I have an old ultrasonic air humidifier that sends out a big electrical spike every time it turns off, making lights flicker and computer monitors restart. I'm afraid of using it because of the risk of damaging sensitive electronics on the grid, especially since I need to have it on a timer, turning on and off many times a day.

Is there anything I can put in between the air humidifier and the grid preventing the spike from going out? I've been told that regular surge protectors aren't going to work.

  • What are you using to measure the spike? – Kris Jun 23 '17 at 11:04
  • @Kris, I haven't measured the spike. – Alex Jun 23 '17 at 11:28
  • is the humidifier on its own dedicated and filtered circuit? – The Evil Greebo Jun 23 '17 at 16:05
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    You have also been told incorrectly. Surge suppressors have no sense of direction - they just conduct when the voltage is too high. But a capacitor as suggested will work faster than a surge suppressor. – Ecnerwal Jun 23 '17 at 22:52

You can try a large capacitor at the proper rated voltage, at least 250 if your device operates at 120 volts. Connect it across the line ahead of the device.

The other option is an isolation transformer between the device and the main line which can help absorb spikes.

Good luck!


Yes. Proper wiring.

The problem here isn't your humidifier. The problem is the circuit it's on, or the building's service, can't handle the load.

Those aren't spikes, they're brownouts. A surge suppressor will do nothing.

This is probably a common consumer appliance and they are sized specifically to work well on ordinary consumer circuits (though they will dominate). So the problem is either

  • a super defective circuit
  • too much other stuff on that circuit
  • this is revealing a serious problem with the building wiring.

So go shut the breaker off that powers this humidifier. Go see what else also turns off. (Save your files). It helps to have a night-light to stick in outlets. You may find your answer.

At this point it may be time to survey your entire house's receptacles as to which receptacles are on which circuit. Happens all the time where people stack every single load on the same circuit while three others go unused, they just don't know any better.

For bonus points, get a $20 Kill-a-Watt meter and test each of your appliances and see how much power they also draw. Make a chart of how many watts (better: VA) you are putting on each circuit.

If getting your circuits sane does not work, then you have a power problem. It is also possible this is a neutral problem, either within a multi-wire branch circuit or for the entire house. The giveaway here will be that your voltage is not 120 (or 230/240 in Europe) - and your Kill-a-Watt will tell you that. We use 120V in America.

At that point expect to see ratty old scary panels and wiring. These things will burn your house down, so it's maybe time to think about a rewire -or if your panel will take them, AFCI breakers.

  • The OP clearly states that the problem occurs when the device switches off, not on, so it's NOT a brownout - it's an inductive surge from a switch being opened. Which is a device problem, not a wiring problem. – Ecnerwal Jun 23 '17 at 22:49
  • @Ecnerwal except there's no way for an ultrasonic dehumidifier to cause a power surge that large. I looked up the specs on them, they are like 40 watts. I suppose if the unit was very broken... – Harper - Reinstate Monica Jun 23 '17 at 22:54
  • Thank you for the and sorry for the late reply. I'm quite certain that the problem is when the timer is turning the device off, not on. I have bought a common surge protector that sits between the unit and the outlet and now I don't experience any more problem. If you have a suggestion for how to measure what is happening, I could try to verify. – Alex Jul 11 '17 at 12:53


  1. Get rid of the old ultrasonic humidifier. If you can't bear to get rid of it, at least stop using this humidifier until you can figure out what to do. You will have to spend a significant amount of money to just try various means of filtering out these spikes. You are risking damaging controls on HVAC, refrigerator . . . any electronically controlled devices.

  2. Try powering the humidifier with a surge protector. Maybe this would filter out these spikes.

  3. Get a better surge protector/UPS for your computer.

  4. Get an Eaton whole house surge protector and install it in or on the side of your electrical panel through a 50-A 2-pole breaker. This would protect against surges from outside (principally lightning) on the incoming power line and prevent a surge generated within one branch circuit from going to all the other circuits.

  • I bought a cheap surge protector and put it in between the humidifier and the outlet, it seems like this have solved the problem. – Alex Jul 11 '17 at 12:56

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