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Proud Mary

Wed Oct 21 2009, 01:41AM

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

Hi everyone,

does anyone know the kind of output to be expected from one of the new microwave oven inverters?

Can it supply continuous current? (I'm not concerned with Tesla use) If so, what?

thanks,

Harry

Conundrum

Wed Oct 21 2009, 05:26PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

i have a broken one here, iirc its the same as a MOT.

the problem seems to be that they overheat really easily and fail, so finding broken ones may or may not be useful.

the transformer is multicore primary around 20 turns, secondary 4*pie wound with around 20ga wire.

on mine the secondary failed near the ground tapoff...

regards, -A

quicksilver

Fri Oct 23 2009, 11:54PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Harry:
I have worked with some but just don't have the background you have to get in-depth with them.
On another working unit I set up an old HV wand and checked it out. It came from a rather new Sanyo I believe and put out 1500 @ 250ma. I used a carefully set stand of clear poly blocks and was quite cautious. It WAS pretty damn hot!There are two switching transistors on HEAVY heat sinks that get very hot. This unit was not pulled from a dysfunctional oven, I therefore believe that they run pretty hot as is. They are T220s (big ones) along with some very useful caps. I may just start gathering up quite a few of these as they contain some damn fine HV components.

Basically I was seeing 1500 @ 250ma: but this was a SMALL home unit. I have access to big restaurant units and would love to see what they have as I saw the unit itself once and it was substantially "beefier" that the residential type.

I pulled the flyback-type tx-former and played with it also. It's an odd design. After pulling the Lutz wire on one unit I experimented with the coil; I have seen this design in monitors and TV's but they seemed to be used as a choke. I used it in place of a AC flyback driven with a simple 12V HF unit and gotta small spark. The driver was of simplest design. Attempting to measure the output was a loss and I only used a cheap DIMM as I think that thing was toasted.

NOTE: I live in the USA therefore I'm putting in 120VAC, etc - The EU stuff may be much hotter.The current did not stay neatly at one figure but altered with time / temp more so than would expect.

Proud Mary

Sat Oct 24 2009, 09:22AM

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

Thanks for that, Mr Quick. Very helpful.

GeordieBoy

Sat Oct 24 2009, 11:13AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Hi Harry, I think the reason you might not have got many replies to this thread is that the way these things work is quite complex.

As far as I know they take in the mains and rectify it, but don't smooth it (because that would cause poor power factor on the mains input side.)

They then chop this up at some high frequency to put it through a ferrite power transformer. This HF is also switched on and off with adjustable duty ratio at a few kHz to achieve the different power settings (0%, 20%, 40%, 60%, 80%, 100% or whatever is on the control panel.) This chopping on and off to achieve the different power settings is just a faster version of the usual on off buzzing that old microwaves used to do. Low frequency burst-fire like that is now difficult to implement without exceeding flicker noise limits for CE approval.

This HF signal is off-course modulated by the rectified mains voltage envelope too because of the lack of smoothing!

The HF output from the transformer is rectified using the HF equivalent of the diode-capacitor voltage-doubler that you normally see in a line-frequency microwave oven power supply. This doubles the voltage but leaves an output with lots of HF ripple on it. This is what is fed to the magnitron.

So... The microwave power output by the magnetron is 2.4GHz amplitude modulated by the HF switching waveform, modulated by the power control signal, modulated by the rectified mains ripple!

This complicated waveshape is probably quite hard to analyse or measure accurately by most amateurs. That's probably why you haven't got lots of simple V and I figures in response to your question.

-Richie,

Proud Mary

Sat Oct 24 2009, 11:55AM

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

Thanks for you helpful insights, Geordie.

D'you think this complex OP waveform could be made into useful DC by a good old-fashioned L(swinging)CLC filter? (My intention to use one to supply HT for power valves?

Wolfram

Sat Oct 24 2009, 12:37PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Harry, is your inverter from a Panasonic oven? These seem to be the most common ones around here at least.

I have found one document with some information on them, here

. Do you know anything about the topology used here Richie? It looks somewhat like a resonant single-switch converter with the top IGBT acting like an active voltage clamp or something along those lines. Sadly, these inverters seem to be of the constant-power type, and the control electronics will shut it off if the load does not draw the current that it expects. Modifications to the control part is not easy either, as everything seems to be controlled by an unmarked custom IC.

Richie, do you think it would be hard to make a new control section for it, so that it could be used to power linear amplifiers, voltage multipliers, VTTCs and the like? What would happen if I applied a constant frequency and duty cycle signal to the gate of the lower switch?

Anders M.

Proud Mary

Sat Oct 24 2009, 02:32PM

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

Anders M. wrote ...

Harry, is your inverter from a Panasonic oven? These seem to be the most common ones around here at least.

That's the one, Anders!

The circuit diagram looks helpful, thankyou

- but look what economists they seem to be - paralleling all those resistors near the magnetron rather than choose a more expensive single one of the right value (or maybe it is to keep the board profile lower for some reason, or spread the heat over a greaert area of the board?)

And to Richie - You're our man here, so what d'you think of Anders suggestion, please?

GeordieBoy

Sat Oct 24 2009, 07:45PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I had not seen that paper before and haven't actually worked on these myself. However, that design looks like an active-clamp resonant flyback converter. (The biggest problem with classic flyback converters at high powers is dealing with energy trapped in primary leakage inductance, and this is likely still quite innefficient!)

The bottom switch turns on and charges up the magnetising inductance of the transformer. Then, when the bottom switch turns off, the top switch turns on and the magnetising inductance forms a resonant circuit with the capacitor in series with the top switch. The natural behaviour of the circuit will be constant power because a fixed on-time of the bottom switch will ramp up the magnetising current to a fixed value storing a given number of joules on each switching cycle. This will be modulated by the mains ripple though of course!

These things are very application specific. A given power supply will most likely be designed around the nuances and quirks of a particular magnetron, and to interface to a given controller. The microwave oven market is also fiercely competitive, so they will be designed down to the absolute minimum cost possible to just manage the intended job.

Microwave manufacturers happily made appliances with big 50/60Hz iron transformers, half-wave voltage doublers and clunky burst-fire power control for many years. It seems nobody cared about the poor power factor, harmonic distortion, and pulsating load they presented to the mains supply. But now emmisions standards like flicker noise and harmonics have made them look elsewhere. The real driving force behind these new inverter driven ovens is standard compliance and safety, not new features, added value or better performance. And the much greater complexity and number of components certainly doesn't help reliability!

If it was me, I think I would just take the transformer and possibly the rectifier diodes and design a new full-bridge converter around those. That would be much more suited to running a linear amp, and I think hobbyists have done this before. Conventional voltage-mode control with current-limiting would give good line and load regulation.

Trying to modify this cheap supply to behave totally differently to how it was designed is probably going to take much more work, than starting from scratch on a more appropriate one.

-Richie,

Proud Mary

Sat Oct 24 2009, 08:48PM

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

I have a couple of dozen 240 to 1kV neon sign inverters, and decided to do with them as Richie has suggested for the MOT inverter- toss out all the application specific parts and just save the output section. In this case, an opto-coupler links the one to the other in the mddle of the board, which invites simply sawing the board in half at that point, but I don't see such a simple option with this Panasonic unit.

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