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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?

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  • $\begingroup$ 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 '15 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? $\endgroup$ – ratchet freak Aug 29 '15 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. $\endgroup$ – Chris Mueller Aug 29 '15 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 '15 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 '16 at 16:57
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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 '15 at 2:35

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