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Monitor Interlook switch on a microwave

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    #1

    Monitor Interlook switch on a microwave

    I *was* trying to determine the cause of a failure in a microwave oven.
    The unit is a Frigidaire Gallery model CGMV175QFA.
    For future reference, the exploded view for this specific model is available here.
    The last page of that document also has the electrical diagram which I am attaching to this message as well.

    The unit had a blown fuse when I opened it. I tested each component individually (magnetron, relays, tracsformers, etc) and all appeared to be working fine (no shorts) but the fuse kept blowing.
    Eventually, I looked at the electrical diagram (attached) and noticed that the circuit has a Monitor Interlook switch (on the right side of the diagram), which, if closed would short the 120v power line. That was exactly the symptom I was seeing and since everything else was in working order, I removed the switch and tested it. Low and behold, it powered up, lit the light when the door was opened, and even the magnetron is working fine! It was fixed!

    So the interlook switch must have been defective, but since this is my first time digging this deep in a microwave, I'm curious: what does a Monitor Interlook Switch do, really? The electrical diagram is clear: it shorts the power line when closed. So other than a fuse blower, what is it supposed to do? And can the microwave work fine without it? Would it be safe to do so?

    thanks for your insight
    Attached Files
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    #2
    Re: Monitor Interlook switch on a microwave

    A quick search reveals that "interlook" probably was a typo and should have read "interlock" instead. It could be that the role of that switch is to fail when a specific condition is detected. but is it good design to have it blow a fuse when that occurs? Is that how all microwave ovens are designed?
    And what exactly is it monitoring? An open door?
    I'm probably missing something pretty obvious, so thanks for shedding some light on this for me

    Comment

      #3
      Re: Monitor Interlook switch on a microwave

      It stops the magnetron being on with the door open.
      https://appliantology.org/blogs/entr...icrowave-work/

      Comment

        #4
        Re: Monitor Interlook switch on a microwave

        Originally posted by diif View Post
        It stops the magnetron being on with the door open.
        https://appliantology.org/blogs/entr...icrowave-work/
        Thanks for the link: very informative article.
        A dead short by design!? That's an effective (and pretty radical) way to implement a safety measure!
        thanks again!

        Comment

          #5
          Re: Monitor Interlook switch on a microwave

          If the monitor switch is bad, it is usually a good idea to replace the interlock switch as well, It's contacts likely stayed closed when the door was opened and the monitor switch did its job but remained shorted.

          Comment

            #6
            Re: Monitor Interlook switch on a microwave

            Originally posted by R_J View Post
            If the monitor switch is bad, it is usually a good idea to replace the interlock switch as well, It's contacts likely stayed closed when the door was opened and the monitor switch did its job but remained shorted.
            There's still something I'm not understanding. On the diagram attached to the first post, the monitor is connected on the 'left' side of the power relay, while the transformer is on the right side. Shouldn't this mean that when the door is closed, the monitor is also closed, thus causing a short?
            If the monitor were on the other side of the relay, I would understand: if the door were opened while the transformer was active it should short, but having the monitor located before the relay does not work in my model, so obviously I'm missing something!

            In the blog article referenced above, the monitor is indeed located after the relay, and that makes sense, but that's apparently not how this unit was designed. To confirm, I disconnecting the PCB assembly completely, and indeed, simply closing the door causes a short. This behaves exactly like what the diagram says, but doesn't make much sense... So again, I'm missing something!

            Comment

              #7
              Re: Monitor Interlook switch on a microwave

              if everything is working correctly, the monitor switch should never be closed at the same time as the interlock switches. In fact, the only situation in which it would be possible for the monitor switch to be closed at the same time as the door switches is if there's been a mechanical failure of the door latch/switches.

              Comment

                #8
                Re: Monitor Interlook switch on a microwave

                Like petehall347 explained above
                When the door is open, the primary interlock switch is OPEN and the monitor switch is CLOSED. When the door closes, first the monitor switch opens, and then the interlock switch closes. This is all to prevent the door being opened while the magnetron would be on. Having the monitor switch before the relay is the best way to do it.
                Last edited by R_J; 04-18-2023, 01:48 PM.

                Comment

                  #9
                  Re: Monitor Interlook switch on a microwave

                  Finally had an opportunity to go back and look at the switch. What I found leaves little doubt as to the cause of the failure!
                  Thanks for help me understand how microwaves are designed!
                  Attached Files

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