a diode rectifier when not powered.

A flywheel diode would not help, but a fast diode (schottky) in series with your pump could, and a capacitor in parallel with the pump would help reduce the voltage spike level.

  • \$\begingroup\$ Am I right in thinking that is a poorly designed pump? Shouldn't it have a flyback diode after the driver? Or is that kind of spike considered acceptable in some scenarios? \$\endgroup\$
    – Dampmaskin
    Sep 6, 2017 at 8:30
  • \$\begingroup\$ Such spikes are pretty much unavoidable, so maybe not. does the manual give any kind of warning? maybe some safety measures? \$\endgroup\$
    – Sclrx
    Sep 6, 2017 at 8:57
  • \$\begingroup\$ Unfortunately there's no manual for the pump, just what the amazon page shows, and I couldn't find a website for the original manufacturer with more information. Thanks for the tips, I'll try out a zener diode (a 12v zener diode happens to be what I have around right now) and/or a capacitor. Do you happen to have any pointers to more information about the pump's driver circuit behaving like a rectifier? That's interesting. \$\endgroup\$ Sep 6, 2017 at 22:58
  • \$\begingroup\$ Try adding something like a 10,000µF 35v cap in parallel with a 22µF 35v cap, both across the leads of the pump and capture the waveform. \$\endgroup\$
    – rdtsc
    Sep 6, 2017 at 23:34

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