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Microwave Oven Inverters - How to run them?

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Kiwihvguy

Sat Aug 27 2011, 10:30PM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Hey,

I recently disassembled an inverter microwave and extracted the components and was wondering if it was possible to run the inverter? It's got quite a good output at 4kV 300mA.
I know it requires a signal input as well as power and will not run unless it has a load (?), so how could I do that?
I would use it to power the magnetron or something of appropriate use for a load. I also know that it is not designed for arcing.

Maybe I could use the original circuitry in the microwave (timer board) or create my own signal input?
This website has some useful info:

Jozef

Ash Small

Sat Aug 27 2011, 11:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If you just want to run a magnetron, why not just run it from an MOT with a smaller capacitor in the doubler, so you can run it in continuous mode (CW) at reduced power?

A TL494 would probably do as a replacement switching unit. The only other thing seems to be current limiting, I assume an inductor would do that.(I think)

Kiwihvguy

Sun Aug 28 2011, 02:22AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

I guess I could do that... I was trying to find a use for this inverter though.

Ash Small

Sun Aug 28 2011, 02:27AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

JozefC wrote ...

I guess I could do that... I was trying to find a use for this inverter though.

Well, as I said, a TL494 should take care of the switching. Do you know what frequency it is designed to run at?

Kiwihvguy

Sun Aug 28 2011, 07:37AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

It runs at about 30kHz, according to the recent link I posted. I don't know any frequency generators that use a TL494, do you have any schematics?

Adam Munich

Thu Sept 01 2011, 05:11PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Relevant:

Conundrum

Mon Sept 05 2011, 08:14PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Heh, I have one of those here.

Still haven't managed to find a way to power it on.

-A

Ash Small

Mon Sept 05 2011, 10:33PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

JÃ³zef C wrote ...

It runs at about 30kHz, according to the recent link I posted. I don't know any frequency generators that use a TL494, do you have any schematics?

Probably more information than you need.

That is the Texas Instruments application note rev D. The rev E is out there (google), so are other circuits.

Others here who have more experience with the TL494 may also be able to help.

Kiwihvguy

Tue Sept 06 2011, 01:10AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Thansk for the help but I haven't constructed a TL494 PWM circuit before, so does anyone have any schematics?

I wonder what signal the timer/control board sends into the inverter. Maybe a waveform that is produced for the period of time the microwave runs at? e.g: set 30 secs in the timer so the control board produces the signal for that amount of time.

Ash Small

Tue Sept 06 2011, 02:21PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

JÃ³zef C wrote ...

Thansk for the help but I haven't constructed a TL494 PWM circuit before, so does anyone have any schematics?

I wonder what signal the timer/control board sends into the inverter. Maybe a waveform that is produced for the period of time the microwave runs at? e.g: set 30 secs in the timer so the control board produces the signal for that amount of time.

I'm sorry, I mis-read the circuit before my earlier post. It was late at night here, so I'd probably had a drink.

Looking at it again, It looks like a half bridge circuit, with Q701 and Q702 being the switches. I was thinking you could just drive them using a TL494, however, the circuit is a bit more complicated than I originally thought. I saw what looked like a totem pole arrangement, consisting of Q704 and Q705, but, on second thoughts, it doesn't look like the normal totem pole arrangement.

It now looks like it's a resonant circuit, with C701 and/or C703 forming a resonant circuit with the primary, but I'm not sure about this. L701 looks like it serves the same purpose as the current limiting inductor in the Mazilli ZVS circuit.

The simplest thing to do would be to try and get it running using everything as it was.

One alternative (as I can't work out exactly how that circuit works) would be to use two IGBT's to construct a 'conventional' half bridge, as in the image below. It would then be fairly simple to drive a half bridge using a TL494 at 30kHz, or thereabouts.

I'm doing this myself (in my case I'm using a full bridge, with four IGBT's, but I might also get an inverter microwave oven and try this myself), If you were to go this route there are plenty of people on here who could advise you (and me).

The value of the capacitors required would be dependant on the leakage inductance and magnetizing inductance of the primary plus the inductance of any ballast inductor (Maybe that is what L701 is, I'm still learning about this stuff, but there is plenty of information available about half bridges).

I'll have another look at this later. I'm interested in this as I'm building a magnetron ion source using a conventional MO, but this might be a better alternative.

Maybe someone else will add something, as I doubt everything I've said is correct.

This would mean scrapping most of the existing circuit (everything except the HV side and mains rectifier

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