Microwave malarky.

[Paul]

To be applied only with the diode out of the circuit.

(+) band +18V (two 9V battery) -------- R1 ------ diode ----+ | GND -----------------------------------------------+

Itest = 18V - 12V / R1 = 6 / R1 [Very approximate equation]

R1 = 100K Itest = 60uA R1 = 10K Itest = 600uA R1 = 1K Itest = 6mA

V_across_diode + Plotting the diode | X forward characteristic | | X | X +-------------------- Itest

Now, reverse the diode so it is reverse biased. No current should flow. But we know that the diode does have some reverse leakage.

2uA * 100K = 0.2V , Vdiode = 17.8V, 0.2V across R1. The only effect from changing R1, is to change the drop voltage value. So across R1, we get the "reverse leakage" while a piddly 18V is applied. When there is 12kV across the diode, that's when the real leakage level is realized.

That's an example of building a test circuit, when your meter is poorly suited to the task. The meter assumes on "diode" range, that the diode is a single junction, with a 0.7 to 1.0V Vf value.

A TV repairman, spins in his swivel chair, reaches for the Tektronics curve tracer, throws the diode on it, and traces the diode characteristic to 100V or so applied voltage. Which would be acceptable for this task. Curve tracers are good to that sort of test magnitude, somewhere around 100-150V or so. And likely with fine controls, because if you were curve tracing one of the mosfets with dual gates, the ones that are "static sensitive", the gate on those blows out at 40V, and you'd be very careful to adjust the sweep parameters.

In this little experiment, we're not pushing the diode to avalanche, as that requires too much test voltage. And would blow out our multimeter :-) We're manually tracing just a portion of the curve. The nine volt batteries in the example, can't supply 350mA to "test at rated spec".

But we can at least see a little bit of "front to back" behavior, with our higher-voltage power source for test. The meter doesn't have 18V to apply to our device. I limited my experiment to 6mA, so the batteries would last.

Paul

[Paul]

My inverter microwave, operates PWM from 70% power to 100% power. It operates in "relay operated ON and OFF" duty cycle, to achieve levels such as 40% power.

The LG microwave at the store earlier this year, is able to use PWM below 70%. It represents an improvement on the range the Panasonic shows.

But at least the earlier inverter based ones, have two operating modes, PWM for continuous adjustment, then interrupted-mode for when PWM can't go low enough.

And being an inverter, there's still a diode to rectify the output. According to this, two diodes, D701 and D702. And the magnetron filament runs off the inverter transformer.

Paul

[Paul]

The fuses are Slo Blow ? You need to match the original type, for correct protection. A regular fuse may blow too quickly, and blow while the oil-filled cap is charging up.

Paul

[Chris Bacon]

They match what came out, and are like these ones:

Maybe "extra quick make sure of the job" types?

[Jethro_uk]

It might be. But as I say 15 years in ????

Also, there aren't many things I am afraid to fiddle with (happily work on gas, petrol and electricity) but f****ng up a microwave is not a risk I like to run.

[Jeff Layman]

The circuitry is also shown in that pdf I linked to. What I find interesting is that magnetron filament voltage must also vary depending on the output from the inverter. It obviously isn't critical at all, and, assuming that at full power its "correct" voltage is being applied, then at lower powers it must run cooler if the inverter voltage, and so the output from the transformer filament winding, must be at lower voltage.

[Adrian Caspersz]

Accidentality, not on purpose...

If you can't see Brian's context, then can we get a bit worried for you and kin?

[Jethro_uk]

I had forgotten that was a risk too ... good thing my instinct was to leave well alone.

Many years ago, I knew a girl who joined the RAF as an engineer. She was always picked to go into radar installations.

(Amusingly after learning about silicon at Uni, she had to be taught about thermionic valves for radar kit ....)

[Tim+]

I didn?t realise it was economically viable to replace magnetrons. I must admit, if our ancient Panasonic ever dies (about 35 years old), if possible I would like it repaired as it has a ?proper? mechanical countdown timer and a real bell that goes ?ding?. ;-)

Tim

[Chris Bacon]

New from China, a fiver. Used in the UK, £6.79. Read the serial number, internet search. Mine's the ubiquitous Galanz M24FB-610A.

[Max Demian]

I don't think it would be economical to replace the glass turntable on mine as I think they charge £25 including carriage and the oven cost £40.

In case I break it, I've lifted the turntable from one someone threw out.

[The Natural Philosopher]

So does my brand new 'catering' microwave.

And the 45? quid cheapo from Tesco that it replaced

[Tim+]

But the new timers are horrible (IMO). Our old one allows you to quickly set minutes and seconds.

(Ours is a Toshiba, not a Panasonic as I originally said). It?s the only microwave we?ve ever owned.

(stock picture, not ours).

Tim

[Jeff Layman]

Why is the turntable made from glass, and not the same metal as the rest of the microwave oven? Glass is pretty transparent to microwave frequencies, and the turntable is only a cm or so above the metal floor, so why not make the turntable from the same metal?

[Chris Bacon]

Yes, that. Mine's a steel one. There's a resistive heating element under, which allows the turntable to be used to make nice pizzas in!

[The Natural Philosopher]

so does mine, to about 10 second accuracy - who needs more?

[newshound]

While I am all for repair in general, IMHO the functionality of "expensive" microwaves (oven, grill, etc) is a waste of time. I think I replaced a fuse in one once, but they are a minefield even for experienced repairers unless you actually have a background in them. So I with TNP here, throw it away and replace with a £50 or less basic. It will go in the "metals" skip at the tip so the steel, stainless steel, and copper will all get recovered and recycled. I don't believe there will be enough on the PCB to be worth recovering, but perhaps as robots get smarter they will get plucked out before shredding too.

[Max Demian]

I suppose it's for ease of cleaning, but they could coat it with Teflon.

There must be a reason - maybe that it usually is just above the floor on a "spider", so a metal turntable would heat up unless recessed in the floor.

[The Natural Philosopher]

I don't think that is the issue. I think the issue is not to shield the underside of the food from reflected microwaves.

[Fredxx]

At a metallic boundary there will zero electric field, I guess lifting the food to be warmed away from the metallic bottom into the microwave field will help to ensure the food doesn't get a cold spot.