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Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
Hey, thanks a heap for the help Ash! That's more than enough, so ta for that.
Basically, you are saying that one option could be remove everything on the inverter board except the IGBTs, inverter transformer, resonant primary cap, high voltage doubler and then drive the IGBTs with a TL494 set at 30kHz? No too complex for me but I may have been unclear before, does anyone have any -TL494 PWM Driver circuit schematics?
I dissassembled another inverter microwave and was thinking of using the control board as the driver which is another option but my Dad is paranoid about me using the original board from a presumably faulty microwave, so I may have to fix something.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Józef C wrote ...
Hey, thanks a heap for the help Ash! That's more than enough, so ta for that.
Basically, you are saying that one option could be remove everything on the inverter board except the IGBTs, inverter transformer, resonant primary cap, high voltage doubler and then drive the IGBTs with a TL494 set at 30kHz? No too complex for me but I may have been unclear before, does anyone have any -TL494 PWM Driver circuit schematics?
I dissassembled another inverter microwave and was thinking of using the control board as the driver which is another option but my Dad is paranoid about me using the original board from a presumably faulty microwave, so I may have to fix something.
Not quite. I don't think you can use the existing IGBT's. According to the article you linked to, Q702 forms part of some sort of snubber circuit (I also agree with this). You may be able to use Q701, but I expect you'd want two identical IGBT's for a half bridge, or four for a full bridge.
The other point I want to make is that in the article you linked to, he didn't seem to be absolutely certain about the 30kHz frequency.
Basically what I'm suggesting is to use the HV circuit and the mains rectifier, then build a complete half bridge or full bridge to drive the primary (you may also need the inductor, L701).
Either way, you could use a TL494 to drive it.
It would probably involve some 'trial and error' stuff, unless someone else can help out, but you could start off with low power, and work from there.
I am interested in doing this myself, as I said earlier. as I need to drive a magnetron at reduced power for an ion source. and this seems to be a good way to go.
I'm a relative newbie myself, but I realise that if you design your own driver circuit you will be able to control exactly what is happening on the HV/magnetron output.
Hope this helps. BTW, what are you planning to use it for?
EDIT: I'll have a look for some 'example' TL494 circuits tomorrow.
Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
I'm a newbie to HV too so it can be difficult to understand some electronics talk that you guys speak. I really want to learn more, especially in the half-bridge, full bridge circuit area because I'm not that experienced. How do they work?. I would like a schematic of a half-bridge and a full-bridge. What's the difference between the two?
Well, constructing a half-bridge or a full is an option but first I want to try and see if I can run original microwave timer board to power the inverter to power the magnetron. If that doesn't work, then I'll have to get started on an IGBT bridge circuit. The inverters have some very powerful IGBTs. At least my has a 1200v 50a IGBT, can't remember which one it was so I'll post the info and description in the next post.
Your question: I was planning to power a magnetron or create the inverter as an HV PSU. Using as it originally was is not an option because something is bound to go pop after a few arcs, so will I have to remove the transformer and connect to a half-bridge(or full)?
I feel slightly guilty about saying that I need some advice on how use a magnetron safely. I know that it needs metal shield around the places where the microwaves shouldn't go. I do stand back at quite a distance (~15m) before powering it up.
I also noticed that I can't hear any noises apart from the MOT humming, but when recorded on my camera, it makes a creepy "bhorrwhowwhowwbrrmbrrm" sound just like other vids on Youtube. Explanation?
I'm just curious, also I'm new to these things but I want to learn. Hope that's fine!:-)
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
I also noticed that I can't hear any noises apart from the MOT humming, but when recorded on my camera, it makes a creepy "bhorrwhowwhowwbrrmbrrm" sound just like other vids on Youtube. Explanation?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Before I go into any further down the full or half bridge route, I took another look at the 'original' circuit, and if you take out the snubber circuit, and IGBT driver circuit, you get this:
This looks a lot like a flyback circuit, with a series inductor.
Can someone more knowledgable than me please comment on this?
Is this a reasonable conclusion to reach?
Is it reasonable to conclude that if you remove the control chip, and drive Q701 at ~30kHz, the inverter will work?
(Maybe using a TL494 and totem pole, and probably leaving the snubber circuit in place)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I aquired this this morning, thanks to a fellow 4HV'er.
The inverter core has a gap of around 3mm, so I think it's pretty reasonable to assume that it is designed to run in flyback mode.
Wikipedia says this: "Newer models have inverter power supplies which use pulse width modulation to provide effectively-continuous heating at reduced power so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating." so it should be feasible to use a TL494 to switch the IGBT (Q701).
I'll have to read up a bit on pulse width modulation, but as far as I'm aware, it can be controlled using a potentiometer. (I'm pretty certain that's the case with a 555, anyway.)
Józef C wrote ...
.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? .
Are you certain it requires a load? Do you mean 'with the doubler connected?' I can't see any reason why the transformer won't draw arcs the same as a TV flyback.
I'll have a play around with it, maybe wind another primary (the primary and secondary are on separate bobbins, I'll post another photo later). I'll also see if I can come up with a TL494 circuit to drive the IGBT in your circuit.
I think this thread has persuaded me to obtain a complete inverter oven and convert it to an ion source using a TL494 myself.
I assume they use the 'active snubber' due to the fact that it is PWM, but maybe someone else could comment?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
There's some enlightening passages in this paper, Ash:
Gyeong-hae Han , Jung-ho Kim , Bum-suk Go The Effective Protection Circuit in Inverter Microwave Oven LG Electronics Inc Home Appliance Laboratory
Thanks for posting that PM. I'm struggling with it a bit, but it looks like there are quite a few differences between the LG and Panasonic circuits anyway. I now know what the current transformer is for in the Panasonic circuit.
I'm wondering if there is another, more expensive, way to prevent the supply irregularities, so that the part of the circuit that shuts down the inverter won't be required. Maybe a resonant transformer to supply the bridge rectifier?
I'm also considering ways to reduce the current to the magnetron. I assume smaller capacitors in the doubler will achieve this, but there might be other alternatives?
(A few things are puzzling me about the PWM. Won't shortening the period that the IGBT is on result in reduced voltage in the secondary, due to the current in the primary not having time to reach a maximum before it turns off? I'll have to read up on this. Maybe by PWM they mean something similar to the interruptor in a DRSSTC, where the power is supplied in pulses? Any comments?)
I'd want to run the magnetron CW at a reduced current if I use it for an ion source.
These circuits are built as cheaply as possible, so it must be relatively easy to add some more expensive components to improve or add more control to them.
Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
"Are you certain it requires a load? Do you mean 'with the doubler connected?' I can't see any reason why the transformer won't draw arcs the same as a TV flyback."
The question in my previous statement noted thast I wasn't entirely sure but it was predictable for a circuit that wasn't designed to be messed around with but I was planning to use the doubler as a voltage stepup. In general, I would take the IGBT and the transformer and connect it to a 555 driver which shouldn't be hard. Maybe I could add a resonant cap across the primary?
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Microwave Oven Inverters - How to run them?
