With most microwaves I've used, I've noticed that once the timer goes off, there is a couple seconds where the "microwave sound" gets louder and it seems to be cooking more intensely before it actually shuts off. Why is this?

  • $\begingroup$ That's strange to me as I've never noticed this myself. All microwaves I've ever used switch off immediately after the timer hits 0. $\endgroup$
    – pauloz1890
    Aug 29, 2015 at 8:31
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    $\begingroup$ Are you sure it isn't the fan making sure the transformer isn't too hot when it shuts down? $\endgroup$ Aug 29, 2015 at 11:28
  • $\begingroup$ I believe ratchet freak is correct; a fan runs for a while after the microwave shuts off to cool the electronics. $\endgroup$ Aug 29, 2015 at 11:47
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    $\begingroup$ "Most microwaves I've used" indicating a sample size of... What? I've never observed this behavior, n ~ 10. $\endgroup$
    – Air
    Aug 29, 2015 at 22:10
  • $\begingroup$ At least one (older?) microwave I've used did this, where it would leave the light and fan on until after it was done with its 'cooking done' beeping. $\endgroup$
    – hBy2Py
    Apr 7, 2016 at 16:57

1 Answer 1


When the timer reaches zero, the microwave tube is turned off. Depending on your house wiring, this large reduction in load may allow the line voltage to rise a bit. The fan, which continues to run for a few seconds in order to get rid of the built-up heat, speeds up a bit as well.

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    $\begingroup$ Also, while the microwave is cooking, it is drawing a lot of current, which reduces the voltage available at the microwave. At timer expiration, the magnetron (the thing creating the microwaves) is instantly shut off, greatly lowering the current, so voltage rises. If a fan is still running, it may spin faster, simply because the magnetron is no longer online. $\endgroup$
    – rdtsc
    Aug 30, 2015 at 2:35
  • $\begingroup$ Reasonable guess. Someone would have to actually test and confirm this theory $\endgroup$
    – user253751
    May 28 at 17:41

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