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| Microwave oven PWM frequency |
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| 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:
--- Quote from: Gyro on July 13, 2020, 12:33:53 pm --- --- Quote from: Zero999 on July 13, 2020, 10:16:43 am ---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. --- End quote --- 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... --- End quote --- 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:
--- Quote from: SeanB on July 19, 2020, 01:43:33 pm ---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. --- End quote --- 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! |
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