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I have a microwave (~1200W when running) and a fridge (~600W when starting, rough/approximate measurement) on the same circuit, and whenever they are both running, the circuit breaker (110V, 15A) trips. Is there any easy way that I can have the microwave always have power, but the fridge only have power when the microwave is NOT running (i.e. heating something)?

I have seen "energy saving" power strips that power stuff down unless when a device is on, but I need the opposite of those - any ideas that I can hack together?

P.S. #1 I am not really concerned about the fridge losing power suddenly or being unable to cool the stuff inside it for a few minutes while the microwave is running.

P.S. #2: I am looking for an automated way, I don't want to have to use a switch or something that requires manual intervention

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  • Intriguing question... Haven't heard about such a device, but try searching for "load balancing" or "current based switch/limiter " – holroy Jul 24 '15 at 16:21
  • Are you on 110/120V power? At 230V, 15A should be plenty to run both. – Hobbes Jul 24 '15 at 17:40
  • @Hobbes yes, 120V – user2813274 Jul 24 '15 at 18:04
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    Maybe you should ask on Electrical Engineering for advice on how to build such a device... – Byte Commander Jul 25 '15 at 12:10
  • Vote to close because it's a wiring issue, there's no lifehack for inadequate house wiring. Just the microwave is too much for a 15A circuit, never mind the same one that runs the refrigerator. – Zeiss Ikon Mar 7 '17 at 15:01
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As you suggest, it is probably possible to rewire a standard current-sensing power strip so that the slave outlet is turned off when the master (microwave) outlet is drawing current. Most relays have single-pole-double-throw (SPDT) contacts, and switching the slave connection to the other side would work as desired.

If you'd rather make your own switch, a schematic from Elliott Sound Products should work. You could put a pilot lamp on the 12-v portion of the circuit to warn when the microwave can't be used, or just leave it out.

One possible issue, though: if the microwave starts while the fridge compressor is already running, back-pressure builds while the fridge is off and it might require a few minutes delay before restarting the motor. Repeatedly switching a fridge compressor motor on and off (short-cycling) can damage it; so add a large capacitor across the base and emitter of Q1 and a diode in series with the base to act as an ON time delay.

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  • The fact that you can damage the fridge should indicate it is better to just use another circuit for one of the device. – Nelson Aug 9 '15 at 22:26
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    The chance damage is unlikely unless the short-cycling is frequent. but the issue can be avoided with a timer on the strips, as is built into most modern air conditioners (appliances more likely to be manually cycled). If @user2813274 had the option of another circuit, one would hope s/he would have used it. – DrMoishe Pippik Aug 10 '15 at 20:32
  • If one is lucky the current from the microwave light coming on when opening the door should be able to shut to fridge down before the magnetron kicks in. Having a minimum OFF time for the fridge will solve the short cycling issue nicely. – KalleMP Sep 3 '15 at 6:49

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