# Why don't microwave ovens operate at partial power? **URL:** \ **Category:** Factual Questions\ **Created:** [March 18, 2015, 4:56pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305 "2015-03-18T16:56:53Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [March 18, 2015, 4:56pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/1 "2015-03-18T16:56:53Z")
Every microwave oven I’ve seen, when set to something other than “full power,” achieves a time-averaged partial power by dithering between full power output and zero power output. Is there something about a magnetron that means it only runs well at full load, or is this some sort of cost-saving measure that allows manufacturers to leave a thyristor out of the design? ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [March 18, 2015, 5:36pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/2 "2015-03-18T17:36:22Z")
AIUI, the power output of a magnetron is determined by its physical properties (size, shape, number and size of cavities). There _might_ be some sort of control circuitry that allows the user to simulate different properties, but compared to the circuitry that manipulates the duty cycle, that would probably be both too complex and too costly to put into a household appliance. ---
**Author:** ![swampspruce](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/swampspruce/32/261_2.png) [@swampspruce](https://boards.straightdope.com/u/swampspruce)\ **Post date:** [March 18, 2015, 5:41pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/3 "2015-03-18T17:41:43Z")
This. ^ ---
**Author:** ![Steve\_MB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steve_mb/32/5339_2.png) [@Steve\_MB](https://boards.straightdope.com/u/Steve_MB)\ **Post date:** [March 18, 2015, 6:11pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/4 "2015-03-18T18:11:57Z")
Actually, some recent models use fast pulse-width modulation to produce an output that is closer to constant partial power than the usual on-off-on-off (basically, making the on-off cycle so fast that the microwave emitter never really quits or reaches full blast). ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 18, 2015, 6:13pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/5 "2015-03-18T18:13:05Z")
Note that the main use of a lower power setting on a microwave is to better match the rate at which heat is delivered to the object (usually to its outer parts) and the rate at which thermal conductivity transmits this heat to the center of the object. One could match these by adjusting the actual rate at which power is delivered and leaving the power on continuously; or one could just cycle the power on and off. Since the choice of method won’t significantly affect cooking time, manufacturers are free to pick the one that saves them the most money. As an analogy, imagine filling a jar with honey by using a funnel with a narrow neck. If you pour the honey in too fast, the honey will start to accumulate in the funnel and overflow. To keep this from happening, you have to do one of two things: dump in a goodly amount of honey at a time and then let the funnel drain, or carefully adjust the rate at which you’re pouring the honey so that the rate at which the honey flows into the bottle is roughly the same at which it’s being added to the funnel. The second method is perhaps more elegant, but requires more concentration on your part, and in the end you don’t get a full jar of honey any faster. ---
**Author:** ![Heracles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/heracles/32/487_2.png) [@Heracles](https://boards.straightdope.com/u/Heracles)\ **Post date:** [March 18, 2015, 8:04pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/6 "2015-03-18T20:04:41Z")
> [@MikeS](#): > > As an analogy, imagine filling a jar with honey by using a funnel with a narrow neck. If you pour the honey in too fast, the honey will start to accumulate in the funnel and overflow. To keep this from happening, you have to do one of two things: dump in a goodly amount of honey at a time and then let the funnel drain, or carefully adjust the rate at which you’re pouring the honey so that the rate at which the honey flows into the bottle is roughly the same at which it’s being added to the funnel. The second method is perhaps more elegant, but requires more concentration on your part, and in the end you don’t get a full jar of honey any faster. Thank you, Sir, for that most delightful analogy. ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 18, 2015, 8:37pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/7 "2015-03-18T20:37:24Z")
> [@kaylasdad99](#): > > There _might_ be some sort of control circuitry that allows the user to simulate different properties And that most certainly would be less efficient. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [March 18, 2015, 10:26pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/8 "2015-03-18T22:26:02Z")
I just bought a new microwave and was surprised that it still regulates the power by pulsing. The old one, that gave good service for 15 years, had various settings shown as percentages on a dial. 25% obviously allowed the power on for 25% of the time and this was easy to hear as it switched on and off. The new one is ‘digital’ and the power is four buttons marked in watts. 900, 600, 300 and 100. I had (naively) assumed that this meant that the power was proportional, but it is clear from listening to it operate, that it is just the same as the old one. ---
**Author:** ![manila](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@manila](https://boards.straightdope.com/u/manila)\ **Post date:** [March 18, 2015, 10:37pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/9 "2015-03-18T22:37:10Z")
[http://what-if.xkcd.com/131/](http://what-if.xkcd.com/131/) The answer ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 18, 2015, 10:54pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/10 "2015-03-18T22:54:45Z")
^ Exactly. My room mate cannot defrost food because she hits the “Defrost” button. I don’t know what power level that is but defrosting (Dethawing as she calls it) should always be done on the lowest power setting to give time for the heat to distribute. And even for reheating, if you think 2 minutes at full power is required, 10 minutes at 20% will bring superior results. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [March 18, 2015, 11:31pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/11 "2015-03-18T23:31:27Z")
Notwithstanding that pulsing is used to simulate partial power (mine does that, too) I’m pretty sure that the “defrost” setting has its own very low power emission. When mine is on “defrost” it cycles roughly about 50% on, 50% off, but it’s clearly much lower power than a 50% power setting. ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 18, 2015, 11:45pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/12 "2015-03-18T23:45:15Z")
> [@wolfpup](#): > > When mine is on “defrost” it cycles roughly about 50% on, 50% off, but it’s clearly much lower power than a 50% power setting. Huh? How is 50% on, 50% off, clearly much lower power than a 50% power setting. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 18, 2015, 11:52pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/13 "2015-03-18T23:52:06Z")
So if it’s possible, one way or another, for a microwave oven to defrost frozen dinner bricks, then why doesn’t the built-in defrost function know how to do it? I’ve noticed that too, with my current μwave and my previous one (by a different manufacturer), that the built-in defrost function works terribly badly. ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 19, 2015, 12:09am UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/14 "2015-03-19T00:09:42Z")
I just tried mine, which is called “Auto Defrost”. After hitting the Auto Defrost button you then have to press 1 - 9. Different numbers give different total time but they do seem to be about 50%. That’s WAY too high. As to “why doesn’t the built-in defrost function know how to do it” it’s because that’s how it’s programmed. ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 19, 2015, 2:56am UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/15 "2015-03-19T02:56:55Z")
> [@dougrb](#): > > As to “why doesn’t the built-in defrost function know how to do it” it’s because that’s how it’s programmed. Let me rephrase that. The programMERS think partially frozen/partially cooked is acceptable for a thaw function. Save yourself the curse of trying to make hamburger patties from partially cooked thawed ground beef. Just manually put it to 10% and start on low time and rinse/repeat as necessary. ---
**Author:** ![FluffyBob](https://avatars.discourse-cdn.com/v4/letter/f/ecb155/32.png) [@FluffyBob](https://boards.straightdope.com/u/FluffyBob)\ **Post date:** [March 19, 2015, 1:23pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/16 "2015-03-19T13:23:37Z")
Panasonic Inverter Microwaves are marketed as using a more advanced power control system. Don’t know if it is simply PWM or something else. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [March 19, 2015, 3:55pm UTC](https://boards.straightdope.com/t/why-dont-microwave-ovens-operate-at-partial-power/715305/17 "2015-03-19T15:55:22Z")
> [@dougrb](#): > > > [@wolfpup](#): > > > > When mine is on “defrost” it cycles roughly about 50% on, 50% off, but it’s clearly much lower power than a 50% power setting. > > Huh? How is 50% on, 50% off, clearly much lower power than a 50% power setting. He means that the defrost mode operates at 50% on, 50% off measured by _time_, but ALSO is operating at a lower output level in \*energy \* while it is on. Contrast this with using the “50% power level” setting which he believes is cycling 50% on and 50% off by \*time \*but operating at maximum energy level when it is on.