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I have a 4.2W 12V electric water pump connected in series with a water float switch as a sump pump, powered by a 1A 12V DC power brick.

The switch has a maximum switching voltage rating of 100V.

Would it be sensible to protect the switch with back-to-back zener diodes to absorb the surge when it switches off, or would that be overkill?

If so, what voltage rating would be reasonable for the zener diodes? I was thinking somewhere above ~18V.

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2 Answers 2

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You can put a free-wheeling diode at the pump leads. Any regular rectifier diode will work. Put the diode in reverse (diode anode on negative side of pump and diode's cathode on positive lead). When switch opens, the diode will dissipate the "spark", thus extending the life of your switch.

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  • \$\begingroup\$ Thank you, I'm sure I've got a 1N4001 somewhere in a box in a cupboard. \$\endgroup\$ Feb 5, 2014 at 22:52
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You can also use a capacitor across the switch if that's any easier. The back emf when the switch opens gets turned into a rapidly decaying sinewave of about 24V initial peak. The rapid decay is due to the circulating current in the motor feeding through a relatively high lossy resistor (motor windings) dissipating energy each time the rapidly decaying sinewave swishes between energy in the motor and energy in the capacitor. Go for something like 1uF and rate it over 50V to be safe.

Some people put a low value resistor in series with the cap and call it a snubber. Something like 100 ohms but, it isn't needed on this application.

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  • \$\begingroup\$ Thank you. I've gone with CAGT's answer as I think I have a suitable diode already. Something about putting a capacitor in parallel with an inductor made me think "oscillator". I had no idea what sort of voltage might be raised: I was thinking hundreds of volts(!). \$\endgroup\$ Feb 5, 2014 at 23:01
  • \$\begingroup\$ Ha ha no it's not as bad as that if you choose the cap to suit the energy held in the motor when it ope n circuits. It's probably only worthwhile using a cap in case s when the motor can act like a generator. With a diode you can damage the power supply but not with a cap. Bit black and white but worth considering for future reference. \$\endgroup\$
    – Andy aka
    Feb 5, 2014 at 23:06

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