Microwave Oven Capacitor Testing

[genuineguy]

Hello Experts
Need help with my experiment to test a Capacitor from microwave oven

Its a High voltage capacitor. 1mfd, 2100V AC.

A DMM test across its terminals show infinite/open resistance.

The capacitor has a 10Mohms shunt resistor internally.

1. Connected 100w bulb in series to the capacitor. AC supply 230v connected. The bulb did not light up. This proves the capacitor is NOT shorted

2. Connected capacitor in series with a diode and 10ohm, 10W resistor. Connected AC supply 230v. I was hoping DC voltage reading of 300+ volts across the capacitor, because the capacitor will charge to peak of 230v rms and hold it there... However, DMM shows 100V DC.

Any corrections to my experiment #2 ? Is the capacitor leaking ?

[bestsystem]

Re: Microwave Oven Capacitor Testing

1. May be shorted with higher resistance so the bulb cannot noticeably light up

2. Calibrate your dmm or get an extra one to cross check

[eccerr0r]

Re: Microwave Oven Capacitor Testing

In experiment #2, what if you tried a known good capacitor or what if there's no capacitor there at all, what is the voltage? Just want to also rule out a weak diode...

The reactance of a 1uF capacitor at your line frequency is a bit over 3K ohms. I'd expect a good capacitor to dimly light up a 100W@230V lamp with a 230VAC source in series. You really need a DC voltage to truly test leakage.

Right now given the information seen here that capacitor is questionable but need to confirm your testing apparatus first.

[genuineguy]

Re: Microwave Oven Capacitor Testing

I connected the diode in series with the 100w bulb. The bulb did light up, it was flickering and somewhat half the brightness than a normal 230v connection. I recognise this light as I have connected diodes in the past to 100w bulb. so i am concluding the diode is good.
I also used a 12V battery to connect to the diode and test using DMM. The forward reverse test did succeed.

So , the diode appears good to me.

I am doing this experiment to confirm my capacitor is good or bad. I do not have a new capacitor. Looks like I have to go buy a new one.

[genuineguy]

Re: Microwave Oven Capacitor Testing

Turns out the capacitor was indeed faulty. I got a new one and the microwave oven is working. I had to replace the magnetron too. Hopefully this sustains and does not fail again.

[PeteS in CA]

Re: Microwave Oven Capacitor Testing

Magnetrons and traveling wave tubes are vacuum tube and have a finite life due to the heater, which is necessary for operation. They also have a high operating voltage, often several thousand volts (the TWTs I worked with at a previous employer had a cathode voltage around -4 KV).