Microwave oven HV cap. Working to shorted overnight?

The HV capacitor is NOT an electrolytic. It will likely fail spectacularly (at least from the cap's perspective). All it would take is a pinhole breakdown between the plates. However, a visible arc is strange. You sure it wasn't the fuse blowing again?

Probably, but it could still fail at full voltage.

BTW, just because your background is in electronics, there is a big difference between a microwave oven with very lethal up to 5000 V and and an AMP or more of available current - and 3.3 V logic even if the power supply can deliver 100 amps. :)

DO make sure you understand the safety issues involved in working on a microwave oven, by far the most dangerous piece of electronic equipemtn in the home to repair.

--- sam | Sci.Electronics.Repair FAQ:

Repair | Main Table of Contents:

+Lasers | Sam's Laser FAQ: | Mirror Sites:

Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

I am going to re-iterate that you FULLY understand the safety when working on a microwave oven. The ONLY technicians that I know of that were killed while working on some equipment were working on a microwave oven.

Even in the off state, the capacitor will hold a total charge at a voltage high enough to immediately stop your heart. Roughly 4.5 Joules of energy at 1.5KV is on that puppy when charged up. The lowest defibrillator setting used in Hospitals today is 2.5 Joules!!!!!

As a result it is my own preference to never work on a microwave oven without a high voltage probe specially made for checking the capacitor voltage and discharging it. With the exception of the over the range hood microwaves, very few are expensive enough to be worth much in the way of a repair.

Doug,

In answer to your question: Yes, I've seen the capacitor go bad instantaneously and will clearly show as shorted in both directions when measured out of circuit using a DVM. (I am assuming your 2 ohms is out of circuit.)

Also check the magnetron resistance and the zener diode bias voltage as well. The only other thing that regularly fails in most microwaves are the door safety microswitches. Good luck and be very careful with the lethal voltages and current generated by the unit.

Bob

Checked cap for charge (dead), metered it,. 2 ohms!.. As a final test, I connected it in series with my bench supply and a #1157 automotive bulb, and it lights.. (it really is shorted.)

Has anyone seen this failure mode before? In my past experience, caps generally fail slowly.

BTW: The HV diode is good, I connected my supply across it with the curent limit set to a few mA, and ti conducts at about 7V.

-- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them.

"zener bias voltage"? Huh? :)

--- sam | Sci.Electronics.Repair FAQ:

Repair | Main Table of Contents:

+Lasers | Sam's Laser FAQ: | Mirror Sites:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

My 15yr old Sears did exactly the same thing a few months ago. Fuse blows like a flash bulb and shorted cap. Here's a good source - reasonable prices and quick service (30% cheaper than Sears)!