Testing a Microwave oven Capacitor

[Matt Bennett]

I've got a microwave oven that keeps blowing it's internal 15A fuse.
After looking at just about everything in it, I'm getting suspicious
of the capacitor. It is a 2100 WVAC, 1uF can made by matsushita.
It is reading as a dead short across it's terminals,and as a dead short
on my capacitance meter (in my digital multitester). On a regular
capacitor, I'd say sure, it's dead, but I am unsure about making
this declaration with this high voltage capacitor. Do high voltage
capacitors act differently with low voltage?

And in case you were wondering, I *do* give microwaves the respect
they deserve.

Matt Bennett
m...@radix.net

[Samuel M. Goldwasser]

If it reads as a dead short out of circuit, it is bad, dead, kaput,
no doubt about it. The only difference between an 'ordinary cap'
and that one is:

1. The working voltage ratings are misleading as the peak voltage
it has to sustain is considerably higher than its working voltage
rating (like 2X or more).

2. There is usually a multi-M ohm bleeder internal to the cap (but
don't count on it to safely discharge - confirm and discharge
using a well insulated high value resistor.

For more info, see:

http://www.paranoia.com/~filipg/HTML/FAQ/BODY/Repair.html

for repair notes including Microwave ovens.

--- sam

[William K. McFadden]

Microwave oven capacitors often have internal bleeder resistors, making
them more difficult to test. I tested one with an digital ohmmeter. At
first it looked like a dead short, then the resistance increased over a few
seconds until it was a couple of megs (the value of the bleeder resistor).

BTW, that particular capacitor had developed a leakage to its metal case,
causing it to arc to the frame of the oven. Insulating it from the frame
cured the problem.

Finally, if you need to replace the capacitor, don't get one from the
manufacturer. They wanted $65 for one. MCM [800-543-4330] sells them for
about $15.

--
Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077
bi...@tv.tv.tek.com, ...!tektronix!tv.tv.tek.com!bill Phone: (503) 627-6920
CAUTION: Don't look into laser beam with remaining eye.

[JaszMan]

Matt Bennett <m...@Radix.Net> writes:

>I've got a microwave oven that keeps blowing it's internal 15A fuse.
>After looking at just about everything in it, I'm getting suspicious
>of the capacitor. It is a 2100 WVAC, 1uF can made by matsushita.
>It is reading as a dead short across it's terminals,and as a dead short
>on my capacitance meter (in my digital multitester). On a regular
>capacitor, I'd say sure, it's dead, but I am unsure about making
>this declaration with this high voltage capacitor. Do high voltage
>capacitors act differently with low voltage?

Some of these capacitors have a high voltage diode built into them. This
may interfere with your readings.

[Samuel M. Goldwasser]

Those are less common than the ones that just have a bleeder. In any
case, he says that it tests as a dead short. This is always an indication
of a bad cap.

--- sam

[ANITA CHAPMAN]

In a previous article, m...@Radix.Net (Matt Bennett) says:

>I've got a microwave oven that keeps blowing it's internal 15A fuse.
>After looking at just about everything in it, I'm getting suspicious
>of the capacitor. It is a 2100 WVAC, 1uF can made by matsushita.
>It is reading as a dead short across it's terminals,and as a dead short
>on my capacitance meter (in my digital multitester). On a regular
>capacitor, I'd say sure, it's dead, but I am unsure about making
>this declaration with this high voltage capacitor. Do high voltage
>capacitors act differently with low voltage?
>

>And in case you were wondering, I *do* give microwaves the respect
>they deserve.
>
>Matt Bennett
>m...@radix.net
>

Well. I'm no expert on HV caps, but I'd say that if it registers as a
short, and it blows fuses, replace it!

[John Fields]

ANITA CHAPMAN (acha...@cln.etc.bc.ca) wrote:

Assuming you know what you're doing, here's how to test the cap:

First, get someone to watch you in case you think you know what
you're doing but you really don't and you make a mistake and have to be
rushed to the hospital.

Next, get a line cord with a plug on one end and bare wires on the other
end and connect one of the wires to one of the terminals of a light socket.
(NOT a light socket that's already connected to the mains, and preferably
one of those nice ceramic ones you can buy at your favorite hardware store)

Next, connect the end of the other wire on the end of the line cord to one
of the terminals of the suspect capacitor.

Next, get a piece of insulated wire and strip a little of the insulation
off both ends, then connect one end to the empty terminal of the suspect
capacitor and the other end to the empty terminal of the light socket.

Next, get a 100 Watt light bulb and screw it into the light socket.

Finally, plug the line cord into a 110 volt A.C. outlet.

If the lamp lights, the capacitor is bad.
If the lamp doesn't light, either the cap is good or you've made a mistake.
If a circuit breaker shuts off the A.C. you've made a mistake.
If a fuse blows you've made a mistake.
If you die you've made a mistake.
Starship

[wa4...@vnet.ibm.com]

I should point out that this work for AC type capacitors. However, if the
capacitor is a DC type (electrolytic) type capacitor, NEVER CONNECT
IT TO AN AC SOURCE!!! Electrolytic capacitors may EXPLODE if connected
to AC! And splashing hot borax solution in your face and eyes is a sure
way to ruin your day.

Dave