Microwave Oven Inverters - How to run them? / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Microwave Oven Inverters - How to run them?

first previous 3 4 5 6 next

Move Thread

LAN_403

Ash Small

Fri Sept 09 2011, 07:02PM

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

Proud Mary wrote ...

Oh golly no!

If you look at the Quick Reference Data box on page 1 of the 1965 Philips datasheet, you'll see that the maximum anode dissipation is 16W. The large figures you have quoted are for TV line output pulse mode only.

Glancing at Note 3 at the bottom of page 3, you'll notice the limitation that Maximum pulse duration is 22% of a cycle and max. 18Î¼s.

Yep, Ive just read that, was about to correct it myself. I'd initially seen the figures 7000V and 250mA and thought 'that's just what I need',

But could I use a higher power valve to do this? controlling current just by changing the grid voltage for a constant DC current at 7000V, or would it vary the output voltage as well? Would I have to pulse the grid on and off?

The articles I've been reading don't all seem to say the same thing.

(I 'think' I need less than 100W, maybe a lot less, but it will be trial and error.)

Proud Mary

Fri Sept 09 2011, 08:11PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

You'll like this one, Ash!

________________________________________________ __________________________________________________ _______________________________________

Rose, C.R.; Warren, D.S.; Performance and test results of a regulated magnetron pulser Conference Record of the Twenty-Third International Power Modulator Symposium 22-25 Jun 1998 pp: 206 - 208

This paper describes the test results and performance of a 5.0 kV, 500 mA, regulated current pulser used to drive an Hitachi model 2M130 2425 MHz magnetron. The magnetron is used to modulate the plasma in a particle accelerator injector. In this application, precise and stable RF power is crucial to extract a stable and accurate particle beam. A 10 kV high-voltage triode vacuum tube with active feedback is used to control the magnetron current and output RF power. The pulse width may be varied from as little as ten microseconds to continuous duty by varying the width of a supplied gate pulse. The output current level can be programmed between 10 and 500 mA. Current regulation and accuracy are better than 1%. The paper discusses the overall performance of the pulser and magnetron including anode current and RF power waveforms, linearity compliance, and vacuum tube performance

_____________________________________ __________________________________________________ _________________________________________________

Helpful hint: Magnetron heap big palaver! You don't need much more than a thermionic filament, a whiff of hydrogen, and some HT to make a modest proton source suitable for small linacs and cyclotrons.

Ash Small

Fri Sept 09 2011, 09:46PM

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

Proud Mary wrote ...

Helpful hint: Magnetron heap big palaver! You don't need much more than a thermionic filament, a whiff of hydrogen, and some HT to make a modest proton source suitable for small linacs and cyclotrons.

Capacatively coupled plasmas do have a number of drawbacks, though, but yes, it is a lot easier to achieve.

Proud Mary

Fri Sept 09 2011, 10:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Capacatively coupled plasmas do have a number of drawbacks, though, but yes, it is a lot easier to achieve.

Oh well, if you're sure you want to go ahead with the microwave thingy, this paper has the full monty - drawings, photographs, experimental details, and theory:

Ceboliyozakha Leonard Ndlangamandla The Design, Development and Fabrication of a Microwave Proton Ion Source at iThemba LABS University of Zululand
November 2006

Kiwihvguy

Sat Sept 10 2011, 03:26AM

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

This whole ting seems to be completely off track.

I wanted to say that I desoldered an inverter transformer with the doubler components and is ready for using with an IGBT and a 555 flyback type driver with an inductor and a cap in front of the inverter transformer.

Ash Small

Sat Sept 10 2011, 09:00AM

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

JÃ³zef C wrote ...

This whole ting seems to be completely off track.

I wanted to say that I desoldered an inverter transformer with the doubler components and is ready for using with an IGBT and a 555 flyback type driver with an inductor and a cap in front of the inverter transformer.

Sorry, I thought you said you were planning to use it to drive the magnetron.

Are you planning to use the original primary?

What voltage are you planning to use?

Kiwihvguy

Sat Sept 10 2011, 09:48AM

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

[/quote1315645063]

Sorry, I thought you said you were planning to use it to drive the magnetron.

Are you planning to use the original primary?

What voltage are you planning to use?
[/quote1315647933]

Originally, I was trying to find a use for this inverter board so means of arcing or using as a PSU for a magnetron would be acceptable either way. I am planning to use the original primary and I guess I could rectify mains then feed that into one of the N-channel 1000v 60a IGBTs (50N322) and use a 555 flyback driver to drive the IGBT as 30kHz as a frequency I could set with my oscilloscope (when I get it). I would put an inductor and a cap in front of the transformer primary to keep "the silicon happy" as bwang said. bwang, can you please advise me on this?

Cheers,
JC

Ash Small

Sat Sept 10 2011, 11:53AM

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

JÃ³zef C wrote ...

I would put an inductor and a cap in front of the transformer primary to keep "the silicon happy" as bwang said. bwang, can you please advise me on this?

I'm pretty certain Bwang was reffering to using a bridge circuit when he suggested using a series cap and inductor.

This won't work in flyback mode.

Running from rectified mains will require something that accomplishes what the protector circuits in the original circuit do, to deal with voltage spikes and interruptions, or the IGBT will die.

I have tried to discuss some alternatives above.

Even using a 1700V IGBT probably won't overcome this.

I did suggest possibly using a 'ferro-resonant transformer' between the mains and rectifier, but no-one else commented on this. from recent reading I've been doing, I think it might be possible to modify an 'old type' MOT to achieve this, but I'm not sure if the core is large enough to accomplish this sucessfully.

I'm fairly certain you will also require some form of 'snubbing' as well.

Then there is the problem of ZVS switching to overcome.

We have identified these problems above already. I'm pretty certain that if you don't address these issues the IGBT won't last long.

Hope this helps.

Kiwihvguy

Sun Sept 11 2011, 02:45AM

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

So I should use a half-bridge instead? Will it still need protection if I am using rectified mains (I presume so).

Yeah, I undertand what you mean. Those IGBTs are probably very rare and expensive, so I would treat them with caution.

OVERVIEW OF MY OPINION: I should use a half-bridge to drive the inverter transformer.

I noticed that on my tranny that there were 3 HV outputs, one was the HV ground (which would be grounded to the chassis of the microwave in order to connect to the magnetron), HV out and another HV out which probably supplied higher current in parallel with the HV.

Indeed 4kV at 300mA is more than lethal, so I will need to treat the tranny with caution. So the secondary HV winding acts as the HV output while the other secondary winding (filament?)provides 300mA to power the magnetron? I notice that older MOTs have a filament winding that provides ~15 amps. Why doesn't the inverter do this?

I also noticed there was a 100M resistor between the two HV outputs (HV out and ground). What purpose does this serve?

Ash Small

Sun Sept 11 2011, 03:45AM

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

You won't get the same voltage with a half bridge. Flyback topology gives higher voltage.

I'll consider the other points tomorrow. It's 4:45 AM here, past me bedtime.

EDIT: Re-winding the primary 'may' overcome this, There are other things to take into account, I think. Personally, I'm looking into the 'ferro-resonant transrormer with snubber on the flyback' route, as I want an un-interupted supply.

first previous 3 4 5 6 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob