Microwave Ovens: why does fan stay for a second after time has elapsed?

When I stop a microwave oven mid-cycle by pressing the ‘Stop’ button, everything just shuts off immediately. But if I let the full time elapse, the noise (I assume it’s a fan) stays on for a bit longer after 0:00 has been reached. It even gets a bit louder just before it stops. Is there a reason for this? I’ve noticed this with a few microwave ovens, but not sure if it’s always the case.

I think you may need to do a more controlled experiment. Because I don’t know of ANY fan that stops immediately after the power is shut off.

Maybe the act of opening the door prematurely or the beeps of the microwave make it sound as if the fan stops immediately?

I just did this a few minutes before I asked the question. I was re-heating my coffee so pressed the 1-minute button, but pressed ‘stop’ after 45 seconds. Also, I never open the door prematurely.

The fan is used to cool the MW tube. If you let the timer run out, the tube is turned off before the fan. If you force the tube off, the fan runs a little longer to cool it down.

A lot of cars do the same thing now. It’s not the same for all MW ovens, or all cars.

Microwave tubes have a heater, which heats an emitter, which emits electrons. The electrons carry heat away from the emitter. If you turn off the electron flow, the heater and emitter overheat, and the tube dies. For this reason, a traditional MW oven can only run the tube at 100%, and the only way to get 50% is to turn the whole thing off, wait a while, then turn it all back on again. An ‘inverter’ microwave oven has variable control of the heater, and so can adjust the heater at the same time as adjusting the electron flow from the emitter. This allows continuous control of the tube from ~25% to 100%. (They typically don’t actually have good control down at the bottom end, because both the heater and the tube loss are non-linear.)

Is it a convection microwave oven? Those have a fan to move heat from the heating element around. It’s a good idea to leave that running for a bit to prevent the heating element from overheating due to lag.

There’s a fan in microwaves whose main job is to reflect the waves around to get a better distribution. It could be the case it also does double duty to draw fumes away from the wave guide (where they can mess up the magnetron). The first duty doesn’t need to run after the magnetron cuts off. The second could be helpful to continue a bit to draw off the fumes.

(And by “fumes” I mean the cruft coming off the food that rides along the steam, etc.)

The stop button is might also be considered a emergency stop button, like fire in the microwave, strange noise, sparks, wormhole formation or gateway to the daemon realm, so cut everything off makes sense. If it completes the normal cycle there is no need to do a emergency stop and better to have a more controlled shutdown, including a bit more venting.

Also consider that a stop request normally means that the door is to be opened, thus no need to run the fan, but if the cycle completes the dish may stay enclosed for a while, so extra venting may be beneficial.

This all makes a lot of sense. Thanks.

I don’t know how many microwaves have a wave stirrer (not sure if that’s the correct term) versus a turntable. Theoretically if you have one you don’t need the other. I think if the magnetron is in the side of the cabinet (behind a metal plate) then you don’t have a stirrer, because the waves are beamed directly into the cabinet, whereas if you have a stirrer the waves are directed to through a “duct” over the top of the cabinet. It’s also my understanding that the stirrer is turned by airflow rather than a motor, so they spin pretty slowly. That airflow would come from the other fan that I think all microwaves have. That fan cools not only the magnetron, but also the massive electrical transformer, and it blows the hot air through the cabinet to add some heat to the food and also expel steam and other gasses.

I think kanicbird is probably right on the reason behind the fan stopping on a pause/cancel/door open command but running long at the end of the cycle. The extra bit of cooling helps the components last longer, but stop means stop right stat now. From a user interface perspective, hitting pause/cancel while leaving “something” running even if it’s just the fan would likely freak a lot of people out, especially when they open the door. Imagine slamming on the brakes in your car, but then it decides to rev up the engine to recharge the battery just then. You’re still stopped, but that would not sit well.

I have an Amazon Basics microwave and its fan shuts off the instant the cycle is done. I suspect it is at least partially to move humidified air out of the chamber. One of the frequent criticisms listed* in the reviews was water puddles found on the floor and as a consequence, I leave the door open after removing the food then close it a little later when I see the light is still on.

*It was also only 700 watts but I found it hard to resist when it was offered for $26.

Wow, that is the first I have ever even heard that existed.

I remember when some car manufacture(s) designs that had the radiator fan run for a bit when one shuts down the car. A lot of people didn’t like that and Ive heard comments like ‘your car is still running’. Perhaps practically good for the engine/cooling system but not that practical in the real world.

Here in Arizona, during the summer my Civic would occasionally cut the fan in after the engine was shut off. The last thirty feet would be a crawl into the carport and I’m guessing the sudden lack of circulation would cause a pulse of hot fluid as the water pump slowed to a stop for a few seconds.

I think now many cars just keep the water pump running, which isn’t AS helpful, but better than nothing, and it’s much quieter.