[TheForge] Microwave Question OT

[Grover Richardson]

-----Original Message-----
From: theforge-admin@mailman.qth.net =
[mailto:theforge-admin@mailman.qth.net]
On Behalf Of David E. Smucker
Sent: Tuesday, February 17, 2004 2:10 PM
To: theforge@mailman.qth.net
Subject: Re: [TheForge] Microwave Question OT


Grover -- thanks for the information.
First I understand about the half wave rectifier -- and that explains =
the
hum at the breaker panel. Think I will just swap out the breaker because
that is very easy for me to do.  (32 years of industrial construction
experience -- and I wired this new house and the new  shop.)  That is =
also
why I know there is nothing else on this breaker.  That being said -- I
think it is more likely that it is a microwave capacitor that is going =
bad.
It is just a lot easier to swap the breaker first.  My industrial =
trouble
shooting says that unless you are damn sure of a cause -- do the easy =
one
first before you do the brain surgery.

Yes.  Absolutely.  Glad you understand half wave too!!

The clunk is right at the very start -- and that is also when the =
breaker
trips.  I can hear the fan start, and the softer hum of the transformer =
and
then hear the louder hum as the  magnetron filaments have warmed up.  =
Yes,
this seem to be a loud hum.  What doesn't seem to make sense to  me is =
why
it only does it sometimes?  The only time it will trip except at the =
very
start is when using it to "defrost" and I have only seen this once or =
twice
and it was when the magnetron was cycled back on. =20

Ahhh.  Ok, they don't do partial power with microwave ovens.  The REAL =
way
they do low power is to cycle the magnetron on and off in about 10 =
second
periods.  So, for 50% power, it's 5 seconds on, 5 off, 5 on, 5 off.  And =
so
on.  Cheap, and the average power into the food is what you want it to =
be.

So the symptom is it kicks the breaker when it first starts up.  This is =
a
capacitor symptom.  Diodes sometimes die, but generally they only get =
"soft"
when they get hot (not at startup), but they usually just short out
altogether.

What usually happens is that the initial shot of Voltage into the =
capacitor
causes it to arc internally.  Since the relay clicks in asynchronously =
to
the line frequency (and Voltage swing) it only gets a real shot of =
initial
Voltage sometimes when you just time the "on" button push with the exact
moment when the line Voltage is maxed at 165 V peak (nominal<G>).  Other
times, it gets hit with a lower input Voltage, and it doesn't arc
internallly.

Now, this is assumption, based upon what I have seen other times.  The =
only
way to know for sure is either to measure it and catch it when it's
mis-behaving, or to replace the capacitor and the symptom goes away.

It is also possible, though unlikely, that there is an arc over =
somewhere
else inside the beast, but outside arcs tend to be more permanent and =
occur
more regularly.

We are the type of folks that mostly use the microwave to heat up coffee =
or
tea, heat leftovers and defrost things.  We very seldom use the mid =
range
settings that cycle to give a lower average output the way the defrost =
does.

Thanks for the warning about the chicken stick -- just like going into =
the
high voltage section in the days when we used to work on TV's.  (I never
worked on TV's but my Dad did a lot in the good old vacuum tube days of =
the
50's and 60's.)  Always good to know that your capacitor has been
discharged.  Oh and for what it is worth Dad always called them =
condensers
-- that is the way he learned it in school.  He was an EE, 1933 =
University
of Illinois.

Again Thanks and work safe,

Dave Smucker

Not a problem.  If you do go inside it, holler direct and I'll give =
pointers
on what to look for, and how to isolate the guilty party.
=20