Why does your Microwave waste half its Power?

The circuit inside a microwave oven is a half-wave doubler, an incredibly inefficient design. How does it work? Why do we put them in microwaves?!

Huge thanks to Mehdi Sadaghdar (ElectroBOOM) for the consult:
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TIME CODES

00:00 Cold Open
00:45 Half-Wave Rectifiers
01:44 Giant Transformer
02:42 Giant Capacitor
04:12 ElectroBOOM Rant
05:35 Low-Voltage Analog
06:32 Diodes
07:36 The Capacitor's Purpose
09:17 Half-Wave Doublers
10:20 Summary
10:54 Outro
11:28 Featured Comment
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Corrections:

01:32 "Utilization" would have been a better word than "efficiency." Half-wave rectifiers have a 50% utilization, not a 50% efficiency. They only utilize 50% of the provided voltage.

01:57 Phone chargers from the 1990s and early 2000s were simple transformers. Modern phone chargers are a more complicated design called an "Switch Mode Power Supply" or SMPS.

[ScienceAsylum]

*Clarification:* By saying half the energy is wasted, I only meant that it was available but unused. In other words, the device doesn't take full advantage of what the wall is providing. It only uses 40-50% of the provided potential. I never meant to imply that the energy was dissipated as heat or dumped somewhere else. "Wasted" and "efficiency" felt like good words for that, but apparently I was wrong given the response in the comments. There's some ambiguity there that I did not anticipate. Oops! In hindsight, I can see how this could be misleading.

*Minor Correction:* Apparently, modern phone chargers are made with SMPS transformers rather than magnetic transformers. I learned about them in the early days of mobile phones when they were all magnetic transformers. I had no idea they changed the design. This doesn't really affect the video, but thanks for the correction anyway. It gives me another rabbit hole to go down.

[ElectroBOOM]

Hey awesome, the scope survived!! oh and good, you survived too! 😁 great video!

[alexrvolt]

Unless I misunderstood something, the half-wave rectifier doesn't actually waste half of the energy (in the sense : dissipating it to heat) ; it just doesn't let power go through 50% of the time, but during the "blocked" half-period, no energy is taken from the source.

[nemurerumaboroshi]

Judging from the amount of damage in the small scale version of the experiment, not trying to mess around with the real thing was extremely wise decision 😂

[NXaiUL]

The collab we didn't know we wanted, but the one we deserved.

[jeffpkamp]

Half wave rectifiers don't "waste" half the energy, they just don't use it, so it stays in the system. Makes a big difference as dissipating half the energy would be a nightmare depending on how much power you are drawing with the half wave rectifiers.

[geppettocollodi]

A half wave rectifier DOES NOT waste 1/2 the energy, even when configured as voltage doubler. If it did the 1000W oven will put 1000W into the food and 1000W into the kitchen cabinet starting a fire.

[bskull]

You are mixing duty cycle (conduction angle) with efficiency. With a half wave rectifier you still can get almost 100% efficiency, you just waste half of your conduction angle, so half of the time your rectifier is not pulling electricity from the wall.

[KeysightLabs]

Imagine being me, waking up to find @ElectroBOOM putting my probes in a microwave

[uwezimmermann]

The half-wave rectifier is not wasting power or energy.
It is just not utilizing the other half-wave. No current is drawn during the other half-wave thus no momentary power is taken from the wall outlet, thus also the integral over all negative half-waves and thus the energy consumed is zero.
The only losses are in the finite forward voltage drop over the diode when it is conducting, the tiny leakage current in reverse direction and the fact that the load does not behave like an ohmic load and thus generates harmonics in the electricity grid.

[arthurmee]

Love the way Mehdi called it a megatron . . 😀

[universeisundernoobligatio]

As a kid 9 year old started playing with electricity by blowing up capacitors for 9 transistor radios with my RC car power supply. Uncle got me the radio on my birthday, only lasted a month before I took it apart.
58 years later I'm a retired electronics engineer, life long electronic hobbyist still fascinated by electronics still blow up stuff.

[j]

Congratulations on 600k! Your uploads are BRILLIANT! Cheers from England.

[rayoflight]

A 900 Watt microwave oven don't waste other 900 Watt as heat.
The half wave rectifier simply doesn't present a load to the secondary winding of the HV transformer, when the diode is reverse biased. This load unbalance cause the magnetic field in the transformer core to be skewed toward one polarity. In theory it could saturate the iron core, but the core is designed to operate that way. The current peaks charging the paper-in-oil capacitor are present only at half wave, 60 times per second in the US. In contrast, a full wave rectifier would work on both half waves, showing a 120 Hz ripple. But the oven is built to be as cheap as possible, by saving either three diodes, or a tap in the secondary winding.
The transformer has another low voltage winding to light up the filament of the magnetron, which is also the cathode from where the electron are emitted and spun into the multiple LC circuit (in a vacuum). These tuned circuits are made of wavy copper. Of the multiple LC resonant cells inside a magnetron, the antenna output can be connected to any of them; the geometry operating the electrons beam rotation under the influence of a static magnet (at a fixed frequency, equal to the resonance of the water molecules) is a kind of black magic. Keep in mind that a magnetron is one of the very few application where a vacuum tube (the magnetron itself) hasn't been replaced with a semiconductor equivalent.
Thank you for the great video!

[AlleyKatt]

Enjoyed seeing a "trained professional" who knows "how to work with electricity without..." so easily fry a diode and bake a capacitor.
Another fun and educational lesson in your library. Loved it.

[MichaelB-wmcg]

A half-wave rectifier doesn't waste half the energy from the cycle, it just doesn't use it.

[artdonovandesign]

Great to see you again, Nick ☺
Mehdi's guest spot was hysterical AND informative- as always.
Three Cheers for Science Asylum and ElectroBOOM!

[kylethompson]

I've always found the cavity magnetron to be the most elegant piece of engineering ever made. So many concepts and scientific fields involved in its inception, but ultimately a hunk of shaped metal that is easy to mass produce. It really is genius, the poetry of science.

[SeanBZA]

The magnetron is in itself also a diode, and a load. The regular diode conducts on the positive half cycle, charging up the capacitor, then, on the negative half cycle, the magnetron diode starts to conduct, with the forward voltage being the 5kV, as power is being generated in the spiralling electron flow inside the magnetron, charging the capacitor to zero, then to -2.5kV at the peak, with the magnetron then seeing briefly the full 5kV. Thus the 2 magnets in the magnetron, there to make the electrons spiral between the hot cathode, and the grounded cold anode. The spiralling electrons create resonance inside the magnetron in the cavities, and this is at 2.4GHz, fed out via a tap in the one cavity via a feedline to the end cap, where it is introduced into the waveguide cavity at the top, and from there through a RF transparent window into the large oven cavity. the magnetron will start oscillating at some voltage between 3-4kV, absorbing the energy out of the capacitor and transformer, so while it only is being turned on for a half cycle, the energy is basically doubled, as the other half cycle is storing energy to be used then.

No Beryllium oxide in the commercial microwave magnetron, it is way too expensive to use in that cheap product, so instead it is either white, pink or purple fused aluminia, aluminium oxide crystal, otherwise called when the crystal is large and not a powder, ruby. Here just a high temperature ceramic insulator. But the magnetron is still dangerous, the 2 strong magnets can shatter if slammed together, and they will attract small metal dust to them.

Also never run a microwave with nothing in the cavity, the energy not absorbed in the load is reflected back into the magnetron, heating it up a lot more thna normal, and eventually the magnetron will become too hot, and the thermal cutout for the magnetron will turn off the microwave, or the transformer, till it is cool enough, and another turns off the microwave if the cavity itself gets too hot.

[AdamV]

0:01 Me who dissasembled 4 microwave ovens:




whats the problem?