Reducing Magnetron Voltage

[Joseph]

Greetings Friends. I am considering easy ways to lower magnetron voltage (but not heater filament) by about 1/3 to reduce output by about 50%, hopefully. The process used to be done by controlling just plain on/off, in other words, by cycling power to the magnetron. Now that the contacts on the timer board have eroded away, I want to do it the way I always thought would give better cooking results since it wasn't being set higher than needed to keep the magnetron on more than half the time. Thusly came the idea for half power.

I see there are ovens that do something like this and that are termed inverter magnetron types, if memory serves me. Best Regards in advance.

[dacflyer]

in a conventional home microwave, the filiment and HV transformer for the MAG. are all in one.
in a commercial microwave ( hobart , they have 2 mags in them. ) they have a total of 4 transformers in them, 2 HV transformers and 2 filiment transformers.
i am not sure you could run a MAG off of lower HV, even tho you have full filiment voltags, but then again i never tried it.
the commercial ovens just keep power to the filiments at all times the oven is on, even in stand by, until a certain time, then it shuts down. like after 10 min of non use.
anyway the filiment stays on in standby, and when the oveen is used, then the HV kicks on to the MAGS. and if lower heat is desired, then the HV transformer is just cycled.

hope this helps..

[Joseph]

Indeed, that does help. I guess those separate filament transformers enable instant heating. I have heard of a case where a guy added another transformer so that the first one was powering the filament and the other provided the high voltage. I haven't ruled out that possibility. Light weight is desirable and at least the filament transformer could be pretty small.

Right now, I have a remote switch wired in series with the primary winding. That way control of the heating is manual, but I don't have to stand next to the oven and wear out the other set of contacts on the timer by turning the knob on and off. So there is the advantage that if I watch it, the cooking can be more finely regulated. The downside is that if my attention wanders, the results are worse.

Another way I am considering is adding an inductance in series with the secondary winding. That could be smaller than a standard microwave transformer, too.

[MrAl]

Hi there Joe,


Dont waste your time unless you feel like doing it anyway

Connect a variac to the INPUT of the microwave oven (120vac side) and lower the voltage to 110vac or even 105vac or even 100vac. The oven will still run but the power will be GREATLY reduced.
The input voltage vs power output is not linear at all, and in fact goes down really really fast when you lower the line voltage, so you only have to lower by 5 or 10 volts to start to see a difference. You can experiment with input voltages and cooking times say to boil a test sample of water (8 oz.) and see how the voltage varies that, but better yet is to cook something like a chicken leg with input voltage at 120v and another one at 100v and see how much difference it makes. It's quite amazing how little you have to change the line voltage to see very low power levels and you dont have to pulse the thing to see this change.
You may even get acceptable operation with a series resistance in series with the line input, but it will have to be low ohms like 1 ohm and it will have to be able to handle 100 watts or so, depending on resistance, so that would be a large resistor. Perhaps connect ten 10 watters in series, with each resistance 0.1 ohms, with free air flow.

[Joseph]

Hi Al,

I didn't try resistors other than up to two 300 watt halogen bulbs in parallel. That apparently dropped too much voltage to allow the filament to get hot enough, or is it that the high voltage becomes too low to let the resonance begin? If the latter is the case, just dropping the high voltage to the magnetron wouldn't give the results I was thinking of.

I also tried placing some different inductances instead of resistances in series with the transformer primary winding. Dropping about 10 to 20 volts seemed to draw less power as observed by less dimming of the compartment illuminating lamp. I had tested some water heating with and without the inductors and was surprised that the radio power seemed not to drop as much as expected. However with the greater voltage drop that I had gotten from the halogen lamps, the water just barely seemed to be absorbing some electromagnetic energy.

[MrAl]

Hi Joe,


Yes it is a fairly exacting thing. The dropping resistance has to be just right, so you may have to use several 0.1 ohm 10 watt resistors in series and try leaving some out, adding some more, etc., until it comes out right.

With my past two microwave ovens (and i have seen this happen this many times over the years) if the voltage drops from 120v to 110v i see some loss of power, where everything takes longer to cook. If i drop it from 110v to 105v i see a lot of power loss, taking much more time to cook, and dropping to 100v i see it taking very very long just to heat something up, and dropping to maybe 95v or so i see the oven stop cooking almost entirely.
This same thing happened with my previous microwave oven too so i saw it happen with two of them already. That means the adjustment range is quite narrow, from some 90v to 110v or maybe 115v. That's the range that takes the oven from full power to no power at all!

One important thing though is that a cup of water is not that good of a test. It has to be when cooking something like a chicken leg. The water boils in almost the same time with several different power microwaves for some reason i cant remember why. I shouldnt have even mentioned using water as although that seems like a great test (because of the predictability of heating water) it does not work with microwave ovens.
Try a chicken leg or maybe hamburger meat in some normal quantity (not a tiny tiny piece) and try settings of 110v and 100v and maybe even 90v. You'll see a big difference, and if you had a variac you could set it more easily.
The resistors might work though so maybe i'll try that with mine some day too, but it has to be adjusted carefully to get the right setting and of course we are assuming your line voltage is stable too, not like mine which varies quite a bit anyway.

[Robert Reed]

"and dropping to maybe 95v or so i see the oven stop cooking almost entirely."

Wow! And I thought I had Power line problems.
BTW MrAl, how did your refrigerator fare during that brownout?

[MrAl]

"and dropping to maybe 95v or so i see the oven stop cooking almost entirely."

Wow! And I thought I had Power line problems.
BTW MrAl, how did your refrigerator fare during that brownout?

Hi there Robert,


Well, it still "worked", that's about all i can say for it hee hee

I've seen the line as low as 90 volts AC in mid summer.

[Robert Reed]

The reason I asked is that one of the marinas I moored in for several summers had terrible line voltage drops due to an inadequate supply line. I secretly climbed the pole one evening and dropped both hot lines (240V) down the pole and connected them to a 4KVA 240v/120V transformer that had been discarded at my workplace.Now I didn't have near as much line drop when the boats refrigerator compressor kicked on (drawing enormous currents on its start winding). Of the 12 boats (all equipped with refrigerators) moored on my channel, mine was the only fridge that survived the summer. The rest all burned up due to never attaining the RPM to get out of their start windings!

[Joseph]

Thanks Al. I have some .5 ohm 25 watt resistors and might try about two of those since the rated power of the oven is just 600 watts.

The thing I was wondering about was why the voltage dropping just a little lowered the output so much. I was wondering if the reason the resonance stops is because the heater fliament gets too cool rather than the magnetron high voltage dropping below some threshhold. If the former was the case I wondered about possible stress on the high voltage components due to the the unloading of the transformer. I sometimes forget to read up on Wikipedia and that mentions that the degree of resonance is greatly dependent on anode voltage.

That is a bummer that your voltage browns out. It's an ironic situation for the power company, it seems. Compressors would tend to stall and increase the loading on the power lines still further.

[MrAl]

Hi again Joe,


Well ok, but keep in mind that 0.5 ohms at 5 amps only drops 2.5v, so you may have to hook several in series, like at least 4, to start to see some cooking time difference. Also, the more added the less current is drawn so it may take quite a few to see a big difference. It may take as many as 10 in series.
Nice thing is, if you have a fairly stable line voltage then you should find a resistance value that works and then whenever you use that value you will get the same cooking power so it should be repeatable.
If you rig up some switches you may even be able to set up your own power selection panel... 75 percent power, 50 percent power, etc.

I never looked into the reason why the power actually reduces like this, i just know it can be a pain when you dont want it to happen.

[Joseph]

Hi Al

Indeed, well put. I had immediately began with 4 of the .5 ohm resistors in series and am guessing that is giving about 75% of full power. I had a switch set up to short across two of them, which I haven't used, but I might try adding 4 more resistors and place switches across sets of four resistors. I found out the resistors are just 15 watt, but that is good because they are smaller but still can dissipate more than enough power.

A good thing is the line voltage here seems very stable.

[dacflyer]

is your timer not capable of power % cycling ? or is a cycling power rating not going to work for you for some reason ?

as i said earlier,, if you keep the filiment hot as it should be,, then you probably can regulate the HV side.. eitherway..the filiment has to stay hot. if you reduce the HV. you should get better results,,

[Joseph]

The timer cycling contacts are toast from it cycling on and off during the life of the oven. I could probably rebuild up the contacts with some flat copper, but it is not the easiest option to work inside the timer because of the required alignment of the moving parts, etc.

I thought about that option of just reducing the high voltage, and it is a certain way to know that the high voltage is staying below the rated voltages of the capacitor and rectifier. However, if it works OK to just reduce the voltage to the primary of the transformer so that the h.v. and filament voltage drop, I prefer that one as being far and away the easiest way to continuously reduce the power output of the magnetron. Altering the h.v. secondary circuit has to be done with extreme caution, too.

Suppose I reduce just the high voltage. I mentioned using a inductor in series with the h.v. winding. Can you think of a simpler, cheaper, easier, and more compact way?

[Joseph]

Al, may I ask why the power to your home fluctuates so much. That is definitely very inconvenient.