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My microwave has multiple power settings, ranging from 500 watts to 1500 watts. My question is how is the power reduced/modulated?

I can think of a few options (but I don't know which one is correct/most common):

  1. Is the microwave constantly switched on and off with different 'duty cycles' (similar to PWM) (different on/off times with longer total 'off time' when the microwave is set to a lower 'power level') in order to create lower power output?

  2. Is the electromagnetic frequency of the photons being emitted by the microwave altered resulting in more or less energy per photon? *

  3. Is the amplitude of the waveform (in other words the amount of photons per second) altered resulting in more total energy per second? *

Have I made any wrong assumptions in the three options listed above? Are all three options above theoretically possible in order to achieve modulation of the microwave's power output? Are any of these options used in a microwave oven in order to handle the modulation, if so which is most common?

According to this forum post photon energy is determined by a photon's electrical frequency and a waveform with same frequency/wavelength and different amplitude has more photons being emitted per second.

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    \$\begingroup\$ It wouldn't make any difference even if the frequency was changed to alter the energy per photon - if the photons were higher in energy but the power input to the oven is the same you just end up with fewer photons; the power absorbed by the food would be the same. \$\endgroup\$ – Kevin White Jan 19 at 23:02
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It's really simple. They have one fixed power level, and turn it on and off. You can hear it when it is on low power. It'll hum louder for a few seconds, then have a longer period when it is a little quieter. On is louder, off is quieter.

On full power, it is on all the time. Lower power switches it off for a short pause with a longer on period. Low power is a short on period followed by a longer off period.

It is sort of pulse width modulation, but real slow.


The frequency of the microwaves is fixed. More or less - it isn't strictly controlled because it isn't critical. The construction of the magnetron sets the frequency - you can't easily vary it. The frequency is more of a range of frequencies.

Modulating the power of a 1000 watt transmitter isn't easy, so the manufacturers don't try. A fixed power level is enough, if you can turn it on and off.

They don't even closely regulate the power all that closely. The transformer is made such that it limits the current to the magnetron, and that's about all the regulation there is.

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  • \$\begingroup\$ could you elaborate on what you mean by "The frequency is more of a range of frequencies" ? Thank you! \$\endgroup\$ – Maarten -Monica for president Jan 19 at 22:22
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    \$\begingroup\$ @Maarten-Monicaforpresident He just means a microwave doesn't output a very exact or precise frequency. It's fuzzy, not sharp and not well controlled, not accurate. \$\endgroup\$ – DKNguyen Jan 19 at 23:00
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Most microwave ovens use the on-off method of control described in other answers. There are microwave ovens advertised as "inverter" products that rectify the incoming power to DC and then invert it to AC at a high frequency. That allows the use of a smaller transformer to boost the voltage to the 2000 volts or so required by the magnetron. The inverter can also modulate the voltage using PWM to control the voltage and thus control the power.

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Most, if not all microwave ovens, use the method 1. At full power it is always on, and at lower levels, it is just turned off for a period of time. The starting and stopping can usually be heard when observing the device operation.

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