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Microwave Oven Inverters - How to run them?

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Ash Small

Thu Sept 08 2011, 09:43PM

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

JÃ³zef C wrote ...

.Maybe I could add a resonant cap across the primary?

The capacitor across the primary makes it 'quasi-resonant' not 'resonant'.

The capacitor and primary resonate at a much higher frequency than the frequency at which the circuit operates at.

I've been considering doing something similar with my transformer, (driving it with an IGBT, or something) but heating could be a problem, depending on how you run it.

I think I'll have to read up on flyback topology again, I'd also thought about re-winding the secondary with thinner wire for more turns/higher output voltage, and maybe putting it under oil, and removing any trapped air using a vacuum pump, for improved insulation. Less turns on the primary should also increase output voltage. Although output current would be less, (which is what I want for an ion source) I could then run it at lower voltage on the primary.....I think. (but I'm not certain).

I think you'd probably also want to switch the IGBT at zero volts (ZVS) unless you were running it at considerably less power.

These are also 'extremely' lethal at full power (300mA).

EDIT: If you use a current limiting resistor you should be able to draw arcs with the doubler connected without any problems.

EDIT EDIT: This explains quasi resonant flyback circuits.

Proud Mary

Thu Sept 08 2011, 11:26PM

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

Ash, this one's got the whole quasi-resonant ZVS doodah, and is specifically aimed at the scientific use of cheap domestic oven magnetrons:

Vasile SURDUCAN, Emanoil SURDUCAN, Radu CIUPA Variable Power, Short Microwave Pulses Generation using a CW Magnetron Advances in Electrical and Computer Engineering Volume 11, Number 2, 2011

Kiwihvguy

Fri Sept 09 2011, 12:05AM

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

Ash, what is an ion source and what is it used for?

Btw, how do you quote other peoples' comments?

Ash Small

Fri Sept 09 2011, 01:34AM

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

@PM, Wow, you never cease to amaze me with the ease at which you seem to find these papers.

Not finished reading it yet, but it looks pretty good.

JÃ³zef C wrote ...

Ash, what is an ion source and what is it used for?

An ion source is basically the same as a plasma generator. My main interest here is to turn hydrogen gas into protons to feed into a particle accelerator, but there are countless different ions. both positive and negative. While there are simpler methods of producing electrons, eg hot cathode, electrons are also generally produced. (That's the really simple answer)

JÃ³zef C wrote ...

Btw, how do you quote other peoples' comments?

There is a 'speech bubble' icon in the top right hand corner of each post. Click on it.

Dr. ISOTOP

Fri Sept 09 2011, 01:55AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

I would advise a half-bridge or full-bridge driver depending on whether you want to save component costs or get more voltage. Flyback mode is pretty hard on your switch.
Place an inductor and capacitor in series with your transformer's primary and drive the circuit at its resonant frequency for 100% duty cycle, or half that for 50% duty cycle. That should get you nice soft switching in your inverter, which keeps the silicon happy =)
The downside of this scheme is you have to variac (or otherwise vary) the bus voltage, as changing the duty cycle will throw your driver off resonance.
One thing to beware of is that because of resonant rise, the transformer will output more voltage than the turns ratio would predict when open circuited, and when shorted will draw tremendous current (the 8:400 transformer I recently wound drew 100A at 140VAC when shorted!)

Ash Small

Fri Sept 09 2011, 02:24AM

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

bwang wrote ...

I would advise a half-bridge or full-bridge driver depending on whether you want to save component costs or get more voltage. Flyback mode is pretty hard on your switch.
Place an inductor and capacitor in series with your transformer's primary and drive the circuit at its resonant frequency for 100% duty cycle, or half that for 50% duty cycle. That should get you nice soft switching in your inverter, which keeps the silicon happy =)
The downside of this scheme is you have to variac (or otherwise vary) the bus voltage, as changing the duty cycle will throw your driver off resonance.
One thing to beware of is that because of resonant rise, the transformer will output more voltage than the turns ratio would predict when open circuited, and when shorted will draw tremendous current (the 8:400 transformer I recently wound drew 100A at 140VAC when shorted!)

While I appreciate that this is excellent advice for some applications, if I want to control/vary the output current won't this just cause more complications?

Personally, I'm only after the nominal 4000V, but I want to control output power (read current).

(I am now planning to follow your advice for my 50kV H bridge, though)

haxor5354

Fri Sept 09 2011, 11:13AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

use a ZVS driver!!

Proud Mary

Fri Sept 09 2011, 02:51PM

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

Ash, here's some practical circuit diagrams of a magnetron PSU.

Magnetron driving power supply circuit
United States Patent 5208432

Ash Small

Fri Sept 09 2011, 06:04PM

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

I've been thinking....If I have a pentode, eg a PL504 (7000V, 250mA) and I run it at 4000V (presumably using a capacitor bank charged to 4000V), I can then control the current to a magnetron, can't I?

(I just found a PL504 in a box of 'assorted treasure' that I recieved yesterday from a fellow 4HV'er)

Could it really be this simple?

(I've never used valves (VT's)before.)

BTW, Thanks for that last link, PM.

EDIT:I think I'm getting confused here, some sources say that the grid controls anode voltage, some say it controls anode current. I need to do some further reading.

Proud Mary

Fri Sept 09 2011, 06:56PM

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

Ash Small wrote ...

I've been thinking....If I have a pentode, eg a PL504 (7000V, 320mA) and I run it at 4000V (presumably using a capacitor bank charged to 4000V), I can then control the current to a magnetron by varying the grid voltage, can't I?

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.

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