Microwave oven PWM frequency

[rea5245]

Anyone want to jump into the middle of a domestic dispute? (*)

My wife, who uses the microwave oven a lot more than I do, says that when she puts something in the microwave oven at low power, she can tell that the microwaves are turning on and off with a cycle time of a few seconds. E.g. she can see water boil for a few seconds, then stop boiling for a few seconds, then start again.

I, while candidly admitting that I didn't know for sure, expressed surprise that it would work that way. I had assumed that the magnetron would be pulsed on and off at at least 1Hz, if not faster.

Yet she does seem to have compelling evidence. Why would the magnetron be cycled at such a low rate? Is it incapable of being turned on and off faster? Or did the designers not think it would matter?

Thanks,
   Bob

(*) Rest assured that no matter who is right, my marriage will not break apart.

[Fred27]

Your wife is right. You can even hear it turning on and off every few seconds - maybe even slower than that.

[Cliff Matthews]

FWIW, listen for fan rpm variations. I've had several since the 80's and don't recall any rate faster than about 0.5hz..

[tom66]

Your wife is correct.

The average magnetron takes about 1-1.5 seconds to warm up from cold because it's a vacuum tube and the heater needs to start boiling off electrons.  You can often hear the tone of a microwave change after the first second of operation.

So any PWM needs to be quite slow.  Also,  the inrush current on the transformer on each cycle would be likely to blow fuses if it happened so rapidly. 

Perhaps keeping the tube warm using a separate supply would be one solution, but magnetron heaters are quite power hungry.  One example I saw was 3.5Vac at 25 amps.  So the additional supply for this is probably not worth it for such a mild improvement.  It is just derived off the main transformer.

Inverter microwaves, marketed by Panasonic and possibly others, still take time to warm up but the inverter directly controls the power to the magnetron.  So the low power setting does not require PWM and the power delivery is constant.  The lowest settings (defrost and warm on my Panasonic oven) still use PWM because magnetrons are not good at producing small amounts of power.  The PWM has a longer period and higher frequency though (about 0.2-0.3Hz).

/rant:
What does annoy me is despite knowing that a magnetron takes a second to warm up - and despite being able to directly monitor magnetron current on the inverter units - no microwave seems to delay the timer if the magnetron is not generating microwaves.   If you want to nuke something for ten seconds, if the magnetron is completely cold it can take 3-4 seconds to start delivering power.  This makes timing inconsistent at the lower end of the scale. 

[Zero999]

She is right. I remember dismantling an old mechanically controlled microwave, when I was about 12 years old and looked at the power control. It consisted of a synchronous motor driving a gear system, which turned on and off a microswitch. Adjusting the power control altered the position of a cam, so the duty cycle was adjusted. The switching frequency was very low, much under 1Hz.

Now there are some inverter microwaves, which don't use a large iron cored transformer, but a switched mode power supply. I believe these either use a higher frequency PWM, or just alter the current through the magnetron, in a linear fashion.

[Cliff Matthews]

You admitted you're not sure and call this a dispute? Nah, my wife's a Cuban surgeon and if anything, she's always sure!

[coppice]

Traditional microwave ovens, with a big heavy transformer, use bursts of microwaves several seconds long. However, many of the inverter driven ovens (perhaps all) rapidly modulate the microwave power.

[rea5245]

You admitted you're not sure and call this a dispute?

I like to add drama and suspense to my dumb questions.

- Bob

[tooki]

Also, there are often two things at work:
1. the duty cycle of the microwave — usually it's 100% at maximum power. At lower power, most microwaves have a PWM frequency in the seconds. Like 10 seconds on, 10 seconds off for 50%. Inverter microwaves (like my Panasonic) seem to both vary the microwave intensity and the duty cycle.

2. The food hitting hot and cold spots in the cooking cavity. Most consumer microwaves use a turntable, and as the cup of water spins around, it hits hot and cold spots. Commercial microwaves and some high-end consumer ones instead use a "stirrer" under the floor of the cooking cavity, which is a slow-turning fan-blade kind of device that accomplishes much the same thing, but with the microwaves moving instead of the food.

[Halcyon]

As tooki mentioned, it depends on the microwave. I have a Sharp "inverter" model at home which makes no difference in sound whether at low power or maximum power. There is no audible on/off at all and liquids, once beginning to simmer/boil, will do so continually (albeit at lower intensity at lower microwave power). That's because the magnetron is always on but at variable power levels.

However more commonly (like all the microwaves we have at work) they regulate the "power" level by adjusting the duty cycle. When the magnetron is on, it's on at full power. So lets say I set the microwave to half power, it will switch on the magnetron for 10 seconds, then off for 10 seconds. You can hear a definite "clunk" as it kicks back in every time. You also see the effect your wife is talking about where the water will go from a boil, to hardly anything at all. On the "low" setting, you might have the magnetron on at full power for 2 seconds then off for 10, for example.

Why? It's cheaper to build a microwave to simply turn the magnetron on and off at regular intervals than it is to include an inverter and more advanced software.

[John Heath]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

[tooki]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

Do you have any sources for this supposed law and its supposed effects?

Microwave ovens predate WiFi by around half a century. I doubt they're bound by laws regarding WiFi.

I'm not even sure what you mean about a microwave operating from unfiltered DC. I mean, the magnetron isn't operating off DC, nor off mains frequency.



As for why microwave ovens and WiFi operate in the same frequency range: it's on purpose, because of the same root cause, though for different intents. It's the frequency range that is best absorbed by water. In a microwave oven, this is desirable to achieve optimum efficiency in heating the food. In WiFi, which operates in the unlicensed ISM band, the ISM band was placed in that range for the same reason: it's absorbed by water. This naturally limits the range of the signals by absorption by atmospheric water vapor, allowing the same frequencies to be reused in relatively close physical proximity to each other.

[Halcyon]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

I think it might be you who has dropped the ball this time. This is the first time I've heard of such a thing as well. This is certainly not the case in Australia. Yes, microwave ovens operate within the same frequency range as 2.4 GHz Wi-Fi however in my experience it's rarely a problem if your microwave is operating / built correctly. Microwave ovens are designed to keep as much of the RF inside the cavity as possible. If it's leaking significant RF, then you have bigger issues than just your Wi-Fi dropping out. Microwave leakage testing can be a bit of a wank (like PAT testing) and there are people making money off it, but significant levels can cause problems. There are a number of standards and studies out there which dictate "safe" limits, but leakage of 5 milliwatts per square centimetre or less is considered acceptable.

I have absolutely no dramas with my 2.4 GHz Wi-Fi when my microwave is operating at full power (1200 watts). When it first starts up and ramps up to full power, it can draw as much as 1900 watts @ 240v. Even old devices manage full signal strength with no perceivable difference in throughput.

My microwave also doesn't switch off the magnetron at those intervals, in fact, it's ALWAYS on, regardless of power setting as it has an inverter.

[Red Squirrel]

I can kind of tell when the magnetron turns on/off.  It's almost like I can "feel" it, not like feel myself getting zapped because that would mean my microwave is leaking, but there's an odd feeling of magnetic force in the area. Hard to explain.   Same thing when there is a power outage.  When the power comes back on I can feel and hear the magnetic force of the initial "hit".  If I'm in bed it will actually wake me up at times, and I look at the clock and sure enough it's flashing 12:00.    Not sure if that's normal though, maybe I'm crazy? lol.

But yeah microwaves just turn the magnetron on/off in couple second intervals based on the setting.  For defrost you actually want that, because any water will heat up, and that water will help melt ice to create more water.  If it was on continuously the water would evaporate.  Ice does not really get affected as well by microwaves so it would take a long time to melt.  I'm not sure what things like the pizza setting or popcorn setting do though, is there actually a science behind those? 

[Halcyon]

I'm not sure what things like the pizza setting or popcorn setting do though, is there actually a science behind those?

They just vary the duty cycle (or in the case of inverter models, the output power) of the oven throughout various stages of cooking based on a program. Some programs might start off slow then ramp up towards the end of cooking, some might stop part way through and prompt you to stir/turn whatever you're cooking. It's really nothing fancy or complicated, they are just based off tried and tested recipes by the manufacturer. The microwave doesn't "know" when the food is actually done, it just bases it off pre-programmed weights, cooking sequences and power levels.

For example, if I tell mine to defrost 500 grams of chicken, it knows based on that weight, it should take 12 minutes and 1 second (at a certain power level) to defrost that mass of particular meat.

My microwave (Sharp Carousel) also comes with its own cookbook and the microwave itself has built-in programs based on those recipes. Most brand name microwaves will have something similar.

[tooki]

They just vary the duty cycle (or in the case of inverter models, the output power) of the oven throughout various stages of cooking based on a program. Some programs might start off slow then ramp up towards the end of cooking, some might stop part way through and prompt you to stir/turn whatever you're cooking. It's really nothing fancy or complicated, they are just based off tried and tested recipes by the manufacturer. The microwave doesn't "know" when the food is actually done, it just bases it off pre-programmed weights, cooking sequences and power levels.

That's not necessarily true. Various microwave oven models over the years have used various sensors to actually try and gauge the cooking progress. For example, a different variant of my Panasonic microwave/convection oven/quartz grill/steamer uses infrared sensors for automated defrosting. And I know there have been microwaves with microphones in them to literally listen to the popping frequency of popcorn to cook it automatically. (Since the way to know it's done is when popping slows to about 2-second intervals.) And of course some models have thermometer probes.

[Halcyon]

They just vary the duty cycle (or in the case of inverter models, the output power) of the oven throughout various stages of cooking based on a program. Some programs might start off slow then ramp up towards the end of cooking, some might stop part way through and prompt you to stir/turn whatever you're cooking. It's really nothing fancy or complicated, they are just based off tried and tested recipes by the manufacturer. The microwave doesn't "know" when the food is actually done, it just bases it off pre-programmed weights, cooking sequences and power levels.

That's not necessarily true. Various microwave oven models over the years have used various sensors to actually try and gauge the cooking progress. For example, a different variant of my Panasonic microwave/convection oven/quartz grill/steamer uses infrared sensors for automated defrosting. And I know there have been microwaves with microphones in them to literally listen to the popping frequency of popcorn to cook it automatically. (Since the way to know it's done is when popping slows to about 2-second intervals.) And of course some models have thermometer probes.

Indeed, I was simplifying it. Mine has "sensor" printed on the front (what the sensor is exactly, I don't know, temperature, humidity? It's not obvious by looking inside the cavity). But despite those sensors, it's largely based on pre-programmed recipes and some fuzzy logic. The oven doesn't truly know when food is cooked or not, it just guesses. I find the auto-defrost function on mine actually a bit too much, it tends to start cooking parts of the chicken as opposed to just defrosting it and the pizzas come out ridiculously hot and over-done.

Having said all that, I suspect they err on the side of "over-cooked" as opposed to "not cooked enough" to meet certain food safety guidelines.

[NiHaoMike]

If regular microwave ovens are "off" a bit more than half the time it's running, why hasn't Wifi in 2.4GHz detect for that and try to put packets in those times? Or does the IEEE expect that most microwave ovens will go inverter and thus make such a mechanism less useful?

[Halcyon]

If regular microwave ovens are "off" a bit more than half the time it's running, why hasn't Wifi in 2.4GHz detect for that and try to put packets in those times? Or does the IEEE expect that most microwave ovens will go inverter and thus make such a mechanism less useful?

They do, in a sense. Wi-Fi won't broadcast if it detects that the channel is "busy". This is why if you have many radios nearby on the same channel, your performance sucks, because it essentially waits for a break in transmission to avoid collisions with other Wi-Fi devices.

Why don't they don't do this with other sources of interference? Probably because it's a little pointless. In the case of microwave ovens, they SHOULDN'T leak enough to cause significant problems with Wi-Fi (see my comments above).

The way TCP/IP is designed, if the client doesn't receive the expected response within a certain time, it just times out, disconnects the session etc... Same for your Wi-Fi access point, it won't just buffer every single TCP/IP packet that comes its way, if it can't deliver it within a timely manner, it won't deliver it at all. If your Wi-Fi router or access point had to wait for all sources of interference to cease in built-up areas, no one would be transmitting anything. Wi-Fi is a half-duplex system, while a radio is listening (or receiving data) it's not transmitting.

Which brings me to another point that I feel I should mention. Most consumer-grade Wi-Fi routers will have an "auto" channel function. Usually, this only operates at the time the router boots to listen for the best free channel. It does not continually monitor the spectrum and channel hop. For it to do this, it would need a dedicated radio that does the monitoring or disconnect all clients from the network, monitor the spectrum, then allow all clients to re-connect (the latter is obviously not ideal).

Most enterprise grade wireless gear will either have dedicated access points connected to the controller through various parts of a building/campus which act as "monitors", that's their full-time job, to provide the controller feedback on channel interference and other radios in that area so it can change the channel on nearby access points. Alternatively some dual-channel access points will have a third and sometimes fourth built-in radio to do this, like the Ubiquiti UAP-AC-SHD.

The auto channel feature on consumer gear is next to useless (like MAC address "security"). Let's just say you had a local power failure and all your neighbours lost power too. Assume your router is the first to boot up and decides that Channel 11 (on 2.4GHz) is the most "clear" so it picks that and starts accepting clients to connect. Then all of your neighbours' routers boot up, some have auto channel (which would at this point avoid Channel 11) but some are configured to only use Channel 11 (which is very common). All of a sudden your "clearest" channel is congested and performs like crap. Your router won't detect this and change channels, it will just continue doing its best in an RF-noisy environment.

[Red Squirrel]

Technically your wifi should not be affected by the microwave.  I don't know how good of a test this is but I did a ping -f to my phone's IP and microwaved a glass of water on high with phone sitting next to the microwave and I had no packet loss.

[John Heath]

I can see there are more than a few who are not buying the microwave oven problem in the 802 IEEE standards for WIFI , XBEE and  blue tooth. I can understand why as you would think they would be smart enough to separate 1000 watt microwave oven frequency from .1 watt communication frequencies. Sadly they did not separate them. Even Sadder if it is your phone that rings when computer communications are down.

[tooki]

Technically your wifi should not be affected by the microwave.  I don't know how good of a test this is but I did a ping -f to my phone's IP and microwaved a glass of water on high with phone sitting next to the microwave and I had no packet loss.

I suspect it depends on several things:
1) the selected WiFi channel
2) the specific microwave oven model
3) the relative location of the WiFi router and the microwave

The last, in particular, I’ve found to make a huge difference: if the microwave oven is between the router and the client, performance will suffer (sometimes to the point of not functioning at all). I just experienced this at a friend’s house. If they’re equidistant but in different directions, it usually works fine.

I can see there are more than a few who are not buying the microwave oven problem in the 802 IEEE standards for WIFI , XBEE and  blue tooth.

Yeah, I dunno. I’ve experienced it enough times firsthand to know it’s a real problem.

That said, I’m still waiting for documentation about these laws you’re claiming.

I can understand why as you would think they would be smart enough to separate 1000 watt microwave oven frequency from .1 watt communication frequencies. Sadly they did not separate them.

I already explained to you why they’re on the same band. It’s not random coincidence!

Even Sadder if it is your phone that rings when computer communications are down.

Huh what?

[richard.cs]

There is a microwave oven avoidance mode in one of the WiFi specs, I think it just keeps the packet lengths down to 8 ms or something relatively simple like that. Note that this is taking advantage of microwave ovens running with half-wave rectified power, but it's not the reason they work that way. Microwave ovens do that because it's cheap. The magnetron produces a fairly pure single frequency tone, but with a weak voltage dependence to it sweeps in frequency during the (slightly less than) half a cycle that it's active and this makes it more interfering than might be expected.

The one dual-magnetron microwave I've had apart runs them on opposing half-cycles. I suspect that this is both to clean up the input current waveform a bit but also to prevent them from injection locking - if they end up locking in frequency you get a single standing wave pattern and the cooking is no more even than a single magnetron microwave (which is one of the alleged advantages). This means that at full power there is minimal dead time.

Interference with WifFi is certainly a real effect, I have a pretty little graph of WiFi data transfer rate against time where you can see the effect of my microwave when on 50% power, the low-frequency PWM is visible in the data rate. This should be entirely expected given that the shielding on a microwave oven often only gives about 30 dB attenuation so gives leakage on the order of a Watt.

[John Heath]

There is also the crowding of WIFI channels. Each year it gets worse. As much as 50 WIFIs all frequency hopping trying to sneak a byte or 2 through a limited shared number of channels. There are free apps for cell phones to monitors how crowded it is.

There is also the window problem if you need to go building to building. Not sure why but there are some types of glass used in windows that acts like a mirror at 2.4 GHz. You can be better off trying to penetrate a brick wall than a glass window. 5 GHz does not help.

[Psi]

Microwaves that control the power level by ~0.25 HZ pwm are pretty annoying.
I would recommend replacing it with an inverter microwave.

When you need to warm up something slowly, like on 'low power' the pulses of full power usually burn/cook it instead.
A good example is anything with egg in it, like warming up hollandaise sauce, it's almost impossible to not ruin it.
But with an inverter oven you don't get these problems.

[SiliconWizard]

Also, there are often two things at work:
1. the duty cycle of the microwave — usually it's 100% at maximum power. At lower power, most microwaves have a PWM frequency in the seconds. Like 10 seconds on, 10 seconds off for 50%. Inverter microwaves (like my Panasonic) seem to both vary the microwave intensity and the duty cycle.

Yeah. Also depends on if it has some kind of "grill" function (usually a quartz lamp or something like that). I have an LG oven that has that. If you use a cooking mode with the Grill function, it will alternate between microwave and grill, making the cycles longer, and you can definitely hear when the magnetron restarts.

[Red Squirrel]

Why is it that they can't just vary the power level though?  It would basically be like a radio transmitter where you can dial down the watts.  I imagine most modern microwaves just use a high voltage inverter to power the magnetron now days, so it should be easy to simply dial down the voltage.  Or is it more complicated than that?

[ogden]

Microwave is good only at one job: flash heating. It is awfully bad for cooking or defrost. It means all you need - max power. Well... unless you are victim of toxic marketing and truly believe that you can *actually* cook something with microwave. In such case you are lost anyway and regulation of microwave power is least of your worries

[bob91343]

I just bought a microwave oven and on reduced power it's a few seconds on and a few seconds off.  Same with the one I just took apart.  That one yielded a nice power transformer to build a hipot tester, which I have done.

It's enough of a cost increment to do reduced power cleverly that I understand why they don't do that.  We are dealing with a kilowatt or so, not trivial enough to use a little FET to PWM it.  Further, there is the filament in the magnetron that needs steady heat, as shown by the fact that the oven doesn't do, say, 12 seconds on and 12 seconds off for 50%, but lengthens the on time to allow for heating filament.

[soldar]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

Rather than get into the technical aspects I will ask for a cite that supports "it is against the law for a microwave oven to run off a filter DC power supply." I am quite sure I know the technical explanation of why it is done this way but let's see the law first. Where can I find it?

[Zero999]

Microwave is good only at one job: flash heating. It is awfully bad for cooking or defrost. It means all you need - max power. Well... unless you are victim of toxic marketing and truly believe that you can *actually* cook something with microwave. In such case you are lost anyway and regulation of microwave power is least of your worries

Well I wouldn't agree with that. Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

The one thing which microwaving doesn't do is brown food. I've just had a microwaved jacket potato and I crisped it up by placing it on top of the toaster for a couple of minutes.

[soldar]

My German-made induction stovetop has ten settings and at the five upper ones it does seem to regulate the power but at the lower ones it will cycle on and off, like a microwave oven, and it is annoying as hell and makes frying an egg impossible.

[thm_w]

My German-made induction stovetop has ten settings and at the five upper ones it does seem to regulate the power but at the lower ones it will cycle on and off, like a microwave oven, and it is annoying as hell and makes frying an egg impossible.

What kind of frequency?
I have a bosch and it does pulse at the lower power settings but its about 0.5-1Hz or so, perfectly fine for cooking. All pots/pans that I have that work with induction are reasonably thick so buffer the heat, and heat distribution is incredibly even. Maybe something else is wrong here.

A typical resistive element heated stove top is much worse, maybe 0.1Hz or lower.

[soldar]

What kind of frequency?

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

Regarding microwave ovens they are good for reheating stuff and that's about it. I have never used one for real cooking.

A typical resistive element heated stove top is much worse, maybe 0.1Hz or lower.

The resistive plates I have do not switch but even if they did it would not matter much because they have great thermal mass and you can turn them off and the pot will continue to boil for 5 or 10 minutes. I have one that combines three resistor elements in different configurations so I can set from 150 W to 1700 W in six steps: https://www.eevblog.com/forum/beginners/calculating-value-of-resistors-in-parallel/msg2086750/#msg2086750

With resistor elements you can also use a triac power control.

[langwadt]

What kind of frequency?

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

get pans with more thermal mass?

how much does the temperature change in those few seconds?

[thm_w]

I did some measurements. Max power consumed is 1600 W. Then it goes down in 200 W steps down to 1000W. Then from there down it cycles on/off at 1200 W and in the one setting I tried at about T=14 seconds which is unusable for low heat. You can't blast 1200 w for 4 seconds and then switch off for 8 seconds to obtain an average of 400 W. It makes it unusable.  Furthermore, even when switched off  the thing is consuming 0.7 Amps of reactive power so I have to disconnect it entirely. It's crap. German crap but crap nonetheless.

ok that does seem like a poor design. Unless they are switching internally with a relay there is no reason to have it that slow and generally a relay wouldn't be used.
Do you pay for reactive power?

The resistive plates I have do not switch but even if they did it would not matter much because they have great thermal mass and you can turn them off and the pot will continue to boil for 5 or 10 minutes. I have one that combines three resistor elements in different configurations so I can set from 150 W to 1700 W in six steps: https://www.eevblog.com/forum/beginners/calculating-value-of-resistors-in-parallel/msg2086750/#msg2086750

With resistor elements you can also use a triac power control.

Well thats a huge problem to me, they heat up, temperature spikes too high, the element shuts off then it slowly goes down again. If I turn the temperature from 10 to 1, I want the temperature to go down as fast as reasonably possible to avoid burning something, and never overshoot.

Triac would be nice, I don't understand why its not used or why it took so long (if modern cooktops are actually using it now).

[andy2000]

...

Perhaps keeping the tube warm using a separate supply would be one solution, but magnetron heaters are quite power hungry.  One example I saw was 3.5Vac at 25 amps.  So the additional supply for this is probably not worth it for such a mild improvement.  It is just derived off the main transformer....

There were a few microwaves that had a separate filament transformer so they could keep the filament hot while pulsing the HV rapidly.  My Amana "Radarange" from the 1970s cycles at about 1-2 Hz, which is fast enough to be useful.  The filament transformer is quite large, so I'm sure it adds significant cost. 

[ogden]

Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

Try steaming. You will never use microwave for vegetables again. All you need - steaming insert for pot you already have. It is hard to manage heat your food receive in microwave because it is proportional to mass/water_content/power ratio.  I want to eat, not math class. Pot, electric oven and multipot are much better at actual cooking. Not to mention pan which as you already mentioned, is uncontested by microwave. Disclaimer: I do love microwave and use it often, but not for cooking.

ok that does seem like a poor design.

It looks like poor design or even failure indeed. My German (LOL) induction stovetop have around 2sec period and it is completely fine. Thou you want heavy pan anyway. For induction stove tops heavy pan is *very* important - because such most likely will have heavy iron inlay which will result in much better heating performance. Well... honestly heavy pan will perform better on any kind of stovetop. Thin pan is good only for high temperature searing but for such you need gas burner.

[soldar]

Do you pay for reactive power?

Good question. As a home consumer I do not pay for reactive power as such ... but (1) there will be some losses in the wiring, maybe a watt or a few watts, that I will pay for and (2) we pay for "capacity" which means the maximum power you can draw at any one time. But this is not measured in terms of real power, it is just a circuit breaker that trips by current. So suppose I have 10 A allowance, if I have some parasitic reactive power connected then I am decreasing the max power I can pull at any one time. And they charge dearly for this power allowance capacity part because it means you pay even if you don't consume at all. You have a second home where you only go a few weekends a year? You will still be paying through the nose.  The crooks in government have us well screwed.

I do not think there is any excuse for a device that is turned off to be using current like that. I have not determined if it is capacitive or reactive. I just see the current in a meter.

So I keep that induction plate disconnected most of the time and hardly use it. I also have a theory that it is not good to apply too much power to a pot because it will deform. The induction plate has a max setting of 1600 W but I would not like to use it above say, half that but I would want it continuous, not pulsing.

Regarding temperature response time:

Well thats a huge problem to me, they heat up, temperature spikes too high, the element shuts off then it slowly goes down again. If I turn the temperature from 10 to 1, I want the temperature to go down as fast as reasonably possible to avoid burning something, and never overshoot.

Gas I think has the fastest response time. OTOH, I don't have a problem with electric plates that have a large mass and longer response time. I just work around it. If I need to lower heat I just take the pot or pan off the plate. Not a problem at all.  In fact, I much prefer the longer response time of a hot plate than the bursts of the inductive plate because I can work around the first but there's nothing I can do about the bursts.

Well, now that I think about it, I do have a steel plate, about 30 cm square and 1 cm thick, that I can put on the induction stove and it just heats up and maintains temperature. But I have hardly used it because I just use the resistive plates and I am not sure if having a very hot plate right on the induction heater might be bad for it. I just avoid using it altogether.

Triac would be nice, I don't understand why its not used or why it took so long (if modern cooktops are actually using it now).

Well, yes and no. Having a single high power resistive element switching on and off seems like a bad design and you can easily set it to max and then control with a triac. But switching high loads with a triac for power control has bad power factor and introduces lots of noise in the line unless you filter really well. The best solution is what I have always seen here which is a plate with three resistive elements which can be switched in different configurations. I think pretty much all cooktops I have seen around here are like that.

[soldar]

get pans with more thermal mass?

Why? Better get a heat source that doesn't cycle.

how much does the temperature change in those few seconds?

So much that it makes frying an egg impossible. Most cooking that requires a certain dosage of heat needs it applied constantly and you can't cycle like that.

You can add thermal mass but I'd rather just cook on devices that don't have the problem in the first place.

That is one reason I only use the microwave oven for reheating things.

[Zero999]

Microwaving is the most healthy way to cook certain foods, such as vegetables, because it doesn't excessively heat them or add fat like frying does or require them to be submerged in water which washes out some of the nutrients.

Try steaming. You will never use microwave for vegetables again. All you need - steaming insert for pot you already have. It is hard to manage heat your food receive in microwave because it is proportional to mass/water_content/power ratio.  I want to eat, not math class. Pot, electric oven and multipot are much better at actual cooking. Not to mention pan which as you already mentioned, is uncontested by microwave. Disclaimer: I do love microwave and use it often, but not for cooking.

Steaming is still not as healthy as microwaving because it's above boiling point, but it does cook more evenly.

What do you use your microwave for if you don't cook with it?

[coppice]

Do you pay for reactive power?

No domestic consumers pay for reactive power, although introducing charges has been discussed in some countries. In some places some large industrial consumers either pay for reactive power, or pay a punishment factor if their power factor is too bad too much of the time.

In most countries there are strict controls over how electrical energy may be charged. For example, most European countries follow the Welmec practices for most forms of legal metrology, including electrical energy.

[ogden]

Steaming is still not as healthy as microwaving because it's above boiling point, but it does cook more evenly.

Steaming in regular pot is precisely at the boiling pot point, at max. I did not advise to use pressure pot to steam vegetables BTW do you know about microwave hotspots - reason why plate or special reflector shall rotate?

What do you use your microwave for if you don't cook with it?

Heating food which for some reason is below temperature of tasty consumption.

[thm_w]

Yeah I'll agree with ogden here, steaming is going to be ~100C max no matter what.
Put a piece of cheese in the microwave for 10 minutes and let me know how black it gets.

Not that the microwave is unhealthy in any way. But generally you need to get up to 150+C? to start running into issues. ie frying, deep frying, BBQ, using trash oil like soybean, etc.

Microwaving did not raise dAGE content to the same extent as other dry heat cooking methods for the relatively short cooking times (6 minutes or less) that were tested.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3704564/
https://suppversity.blogspot.com/2014/12/the-quest-for-optimal-cooking-oil-heat.html

[Red Squirrel]

Microwaves arn't really good at cooking nor are they really meant for it.  They are for reheating.  For real cooking you want a source of radiating heat like from a fire or a hot stove element, or even just hot air/steam.   The way microwaves cook is just different and you'll never get the same kind of result from it.   It also depends on the type of food though, some foods it won't matter while others it will. 

Putting eggs in the microwave for example is a really bad idea (but it's fun as long as it's not your microwave.  )

[Zero999]

Steam is above 100°C.

There are plenty of foods which microwave well. Of course putting a whole egg in the microwave will end badly, but scrambled egg is fine and the result will be indistinguishable from the stove. I also microwave porridge, which is more efficient than the stove for a single portion.

There is one food I only will cook in the microwave: onions. I stab the onion with a knife a few times, put it on a plate and zap it for a couple of minutes at full power. I leave it to cool for a bit, squeeze the onion and the cooked bit in the middle pops out. If I want to brown it, I'll put it in a frying pan or under the grill for a couple of minutes. I do this because I react badly to chopping raw onions and blasting them in the microwave neutralises them, making them safe to cut up.

[ogden]

Steam is above 100°C.

Compare water boiling point (99.97 °C at 101.325 kPa) with steam equilibrium data and you will see that you are mistaken: https://en.wikipedia.org/wiki/Water_(data_page)#Water/steam_equilibrium_properties

[Zero999]

Steam is above 100°C.

Compare water boiling point (99.97 °C at 101.325 kPa) with steam equilibrium data and you will see that you are mistaken: https://en.wikipedia.org/wiki/Water_(data_page)#Water/steam_equilibrium_properties

Yes, I was mistaken. I knew that chart and that water never exceeds 100°C. What I missed and I've now realised is: as long as there's boiling water, underneath the steam, then the temperature of the steam will never exceed 100°C, because the boiling water protects it from further temperature rise. It isn't the same as steam inside a conventional oven, which can easily exceed the boiling point.

I remember having this sort of discussion with a colleague who would always hold the switch down on the electric kettle after it had already boiled, in the hope to heat the water more. I explained to him that all it does is waste energy and produce more steam. At the time he didn't seem to accept it, but I did notice he didn't bother the next time he boiled the kettle, it might have sunk in.

Anyway, I can't say that I'll bother with steaming my vegetables, as I don't think it's worth the bother for one person, but if I'm cooking for more than two, I certainly will.

[ogden]

I remember having this sort of discussion with a colleague who would always hold the switch down on the electric kettle after it had already boiled, in the hope to heat the water more.

LOL BTW you can tell him that holding switch down may work when water is completely evaporated and steam is all that's left.

Anyway, I can't say that I'll bother with steaming my vegetables, as I don't think it's worth the bother for one person, but if I'm cooking for more than two, I certainly will.

Remember please - microwaving is not cooking [kidding] What you eat? Even - chicken needs more time than vegetable steaming. Small pot + glass of water = vegetables for two in < 30 minutes. Just saying. It's like thread about 60fps versus 120fps monitors where people who never experienced 120fps, have strong opinion that it sux

[Zero999]

I often eat things which are quicker to cook than that, such as fried eggs or some fish I already have cooked and just needs reheating.

I have steamed vegetables before and although I agree it's better, than microwaving, I don't think the difference is great enough to warrant the extra time and cleaning of kitchen utensils, for a single portion. I like the convenience of being able to put the vegetables in a bowl or mug, microwave them at full power for a minute, stop, stir and microwave them for another minute.

[ogden]

I often eat things which are quicker to cook than that, such as fried eggs or some fish I already have cooked and just needs reheating.

Fish which is already cooked, can be cooked quicker than that. Good one 

I have steamed vegetables before and although I agree it's better, than microwaving, I don't think the difference is great enough to warrant the extra time and cleaning of kitchen utensils, for a single portion.

Agreed Cooking/eating is not "discrete" if I can say so. As long as your microwaved, steamed or caramelized in pan vegetables are what you like - enjoy!

[Ilima]

Actually, pulse width modulation.... is 100% on, then completely, 0% off, then on, then off, and repeat.

The trick is, they just turn on and off much faster than relays can manage by using more advanced switches (usually semiconductors).

So, no. PWM controlled microwaves do not output 495 watts for 100% of the time. Rather, over the course of, say, one second, they switch on and off rapidly enough that the sum energy output for that second is 495 Joules.

[David Hess]

Microwaves that control the power level by ~0.25 HZ pwm are pretty annoying.
I would recommend replacing it with an inverter microwave.

But they work pretty well and without the added complexity of an inverter, they are more reliable.

I can see there are more than a few who are not buying the microwave oven problem in the 802 IEEE standards for WIFI , XBEE and  blue tooth. I can understand why as you would think they would be smart enough to separate 1000 watt microwave oven frequency from .1 watt communication frequencies. Sadly they did not separate them. Even Sadder if it is your phone that rings when computer communications are down.

I have never had a microwave which did *not* completely trash 2.4 GHz WiFi.  But the frequency selection for WiFi was no accident.  The ISM bands were available *because* industrial and commercial devices like microwave ovens render them useless for reliable communications.  If microwave ovens were not polluting the 2.4 GHz band, then the 2.4 GHz band would not have been available for WiFi and general use.

[tooki]

Microwaves that control the power level by ~0.25 HZ pwm are pretty annoying.
I would recommend replacing it with an inverter microwave.

But they work pretty well and without the added complexity of an inverter, they are more reliable.

The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

[coppice]

The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

When I bought a Panasonic microwave oven 20 years ago they were not inverter types and they did last well. The early inverter ones had a poor reputation for reliability and longevity. I don't know if that has improved.

[David Hess]

When I bought a Panasonic microwave oven 20 years ago they were not inverter types and they did last well. The early inverter ones had a poor reputation for reliability and longevity. I don't know if that has improved.

Exactly, that is what I found when I looked into it last year.  And replacement inverters are either expensive or not available.

[NiHaoMike]

I wonder if it could be possible to hack an inverter microwave oven to be more Wifi friendly by preferring to operate when there is no Wifi activity and/or by broadcasting RTS/CTS packets (using an ESP8266 or similar) to tell Wifi networks to avoid sending data when the microwave wants to operate.

[tom66]

The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

When I bought a Panasonic microwave oven 20 years ago they were not inverter types and they did last well. The early inverter ones had a poor reputation for reliability and longevity. I don't know if that has improved.

Our is 8 years old and still going strong.   Really like the "turbo bake" feature where it can use the oven and grill simultaneously for 3000W cooking - makes for some really nice roast potatoes with a crispy top but cooked through. 

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

[Yansi]

A microwave oven is switching on and off every 8 m seconds as well as every second or so. The reason for this is it is against the law for a microwave oven to run off a filter DC power supply. It must be a unfiltered supply raw off the full bridge rectifier thus on and off every 8 m second. The reason for this law is to sweep a gigantically stupid law passed before under the rug. The gigantically stupid law was to have a 1000 watt microwave  and a tiny 100 m watt WIFI in the same frequency range. The solution , equally as dumb , was to out law filtered power supplies in microwave ovens so that the tiny 100 m watt WIFI can sneak a few bytes out in a 2 m second window between power cycles of a monster 1000 watt microwave. Is it just me or did someone drop the ball here? Thank god for 5 GHz WIFI.

Seems a pretty much nonsense my friend. 

[Yansi]

I wonder if it could be possible to hack an inverter microwave oven to be more Wifi friendly by preferring to operate when there is no Wifi activity and/or by broadcasting RTS/CTS packets (using an ESP8266 or similar) to tell Wifi networks to avoid sending data when the microwave wants to operate.

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

[NiHaoMike]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

I don't see why you couldn't "emulate a busy network" using a separate transmitter and take advantage of the collision avoidance mechanism that Wifi uses. (Make the oven appear like another network that nearby networks should avoid colliding with.) Actually, come to think of it, you don't even need an inverter microwave to do it to some extent. The inverter just gives you more flexibility in being able to turn the output on/off at will. With a conventional microwave, you could add a separate filament transformer and then gate the HV supply on and off with a SSR.

[tom66]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

I've not, actually.  Is there a specific one I should use?

[Yansi]

The only failure is the plastic that's glued to the front of the door has peeled off. Funnily enough the same failure has happened to the one in the work kitchen (same generation, different model.)  I have yet to find an adhesive that survives the high temperature of the oven's operation so I haven't been able to glue it back on permanently yet.

Have you tried JB Weld?

No, because inverter microwave just means that the bulky transformer supply was replaced by a high frequency inverter. The magnetron is still there, spewing all sorts of shit somewhere in the 2450 MHz or so. There is no control over the frequency possible. Neigher does the magnetron run on DC voltage, it still uses the pulsed rectified voltage from mains. There would be no benefit in running the inverter from DC, in fact that would be an disadvantage.

I don't see why you couldn't "emulate a busy network" using a separate transmitter and take advantage of the collision avoidance mechanism that Wifi uses. (Make the oven appear like another network that nearby networks should avoid colliding with.) Actually, come to think of it, you don't even need an inverter microwave to do it to some extent. The inverter just gives you more flexibility in being able to turn the output on/off at will. With a conventional microwave, you could add a separate filament transformer and then gate the HV supply on and off with a SSR.

It already appears as a busy network channel already. It doesn't matter how did a packet got corrupted.

Why would you add a SSR? What for? To further make more EMI/RFI and increase the apparent power input?

Microwave oven designs are already very optimal.  Adding unnecessary shit doesn't make it better.

[chris_leyson]

The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

Modern microwave ovens fitted with inverters such as Panasonic, Bosch and Siemens will generate continuous microwave energy from 1kW down to 360W or so, below that they switch over to PWM. By clever and subtle design an inverter will control a magnetron over a 1:3 power range. If they come with a grill or convection fan they can be used with or without additional microwave heating and actually produce some very well cooked meals. I know because I help design them.

The older ovens with transformer driven magnetrons are really crude by comparison and there isn't a great deal of food stuff that you can cook in the traditional sense. My old school cheap popty ping, or is it popty beep these days, is just used for warming.

[tom66]

One thing I *don't* like about our Panasonic combi microwave-inverter is the fan.  It runs for about a minute to "cool down" even if you only used the oven for 10 seconds to briefly zap something, in which time almost no heat is produced.  There seems to be no temperature monitoring, or estimated power dissipation, in the control logic. The fan is noisy and while it's not a massive problem, it strikes as something that could have easily been addressed in the software.

[NiHaoMike]

I've not, actually.  Is there a specific one I should use?

Just plain JB Weld is rated to quite high temperatures.

It already appears as a busy network channel already. It doesn't matter how did a packet got corrupted.

Wifi has a mechanism for APs to tell each other that they're going to transmit in order to avoid collisions. If the microwave could act like an AP and tell nearby networks to avoid transmitting for some time, they'll wait until the microwave has finished outputting its pulse before trying to send data, thereby avoiding retries.

[tom66]

I recall an industrial (i.e. chef's) microwave that used 915MHz RF.  I don't know if that's common in industrial ovens or not, or if it's any better at heating food, but it was interesting.  Probably only legal in the USA as 915MHz is protected in Europe.

[David Hess]

900 MHz works fine for heating food and probably even better than 2.4 GHz.  Early microwave ovens in the US were 900 MHz.

The problem with 900 MHz is that there is no room for a choke ring in the door so metal finger stock is used instead to seal the door from RF leakage.  But the metal finger stock proved to be too easy to damage, especially in a kitchen, leading to problems with leakage.

[tom66]

I've never heard of a choke ring.  I'm assuming that's a tuned antenna of some kind inside the door that helps reduce the emissions by shorting leakage?

[David Hess]

I've never heard of a choke ring.  I'm assuming that's a tuned antenna of some kind inside the door that helps reduce the emissions by shorting leakage?

It is something like that.  There is an open quarter-wave or shorted half-wave channel along the door seam which causes an out of phase reflection of the 2.4 GHz energy canceling out leakage.  Since it only works at 2.4 GHz and harmonics, this is why microwave ovens are not cheap substitutes for Faraday cages.

[tooki]

The primary manufacturer of inverter microwaves is Panasonic, whose microwave ovens have a great reputation, and AFAIK no issues whatsoever with longevity. (They’re all I’ve used for 20 years, and I’ve never had one die. I’ve only gotten new ones because of moving to a country with different mains voltage, or to upgrade to one with new features.)

When I bought a Panasonic microwave oven 20 years ago they were not inverter types and they did last well. The early inverter ones had a poor reputation for reliability and longevity. I don't know if that has improved.

Weird. The Panasonic I bought in 2000 definitely was an inverter model, and I had it until I moved overseas in late 2008, at which point I gave it away.

[Gyro]

By coincidence, we replaced our Panasonic Inverter Microwave today. From the IC datecodes the old one appears to have been manufactured in 2005. I remember it was still the only brand offering inverter models. The only hard failure over its life was a failed turntable motor (a cheap and easy replacement due to the snip-out panel in the bottom).

Anyway, it had recently begun to suffer dropping power as cooking time increased, with the odd cracking sound. I had it apart a couple of times but couldn't see any problem - other than the filament glow of the magnetron appearing to dim slowly over several minutes. I wasn't sure if this was due to the inverter or the magnetron (inverter seemed more likely) so effectively BER.

With the nice new one in place, it was time to 'scavenge' the old one. This time I removed the magnetron and found cracked magnets and loose cooling fins that I hadn't noticed previously - a sure sign of overheating. I'm kicking myself now, because I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it! Still, the new one is very shiny, no dents and we don't need the same volume these days (it's still 950W though).

I found the attached Panasonic training guide on the web today. Apparently the inverter controller senses the Anode current using a current transformer to control the power output. With the damaged magnetron, it was presumably dialling back the power as the (non-microwave-generating) anode dissipation increased, hence the slowly dimming filament. Ah, hindsight!

From the training guide, it appears that the PWM frequency varies between approx 20kHz and 45kHz depending on output power, the whole thing running on 100Hz un-smoothed bridge rectified mains. Surprisingly there was only a single Electrolytic in the whole thing, a small Panasonic one on the controller logic supply, rated at 105'C despite having good ambient airflow from the cooling fan (take note treez).   I was expecting it to be chock full of the things!

Now, what to do with a 1kW rated IGBT driven ferrite cored transformer (once the nasty 2kV secondary is unwound)? 

[coppice]

I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it!

Wow, that's a heck of a markup. You can get those magnetrons from TaoBao for about 20% of that price.

[Gyro]

Yes, almost certainly - It was a Scrooge's remorse search for a 'genuine matching Panasonic part number label' made in China magnetron, versus a TaoBao made in China magnetron. 

[Red Squirrel]

Real men just wind their own magnetrons.

Actually, what part normally fails in them?  Is it usually the windings, or more involved?  I know they use some rather special (and dangerous) materials so probably don't want to handle them more than you need, but I suppose in theory one could rewind one.  There's a pretty serious science to it though, to get the right resonance etc. So probably would not recommend. 

[Zero999]

By coincidence, we replaced our Panasonic Inverter Microwave today. From the IC datecodes the old one appears to have been manufactured in 2005. I remember it was still the only brand offering inverter models. The only hard failure over its life was a failed turntable motor (a cheap and easy replacement due to the snip-out panel in the bottom).

Oddly enough a friend of my mum's had a similar problem with her microwave, so I ordered a second hand motor off ebay or Amazon (I can't remember which) and it's still working six years later.

Anyway, it had recently begun to suffer dropping power as cooking time increased, with the odd cracking sound. I had it apart a couple of times but couldn't see any problem - other than the filament glow of the magnetron appearing to dim slowly over several minutes. I wasn't sure if this was due to the inverter or the magnetron (inverter seemed more likely) so effectively BER.

With the nice new one in place, it was time to 'scavenge' the old one. This time I removed the magnetron and found cracked magnets and loose cooling fins that I hadn't noticed previously - a sure sign of overheating. I'm kicking myself now, because I could have got a brand new Panasonic magnetron for £44 on ebay and probably got another 10 years of life out of it! Still, the new one is very shiny, no dents and we don't need the same volume these days (it's still 950W though).

I found the attached Panasonic training guide on the web today. Apparently the inverter controller senses the Anode current using a current transformer to control the power output. With the damaged magnetron, it was presumably dialling back the power as the (non-microwave-generating) anode dissipation increased, hence the slowly dimming filament. Ah, hindsight!

From the training guide, it appears that the PWM frequency varies between approx 20kHz and 45kHz depending on output power, the whole thing running on 100Hz un-smoothed bridge rectified mains. Surprisingly there was only a single Electrolytic in the whole thing, a small Panasonic one on the controller logic supply, rated at 105'C despite having good ambient airflow from the cooling fan (take note treez).   I was expecting it to be chock full of the things!

Now, what to do with a 1kW rated IGBT driven ferrite cored transformer (once the nasty 2kV secondary is unwound)? 

Do you know how much you could get for it second hand, if you repaired it? I wonder if it's worth it, for the satisfaction, even if you break even.

[coppice]

Real men just wind their own magnetrons.

Actually, what part normally fails in them?  Is it usually the windings, or more involved?  I know they use some rather special (and dangerous) materials so probably don't want to handle them more than you need, but I suppose in theory one could rewind one.  There's a pretty serious science to it though, to get the right resonance etc. So probably would not recommend. 

Winding? I think you've got the wrong device. Its vacuum equipment you need to make your own magnetron.

[Gyro]

Do you know how much you could get for it second hand, if you repaired it? I wonder if it's worth it, for the satisfaction, even if you break even.

It is tempting, the going rate on ebay looks like about £45 for a pre-owned one. Balancing that is the worry about a potential buyer claiming that it's faulty when he puts it in his mis-diagnosed microwave and demanding a refund.

On the other hand, it is an interesting looking board with a simple interface, Mains in and a 3 wire opto isolated interface - Gnd, input control, and output power sense signals. The primary side is obviously not isolated, but the secondary side is grounded. I have no appetite for 4kV at 300mA, so that would have to go, but it has a nice high current 3V filament winding. It might possibly be a candidate for an induction heater.



Maybe time for a little research before I decide. 


EDIT: Ah, sorry, you meant the Microwave. I'm not sure if I'm a bit too twitchy, but I'd be worried about selling on a microwave appliance that I'd had apart and done magnetron related work on. Likewise a charity shop. The new Panasonic one only cost £144, and you can get cheap ones for less than £70, so by the time you factor in cost of a new magnetron and shipping a bulky 30kg lump...

[Gyro]

Just as a follow-up. I found the attached PDF which relates to this specific inverter board - and somebody evaluating it for a valve linear amplifier PSU. It gives a good level of detail on operational characteristics and drive requirements (a 555 timer will do).

I'm not clear on the topology - I'm not sure that it counts as a forward converter, despite having two IGBTs (of different sizes but both very high rated) as the emitter of the top (smaller) one is capacitively coupled to the primary and the core is gapped. On later inverters, Panasonic went to a single IGBT flyback converter so this is a bit of an early iteration (circa 2000). It doesn't appear to mind its output rectifier polarity being switched either.

From the figures in the PDF, it is a constant power inverter rather than constant voltage or constant current, and seems to be able to maintain a constant power output to remarkable accuracy (1W mentioned) over a considerable load variation.

The usual safety warnings should apply to anyone reading the PDF, the output is truly lethal and I'm certainly not planning on any lethal HT PSUs. The transformer does appear to be a split (concentric) bobbin though, so I am tempted to try winding a replacement higher current, low voltage secondary.

EDIT: https://streampowers.blogspot.com/2012/09/panasonic-microwave-oven-inverter-hv.html    (same author, less detail)

[Gyro]

I seem to be doing a lot of following up here...

Anyway, definitive proof of the failing Magnetron, artistically arranged with evidence of the true extent of the magnet cracking:



If you look carefully you can see that alternate anode segments, the ones on the antenna driving ring, have been reduced to little shiny molten blobs (melting copper in a vacuum leaves a really nice finish!). Magnetron internals are certainly things of beauty, the closer you look, the more internal structure you see.

It's surprising that the magnetron was still producing a decent amount of power from cold. Presumably the blurring of anode segment edge definition resulted in excessive anode dissipation over cooking time - it's hard to know whether the segments were re-melting each time, there had been no specific abuse, other than the passage of time and the cracking of the magnets.

On the subject of inverter Induction heater construction, I found a 'proof of concept' video on youtube. The inverter uses a newer transformer format, but it is still the older dual IGBT version of the circuit. I can't say that I approve of the isolation between the primary and improvised secondary, but it seems to work rather well - especially later in the video where he adds an additional impedance matching transformer. It seems that the normal 'self tuning' royer converter isn't that important and that fixed frequency operation is sufficient, certainly saving some on some very highly stressed capacitors.



P.S. In the spirit of staying vaguely on topic, the PWM frequency is 30kHz. 

[Gyro]

Out of interest, I remembered that I still had the core of the magnetron from our previous microwave, which died of control panel failure around 15 years ago. I had salvaged the (intact) magnets and thrown it in a drawer! Irrc, this one was rated at 800W (despite being physically the same size as the 950W Panasonic one), and driven by a standard MOT.

Here's a photo of the Anode - showing absolutely no signs of wear:



Together, they cover over 30 years of microwave ownership. Given the lack of abuse,  I have the uncomfortably feeling that, if it hadn't failed for other reasons, the original monster would still be lurking in the kitchen!

I'm bound to conclude that modern inverter units do put significantly more strain on their magnetrons than the older generation. My instinct would have been exactly the opposite, with slamming the magnetron on and off every few seconds with the filament having to warm up each time being much harder on them than a slowly varying PWM supply.

[SeanB]

I find that the magnetron typically fails by either going open circuit on the filament, or by overheating from a clogged cooling fan. Typically the common faults are the door switches, the latches for them, and the diode failing, along with fuses failing from thermal cycling.  I have a largish collection of slightly used magnetrons, from ovens that failed mostly from switches and cavities rusting out, and in general with the common microwave irrespective of the make you typically find 3 different magnetrons in use, only varying in the orientation of the input power connector and the mounting holes for the magnetron, and then you find the shrouds used vary if there is a thermal switch on the shroud or on the magnetron body, though they all have the holes for it already there. Other than that no difference in them, they all use a similar size capacitor, diode and perhaps a high voltage fuse.

Power wise you cannot operate a magnetron at low power easily, you need a certain anode current for it to oscillate, below that it simply is a power diode, and the frequency stability varies with voltage, temperature and anode current, which is why the ISM band at 2.4GHz is so wide, so that the manufacturers can keep the tolerances of mass produced punched plates inside down. 2.4GHz chosen as you could get it back then with cheap components and magnetics, as the first magnetrons used Permalloy magnets, so had to be kept large enough to keep the magnets cool. Ferrites made the magnetron assembly smaller, but the actual body size didn't change, just the magnetic circuit. Probably about the only thing you find that has not been made cheaper by using copper coated steel, or CCA, though the transformers certainly have been optimised to the bare minimum volume and windings, relying on the forced air cooling to survive, along with class H insulation.

[Zero999]

Do you know how much you could get for it second hand, if you repaired it? I wonder if it's worth it, for the satisfaction, even if you break even.

It is tempting, the going rate on ebay looks like about £45 for a pre-owned one. Balancing that is the worry about a potential buyer claiming that it's faulty when he puts it in his mis-diagnosed microwave and demanding a refund.

On the other hand, it is an interesting looking board with a simple interface, Mains in and a 3 wire opto isolated interface - Gnd, input control, and output power sense signals. The primary side is obviously not isolated, but the secondary side is grounded. I have no appetite for 4kV at 300mA, so that would have to go, but it has a nice high current 3V filament winding. It might possibly be a candidate for an induction heater.



Maybe time for a little research before I decide. 


EDIT: Ah, sorry, you meant the Microwave. I'm not sure if I'm a bit too twitchy, but I'd be worried about selling on a microwave appliance that I'd had apart and done magnetron related work on. Likewise a charity shop. The new Panasonic one only cost £144, and you can get cheap ones for less than £70, so by the time you factor in cost of a new magnetron and shipping a bulky 30kg lump...

C704 & C705 appear to be the wrong values. If they were really 8200µF, at that voltage, they'd each be the size of the entire microwave.

[Gyro]

Haha, you've got sharp eyes! I managed to fight my way close enough to the PCB to confirm that they are indeed pF. 

[Gyro]

I find that the magnetron typically fails by either going open circuit on the filament, or by overheating from a clogged cooling fan. Typically the common faults are the door switches, the latches for them, and the diode failing, along with fuses failing from thermal cycling.  I have a largish collection of slightly used magnetrons, from ovens that failed mostly from switches and cavities rusting out, and in general with the common microwave irrespective of the make you typically find 3 different magnetrons in use, only varying in the orientation of the input power connector and the mounting holes for the magnetron, and then you find the shrouds used vary if there is a thermal switch on the shroud or on the magnetron body, though they all have the holes for it already there. Other than that no difference in them, they all use a similar size capacitor, diode and perhaps a high voltage fuse.

Power wise you cannot operate a magnetron at low power easily, you need a certain anode current for it to oscillate, below that it simply is a power diode, and the frequency stability varies with voltage, temperature and anode current, which is why the ISM band at 2.4GHz is so wide, so that the manufacturers can keep the tolerances of mass produced punched plates inside down. 2.4GHz chosen as you could get it back then with cheap components and magnetics, as the first magnetrons used Permalloy magnets, so had to be kept large enough to keep the magnets cool. Ferrites made the magnetron assembly smaller, but the actual body size didn't change, just the magnetic circuit. Probably about the only thing you find that has not been made cheaper by using copper coated steel, or CCA, though the transformers certainly have been optimised to the bare minimum volume and windings, relying on the forced air cooling to survive, along with class H insulation.

Agreed, most failure modes seem to be surprisingly trivial considering the power conversion that is going on. As I mentioned above the only failure previous to the magnetron melt-down had been the turntable motor... and the previous one, a fuse due to an accidental door latch shorting switch bounce. Both Magnetrons were ferrite magnet, from the feel of the fragmented ones, I suspect that they may have been stronger, hard to tell now. I suspect that ratings have climbed a bit on the basis of experience and expected appliance life - 15 years a time isn't that bad. On the melted one there was minimal dust contamination, I suspect that a factor may be differential expansion of the copper anode and aluminium fins - there were at least a couple that were a loose fit.

From what I've read the Panasonic inverters run variable power down to about 50% and then then cycle on and off to get the lower outputs (down to 100W). One thing have noticed is that the current models have a thermistor screwed to the magnetron body, together with a thermal switch - the failed one had neither, so had no feedback of magnetron temperature or airflow.

One failure that does surprise me is the pictures I've seen of magnetrons is the meltdown of the metal antenna cap. The one on my melted anode magnetron was still in perfect shape but there are many images of burnt antennas, eg:



This is presumably due to arcing within the waveguide, maybe from energy being reflected by running empty or contamination? It's hard to think that anode damage wouldn't also result. It seems to be frequent enough failure for replacement caps to be available in large quantities on ebay and the usual Chinese sites!