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Fun with the Panosonic High Frequency MOT

RogerInOhio, Sun Dec 13 2009, 08:41PM

I have been investigating different ways of generating high voltage at a frequency in the range of 20khz to 50khz for a C and W voltage multiplier so I decided to hook up an inverter power supply out of a Panasonic microwave oven that someone had given me just to see what it would do.

I must say form the start that if it wasn't for the information on Dave Smith's web site (

) I would have been totally lost. To make the power supply unit come on you first need to have line voltage hooked up to the heavy blue and gray wires coming out of the plug on the low voltage side of the board and then you need to apply a 1.5 to 3vdc signal voltage between the brown and the yellow wires coming from the smaller plug.

Once the unit is running it seems to have two different modes of operation which depend on loading. From zero load up to about 200 watts the voltage stays constant but when you get above 200 watts look out!!! Once you pass this threshold the Power supply will try to put out a constant current of about 300 ma to what it thinks is the magnetron so if the current is too low it will raise the voltage. If it raises the voltage as much as it can and still doesn't have the right current it will shut down. Unfortunately this constant current mode isn't desirable for most power supply applications.

In the pictures below I show the inverter with a load hooked up directly across the output of the transformer which puts out about 2000volts at 30khz. The 300watt light bulb uses a ferrite step down transformer which was a project in itself. It has a 275 turn primary and a 15 turn secondary. Also you can see a 10,000 ohm resistor and a 650 pf capacitor hooked up to it as well. With all this stuff loading it the inverter will run fine in constant current mode.

Re: Fun with the Panosonic High Frequency MOT
Henry H, Mon Dec 14 2009, 12:58AM

Are you giving it a DC voltage as the control signal, or a PWM command as used in the microwave?

I have a couple of these. It's pretty amazing what they can do, and the lengths the manufacturer goes to to reduce cost (phenolic single-sided PCB etc). I highly recommend using forced air on the heatsink - the fan from the oven works pretty well.

Re: Fun with the Panosonic High Frequency MOT
RogerInOhio, Mon Dec 14 2009, 02:05AM

Are you giving it a DC voltage as the control signal, or a PWM command as used in the microwave?

I'm giving it a DC voltage witch I figure must be telling it to run at the highest power level. Yes the heat sink dose warm up pretty quick so I avoid letting it run too long. If I ever come up with a useful application for this thing I will get a fan.

Re: Fun with the Panosonic High Frequency MOT
RogerInOhio, Tue Dec 22 2009, 12:43AM

I have solved the problem I had with the inverter power supply going into constant current mode. All I had to do is shunt out the primary and secondary of the current transformer as shown in the first picture below. Now the power supply puts out a constant voltage no mater what kind of load I have on it as long as I don't over load it.

I had wanted to hook up a fly back transformer to this thing but good flybacks are hard to find these days and if you get one off of Ebay it cost a fortune. If only I had gotten the fly back transformers out of all the old TV sets I came across back in the 70s and 80s I would be a rich man today. Since I had no fly back transformer I decided to try winding my E core transformer as a high voltage transformer even though there isn't much room for a well insulated secondary winding. It ended up having a primary winding of 225 turns of # 26 wire and a secondary winding of 1200 turns of #30 wire and the whole thing runs in an oil bath.

With that turns ratio I was expecting to get about 1,100 volts but the ark will jump an inch and the current is so strong it melts the 18 gauge wire. It is scary!! I don't dare let it run more than a few seconds on tell I can wire in some kind of ballast.

Re: Fun with the Panosonic High Frequency MOT
Zeus, Tue Dec 22 2009, 01:19AM

You most likely have an air gap in your EI core so it may be running as a flyback. Also try Uzzor's website

for big home made transformers.
[Curse links]

Re: Fun with the Panosonic High Frequency MOT
RogerInOhio, Tue Dec 22 2009, 10:49AM

Actually it doesn't have an air gap. Thanks for the link to Uzzor's site. He has some cool stuff.

Re: Fun with the Panosonic High Frequency MOT
Zeus, Tue Dec 22 2009, 11:58PM

Maybe it is tuned by the act of some HV deity, unless you planned it that way?

Re: Fun with the Panosonic High Frequency MOT
RogerInOhio, Wed Dec 23 2009, 12:42AM

The ark across the secondary of the ferrite transformer pretty much acts as a short circuit so the big miracle here is that the Panasonic inverter doesn't go up in smoke. I have been trying lots of things here without really knowing what I'm doing and so far I have been lucky.

Re: Fun with the Panosonic High Frequency MOT
LutzH, Wed Dec 23 2009, 09:48AM

Hey really cool!!!! Very interesting I was told these run at about 30KHz, and the power level is adjusted via pulse width modulation. The constant current mode would be great for capacitor charging, maybe let the laser folks know, the voltage is perfect for flashlamp pumped lasers.

I have one of these, and I just found out its overpowered for running an microwave ion source and thus useless for this app. But eureka!!! Now after reading this post it confirms my suspicion that they can drive a VM, the question is whether to rewind the on board ferrite tranny? Or just use fewer turns on the HV transformer that you are feeding. The constant voltage operation would be perfect for a CWVM setup. thanks for this post it is very useful. Would you be willing at some point to post a schematic with all the step by step mods for constant voltage and constant current operation? This would be the biggest thing in power supplies for quite a while now. Ham's, Laser Folks, HV Folks, and especialy the x-ray crowd would love this. I just had the idea to use one of these, to drive the HF Mammo X-ray transformer which I have. Thanks again, a very useful posting. Yes I know there are so many posting on HFHV, but this one is so simple, which is the beauty of the thing.

Take Care.....Lutz

Re: Fun with the Panosonic High Frequency MOT
RogerInOhio, Thu Dec 24 2009, 01:32AM

Hi Lutz, I thought about rewinding the on board tranny too but the way it is made it didn't look to me like there would be enough room for a reliable high voltage winding. The primary coil of my ferrite step up transformer has just a little more than the minimum number of turns to keep the core form saturating so I couldn't cut back on that any. The only mod I had to make to get the inverter unit to run in constant voltage mode all the time was to solder jumper wires across both sides of the small current transformer on the board.

In a recent post I had mentioned that I was surprised that my ferrite transformer was producing one inch arks when the output voltage was only suppose to be 11000 volts. I have found from looking at Uzzor's sit that when there is no load on the circuit the voltage can go up a lot higher than normal so I put in a small load (15000 ohms) across the output of the inverter and now the furthest the ark will jump is 9/16ths of an inch. I also put 2000 ohms in series with the primary of the transformer to limit the current.

Re: Fun with the Panosonic High Frequency MOT
Zeus, Thu Dec 24 2009, 02:16AM

I'm looking to get one of these from a repair shop, so I'll have spares if I cook it.