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Microwave Oven Inverters - How to run them?

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Kiwihvguy

Sun Sept 11 2011, 06:18AM

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

What's the power difference between a half and a full bridge? Is the full bridge able to withstand more power?

How will I not get a higher voltage with a half bridge?
Also, as seen in the schematic you said that the transformer was driven by an IGBT then another one in a snubber circuit (what's a snubber circuit?).

Basically, I am asking for some advice on how to power this tranny from an inverter in a similar way as seen in the schematic using the IGBTs.

Cheers,
JC

Ash Small

Sun Sept 11 2011, 11:47AM

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

I suggest you use google or look in the 4HV Wiki.

Full and half bridges are 'push-pull' circuits. They will give more current at lower voltage compared to a 'flyback' circuit.

I wouldn't recommend a bridge as a first circuit.

The snubber prevents transient high voltage spikes in a flyback circuit which will probably kill the IGBT.

The original circuit is a flyback circuit.

Kiwihvguy

Sun Sept 11 2011, 08:38PM

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

Ash Small wrote ...

Full and half bridges are 'push-pull' circuits. They will give more current at lower voltage compared to a 'flyback' circuit.

I wouldn't recommend a bridge as a first circuit.

More current sounds fantastic to me. I've flybacks with 30kV+ but I would fancy a circuit that makes huge arcs.
I have already made a 2N3055, 555 and ZVS driver. I don't to worry about the half-bridge now because I'm already experienced with flyback drivers.

Hvwiki got some good schematics of half and full bridges. What are capacitor values in the half-bridge circuit? Is a 555 or a TL494 suitable for the driver?

Thanks for the help Ash! I really appreciate it, let's hope I can get started on this soon.

Ash Small

Sun Sept 11 2011, 09:06PM

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

JÃ³zef C wrote ...

Hvwiki got some good schematics of half and full bridges. What are capacitor values in the half-bridge circuit? Is a 555 or a TL494 suitable for the driver?

The capacitor values depend on the inductance of the primary and other factors.

The schematics in the HV Wiki only outline the 'basic circuit', if I remember correctly.

TL494 is perfect for the job.

Depending on the IGBT's you use, there are various methods to drive the high side IGBT's (ones at the top).

Others, for example, Bwang, are better able to advise on these than I am.

There are other 'push-pull' circuits available that 'might' be usable, and much simpler to build. (The Mazilli ZVS circuit is one example, but these would require a centre tap on the primary, which should be relatively straightforward to achieve)

All the above mentioned circuits will allow you to vary output voltage by varying input voltage.

Kiwihvguy

Sun Sept 11 2011, 10:45PM

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

I'm happy to stick with a half-bridge. I'm looking for some nice arcs from the inverter tranny and also some monster big, fat, hot arcs using a flyback.

Could somebody give a full schematic of a half-bridge? I've looked on Youtuube, asked people for it but i can't find anything. (apart from one which uses the IR2153 but it's unstable)

Ash Small

Mon Sept 12 2011, 12:18AM

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

JÃ³zef C wrote ...

Could somebody give a full schematic of a half-bridge? I've looked on Youtuube, asked people for it but i can't find anything. (apart from one which uses the IR2153 but it's unstable)

Two things.

First, I don't know what effect, if any, the airgap will have when driven by a half-bridge (see photo in earlier post).

The air gap is on a leg between the primary and secondary.

Second, I doubt anyone here can post a complete circuit for you.

You'll have to start yourself, maybe by working from the basic circuit and choosing suitable IGBT's, then posting it and working from there. You'll also have to design the gate driver circuit. I imagine you'll want to research 'soft switching' as well.

A number of the tesla coils on here use half bridges. You may find some links to schematics there.

I'm still in this position myself, as are a few others here.

Probably best to start a new thread specifically on the subject of 'your' proposed circuit. (It will be your design).

Kiwihvguy

Mon Sept 12 2011, 02:57AM

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

Sorry about asking for the schematics, I didn't realise that there were no fixed or official half-bridge schematics out there. At least I've got some 1000v 60a N-channel IGBTs for the job, so that's a start. I hunt for some more from inverters.

I can ask a friend of mine for some advice and mentoring on this.

I'm looking for arcs monster big, fat, hot arcs like this:

I'll ask Jan Martis (Dr. Kilovolt) if there are any distinctions in his half-bridge that makes it have such a powerful output i.e: aspects that are different from other circuits.

EDIT: The IGBTs are GT50N322A.

astralhighway

Mon Sept 26 2011, 06:30PM

Registered Member #4107 Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53

Hi, interesting as your proposal is, I wouldn't advise powering the magnetron. I can't see a safe way to do this, and why would you want to do that? The only effects you can safely produce are with the original waveguide and the magetron intact inside the / behind the door of the oven. Seriously. You don't want this wavelength bouncing around your room and back at you.

What are you proposing to do with it?

I am interested in how you get on, don't mean to be discouraging. Particularly as I've never seen an inverter-powered microwave oven, only the traditional, previous generation with MOT and doubler.

Kiwihvguy

Mon Sept 26 2011, 07:28PM

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

I know how you feel about me using magnetrons anyway. I got bored of them and stopped doing a while ago.

I agree, it'd be hard to use an inverter board to power a magnetron. Instead I just took the tranny off and the IGBTs.

The IGBTs are a bit too slow to use but I'll keep them for later.

astralhighway

Tue Sept 27 2011, 03:00PM

Registered Member #4107 Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53

Jozef, I don't think the IGBTs are slow, as you say. They have rise times that are plenty fast enough. If you're after 'monster, fat arcs', you'll be disappointed with results at 4KV. This inverter offers a dangerously high current to mess around with, yet, it's not going to give you spectacular arcs. If you want astonishing, thick and interesting arcs, you'll need to investigate Tesla Coils in the page dedicated to these. Tesla coils discharge into free air - you don't need to offer a point at ground potential to get an arc. Watch your eyes with UV from any plasma arc.

Please be very careful. I get the impression from some of your other threads that you're getting shocks quite often. This is a very bad sign as just one shock with some set ups is instantaneously lethal. Take care out there. Always operate with one hand behind your back and think twice before touching anything. Where possible, design a rig that will neutralise any residual stored potential, especially in capacitors, real or virtua.

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