Hi,
I have to fix our broken microwave oven. I opened it and found a shorted capacitor. Anyway, I also found a rather weird magnetron HV supply, it's basic circuitry is like the one given in the German Wikipedia:
Translation: Netz = mains, Heizung = heater, Gehäuse/Erdung = ground.
Using the same components, I would have expected the capacitor in parallel with the magnetron and the diode between the transformer and the magnetron/capacitor. I'm lost now and don't have any clue how the arrangement works, as the magnetron basically is a diode wired in antiparallel with the SS one.
Another question: How critical is the capacitance value? The defective one in our oven is 0.87 µF, but I only find either 0.85 or 0.9 µF capacitors. Which one should I chose as the replacement?
Best regards!
I have to fix our broken microwave oven. I opened it and found a shorted capacitor. Anyway, I also found a rather weird magnetron HV supply, it's basic circuitry is like the one given in the German Wikipedia:
Translation: Netz = mains, Heizung = heater, Gehäuse/Erdung = ground.
Using the same components, I would have expected the capacitor in parallel with the magnetron and the diode between the transformer and the magnetron/capacitor. I'm lost now and don't have any clue how the arrangement works, as the magnetron basically is a diode wired in antiparallel with the SS one.
Another question: How critical is the capacitance value? The defective one in our oven is 0.87 µF, but I only find either 0.85 or 0.9 µF capacitors. Which one should I chose as the replacement?
Best regards!
MOT are used for various high voltage experimenters.
One caution is most people start with neons transformers
or car coils and voltage is high, current very low.
MOT is high voltage usually 6 to 8 K
but the current is very very high.
and they knock people flat dead.
Not like tube builders or car coil
voltage. where guys get zapped and
ignore high voltage warnings.
A MOT will flat out kill you.
Sometimes the magnetron is dead
and will take out the power supply with
it.
Cap needs to handle high voltage high current.
I would check how to test the magnetron
cause it is likely dead. And repairing the supply
would be useless.
Capacitor be 20% tolerance
so posted values tolerant.
voltage and current more a issue.
and a basic resonant circuit will have
fly back protection diodes. and the transformer
core shunted for current limit
Did I mention a MOT will actually kill you
One caution is most people start with neons transformers
or car coils and voltage is high, current very low.
MOT is high voltage usually 6 to 8 K
but the current is very very high.
and they knock people flat dead.
Not like tube builders or car coil
voltage. where guys get zapped and
ignore high voltage warnings.
A MOT will flat out kill you.
Sometimes the magnetron is dead
and will take out the power supply with
it.
Cap needs to handle high voltage high current.
I would check how to test the magnetron
cause it is likely dead. And repairing the supply
would be useless.
Capacitor be 20% tolerance
so posted values tolerant.
voltage and current more a issue.
and a basic resonant circuit will have
fly back protection diodes. and the transformer
core shunted for current limit
Did I mention a MOT will actually kill you
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What does MOT stand for?
Anyway, the circuit in the opening post looks like a voltage doubler without smoothing to me. Keep in mind that the magnetron only conducts in one direction.
When the voltage across the secondary winding of the transformer goes positive, the capacitor gets charged via the semiconductor diode. When it then goes negative, the magnetron gets the sum of the transformer's secondary voltage and the voltage across the capacitor.
Anyway, the circuit in the opening post looks like a voltage doubler without smoothing to me. Keep in mind that the magnetron only conducts in one direction.
When the voltage across the secondary winding of the transformer goes positive, the capacitor gets charged via the semiconductor diode. When it then goes negative, the magnetron gets the sum of the transformer's secondary voltage and the voltage across the capacitor.
The original capacitor specifications are: 2100 Vac 0.87 µF ±3%. So I don't think the secondary voltage is higher than about 2 kV. Anyway, I understand the warnings and promise to let my fingers out of the oven when it is working.
Thanks, Marcel. Voltage doubler without the smoothing capacitor, that's it!
Follow-up question: Do I need to replace the SS diode also? It measures strangely: No conduction in both directions. Anyway, before I detected the short in the capacitor, I had replaced the open mains fuse, and on turning the unit on this fuse blew again, and with it the in-house circuit breaker also tripped! So, how to check this diode properly?
Best regards!
Edit: Checked the diode using a mains bulb tester and an isolating transformer. The bulb flickered, so the diode is likely ok.
Thanks, Marcel. Voltage doubler without the smoothing capacitor, that's it!
Follow-up question: Do I need to replace the SS diode also? It measures strangely: No conduction in both directions. Anyway, before I detected the short in the capacitor, I had replaced the open mains fuse, and on turning the unit on this fuse blew again, and with it the in-house circuit breaker also tripped! So, how to check this diode properly?
Best regards!
Edit: Checked the diode using a mains bulb tester and an isolating transformer. The bulb flickered, so the diode is likely ok.
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Like a BY140. These will read open circuit on any typical DVM on any range. A proper meter 😉 (AVO 8) will show around 10 megohm when checked in the forward direction on the highest ohms range.
It's an all plastic device and, as Marcel said, most probably consists of multiple diodes connected in series. But no selenium ones, like in those TV high voltage rectifiers, and also no ceramic tube 😉:
Best regards!
Best regards!
Around here a new microwave oven can cost from $75 to a bit over $300.
A life is valued at $200,000 or more. So if the chance of you being killed servicing the oven is more then 300/200,000 that would be a bad choice.
I suspect the chance of you being injured, even if careful is greater than .15%.
If you do want to proceed I would suggest using a high voltage insulation tester to be sure that was not the failure mode.
Of course it is possible a new oven is beyond your financial means. Then look for a used one on the internet. I often find free ones.
https://hackaday.com/2022/05/02/the-most-deadly-project-on-the-internet/
https://www.quora.com/Can-you-really-die-trying-to-take-apart-a-microwave-oven-Who-died-from-that
By the way some folks get killed taking apart a microwave oven from the charge left on the capacitor.
A life is valued at $200,000 or more. So if the chance of you being killed servicing the oven is more then 300/200,000 that would be a bad choice.
I suspect the chance of you being injured, even if careful is greater than .15%.
If you do want to proceed I would suggest using a high voltage insulation tester to be sure that was not the failure mode.
Of course it is possible a new oven is beyond your financial means. Then look for a used one on the internet. I often find free ones.
https://hackaday.com/2022/05/02/the-most-deadly-project-on-the-internet/
https://www.quora.com/Can-you-really-die-trying-to-take-apart-a-microwave-oven-Who-died-from-that
By the way some folks get killed taking apart a microwave oven from the charge left on the capacitor.
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Extreme caution is mandatory in order to discharge the capacitor before any servicing is done.
An insulated resistor (I forget the high value) is recommended.
Also any residual charge in the magnatron is a good idea to check.
I bought my countertop Sharp Carousel microwave in 2003, and it's still going strong, used every day.
An insulated resistor (I forget the high value) is recommended.
Also any residual charge in the magnatron is a good idea to check.
I bought my countertop Sharp Carousel microwave in 2003, and it's still going strong, used every day.
Given the safety policies of this forum which I fully support, the best advice is ditch the microwave. Do not fiddle around with circuits that go to kV.
A television high tension supply can't deliver much current, a microwave supply can. It's about as dangerous as a direct-drive amplifier for electrostatic loudspeakers.
As I've written above, the defective capacitor is shorted. Zero ohms! Anyway, before I noted that, I actually shorted it's terminals to avoid getting zapped, injured or even killed.
Last year I scrapped two microwave ovens. The diodes shown in the pic are from these. Unfortunately I discarded both capacitors, as they've got in-built bleeding resistors to avoid electrical shocks, thus being of virtually no use for me. As far as I see it, this is a common feature with such caps.
Best regards!
Last year I scrapped two microwave ovens. The diodes shown in the pic are from these. Unfortunately I discarded both capacitors, as they've got in-built bleeding resistors to avoid electrical shocks, thus being of virtually no use for me. As far as I see it, this is a common feature with such caps.
Best regards!
Yes. The safety aspects can never be understated with a microwave PSU.
I'm in that boat too but the HV supply in microwave is a totally different animal to the low current EHT in a TV. We used to rent out microwaves back in the day... urghh 😀 and had to have some official training before being allowed to work on them either in the workshop or in a home.
Talking of EHT reminds me of the EHT meter a place I worked at had. It was a moving coil meter with a tall resistive stack mounted on top... wow, a picture of one online. Now this one has had a plastic cap fitted on top of the, ours was just erm... like the bottom of the stack. An open metal ring at 25kV. Place it on top of TV and just make sure you don't get your head to close 😱
My last microwave tripped the kitchen circuit breaker and blew an internal fuse. I didn't think it could do both.
Microwaves are throwaway items ($100 USD) and built accordingly.
Ed
Microwaves are throwaway items ($100 USD) and built accordingly.
Ed
@Mooly "I'm in that boat too but the HV supply in microwave is a totally different animal to the low current EHT in a TV."
Yes it is.
We serviced microwave ovens in the shop too.
But when they became less expensive to buy, we dropped servicing the cheap countertop ones, only an occasional expensive Over-The-Stove types.
Yes it is.
We serviced microwave ovens in the shop too.
But when they became less expensive to buy, we dropped servicing the cheap countertop ones, only an occasional expensive Over-The-Stove types.