Measuring the voltage and current of a microwave oven magnetron--- make money online

I took apart my microwave oven and measured the voltage and current supplied to the magnetron. The device appears to start conducting current at 4KV, and will allow lots of current to flow once this threshold voltage is reached.

My current probe is pretty cheapo, so I wouldn't trust its measurement too much, but the final determination of 1300 W average seems pretty spot-on.

hey everyone I'm out in the shop and I

wanted to play with magnetrons so I took

my microwave apart and here's the

magnetron here this is the device that

actually creates the microwaves that

cook food and what I want to do is

characterize the voltage and current

requirements of the magnetron so I've

got some projects in mind that will come

up later but right now I'm just

measuring one of these to see what it's

how it works so a magnetron is a vacuum

tube with a filament and an anode and it

works by swirling around electrons

there's two large magnets if you've ever

taken one of these apart you'll see

there's two doughnut shaped magnets in

there and the magnetic field causes the

electrons to swirl around in a spiral

and the speed at which they're swirling

around past the number of cavities in

there determines the frequency of the

microwaves generated so pretty much all

modern ovens operate at 2.45 gigahertz

and the frequency is determined by the

number of cavities that you've got and

the magnetic field strength so here's a

terrible little schematic diagram in the

center there's a coiled filament and the

filaments are quite heavy duty in these

tubes because they're transmitting so

much power so the filament is heated up

by about maybe 4 volts AC it may be 5 to

10 amps and so it's a really powerful

film and maybe it's only 5 amps but it's

a lot for a filament it's dumping out

quite a lot of power and the electrons

are emitted from the hot filament and

they're attracted towards this outer

ring of cavities but as the electrons

make their way from the filament towards

the anode they actually start spiraling

around because this entire cavity is in

a magnetic field so as the electrons are

emitted from the centre and spiral

around they move past these cavities and

the speed at which they move past the

cavities is determined by the magnetic

field strength

so if you if you take a little tap off

one of these cavities you can draw off

some RF energy so basically just it's

sort of like blowing wind past a you

know like a reed or something or blowing

wind past an open pipe you get some

resonance going on in here and you can

take off some of the energy so this I

mean these things are you know a dime it

doesn't even get a microwave oven for

well under a hundred dollars now and

this you can you know it's over a

kilowatt of transmit power so these

these devices are actually very simple

in operation schematically they look

like this here's your two filament leads

coming in and that's the anode and this

is this little it's it's almost a

symbolic representation of this little

take off line so that's actually how you

get the RF energy out of the microwave

the circuit is almost ridiculously

simple pretty much all microwaves that

I've seen are set up just like this so

when the microwave you know decides that

it should be running you've passed all

the interlocks and the the cook timer is

running it puts power onto this

transformer 115 volts line voltage onto

this transformer and one secondary on

the transformer is high-voltage probably

about 2,000 volts maybe 2,500 open

circuit or something like that and it's

set up with this simple voltage divider

so during half the cycle this capacitor

gets charged up and during the other

half of the cycle this capacitor is put

in series with a negative voltage on

this line here so when when this line is

positive and this is negative the

capacitor is charged through the diode

then when the when the phase changes to

negative here then this capacitor is in

series with the negative voltage on this

transformer and it pumps negative

voltage into the magnetron tube while

this is all going on the film has just

kept hot all the time so the filament

has its own little winding on the

transformer

and it's it's three or four volts AC you

know five or ten amps or something like

that so what this means is that the

magnetron is actually not transmitting

half the time so if you have sixty Hertz

power during the negative half of the

cycle this isn't doing anything at all

it's just charging up this capacitor and

during the other half then the magnetron

is actually transmitting so you'll see a

little later in the video where I sort

of surprised myself I didn't even

realize this myself until I did some

more careful measurements so anyway so

I'm really interested in doing is

finding out what this graph looks like

for a magnetron if it's current versus

voltage if you put a little bit of and

let's assume the filament is already hot

and up to temperature you can put a

little bit of voltage on it

a little more a little more eventually

you know no current really flows until

you get up to about four kilovolts and

then suddenly you get tremendous current

gain and as far as I know this line just

continues essentially straight up so you

can never have a voltage drop greater

than maybe four kilovolts for this type

of magnetron you can just keep dumping

more and more current through them so

for my application I want a really high

pulse of power so what I'm going to do

is charge up a capacitor bank and dump

the capacitor bank through the magnetron

and as soon as the the applied voltage

goes over four kV you should get a

transmitted pulse and you can get much

much higher peak powers then you could

just by supplying it with a transformer

like this

okay here's my next setup so I took the

filament wire off the capacitor terminal

and soldered it in line to the high

voltage wire going to the magnetron so

this way I could keep the filament

currents out of my way I can measure

just the magnetron current going in

emission current so I've got the high

voltage probe hooked up to the line and

I've also got the current probe clamped

on to the wine should be sensing just

the emission current so let me point

this at the scope and you can see what

the start-up behavior is like okay I put

this on auto and I'll start the

microwave now

so you can hear it takes a few seconds

for to stabilize and after it comes up

to power it sort of the curve is quite

repeatable so you can sort of trust

what's going on here so I'm just going

to press single and grab a single

acquisition and we've got the same

really unusually square looking square

wave zero volts negative for kilovolts

and on the current side I switched this

to a hundred millivolts per division and

the zero is here so in the positive or

in the zero part of the waveform there's

zero current as we'd expect and in the

negative part of the waveform it kind of

gets down to about an amp and then

trails back off and it spends most of

its time at about negative one amps one

amp which is still way too much that

would mean this this thing is drawing

three kilowatts input which is far more

than it and it is probably three times

less than that so I'm still trying to

figure out how to get a resistor into

the high side the problem is that you

know I even I even put a 1 ohm resistor

there that you can see I really don't

want to float this I mean even if I had

an isolation transformer big enough to

handle the microwave floating this thing

would mean that the entire case would be

four thousand volts different from the

from ground from real earth ground so

I'd have to connect that to my scope

I could also float the scope but then

that means that the scope case would be

three or four thousand volts above

ground which were below ground whatever

it is it's going to be bad so I'd rather

not do that either so now that I look at

it this is actually probably correct

I changed the scale on the current probe

so that now we've got one volt per amp

or one division per amp on the current

trace which is one the voltage trace is

still two and it's two kilovolts per

division and a is the product of the two

so a is power

and I used the scopes measurement

feature to get the mean value for power

across one full cycle so it's gated on

these two bars here and lo and behold

1.33 kilowatts so that's actually about

right microwave ovens are you know 1200

kilowatts in and the magnetron is

probably outputting 7 to 800 watts of

microwave power but yeah 1200 1300 watts

in that's that's pretty reasonable so

what's interesting is that the current

traced lags behind the voltage trace by

quite a bit almost a full division here

and we're at 2 milliseconds per division

so I'm not sure that this actually might

be an artifact of the current

measurement probe there's probably some

low-pass circuitry in the probe that's

just causing some delay there I find it

hard to believe that the voltage jumps

right up to 4 kilovolts and then there's

2 milliseconds later the current starts

rising I don't believe that so there's

probably some inaccuracy in here but I'm

still surprised that the number there is

as close as it is to reality so what I

forgot to mention was that in this part

of the cycle when the magnetron is

actually firing sure maybe it averages

you know 3 3 kilowatts but when you

average across the whole cycle or down

to below one-and-a-half kilowatts which

which makes sense so in the when the

microwave is running half of the cycle

is used just to charge the capacitor and

then the other half of the cycle is when

the thing fires and it's drawing power

plus using the power that was stored in

the capacitor so really you could say

that your microwave oven is a four

kilowatt or three kilowatt device it's

just only running half the time

basically okay I hope that was helpful I

will keep you posted on my high-power

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