Substitution of high-voltage capacitor when repairing a microwave oven
Quick Fix
Hi, John. The source impedance of the power transformer causes the variation
in the capacitor to have a more linear effect in Controlling the power.
The more current we draw from an unregulated power transformer, the less
voltage is available and vice versa. Therefore, the "10% difference in C
would" NOT "make 20% difference in the time to cook a chicken". The
tolerance in these capacitors when purchased new is about 5%.
However, if you want the inaccuracy to work in your favor, if you cannot get
the exact cap, get the one that is 5% larger.
Respectfully
Roger
John Stagg <jo...@tt-tech.se> wrote in message
news:3707CDEA...@tt-tech.se...
> The following contribution refers to the advice given in the article
> "Troubleshooting and Repair of Microwave Ovens INDEX V2.21 (7/30/96)",
> chapter 9.5).
>
> When recently forced (by pressure from the wife and kids) to quickly
> repair our microwave oven, I referred to the excellent article mentioned
> above, part of the Repair Briefs authored by Samuel M. Goldwasser. With
> help from the brief, I could rapidly locate the fault which was a short
> in the HV diode (unfortunately located _inside_ the capacitor). So the
> capacitor needed to be ordered from a local electronics shop, and after
> a discussion with the dealer about the capacitance (there were doubts
> about the availability of a component with the exact value), I decided
> to take the dust off my engineering education and see if I could come to
> the same conclusion as stated in the brief.
>
> What spurred me to do this was the fact that the dealer had a range of
> capacitors that only differed in value by a few % of a uFarad, i.e.
> 0.90, 0.95 etc. and the brief gives the impression that such differences
> result in almost undetectable power differences. The reasoning used in
> the brief appears at first sight to be a sound way of getting a
> "ball-park" feeling of how big a roll the capacitor plays - look at its
> impedance and compare it with the effective "impedance" of the
> magnetron. OK, we're mixing phases a bit but as already said, we're only
> trying to get a feeling for it. But then, when I started thinking more
> about the actual mechanics of what's going on in the voltage doubler, it
> soon became apparent that the set-up it just too non-linear for that way
> of reasoning.
>
> So I attacked the problem from another aspect - look at the drop in
> capacitor voltage during the duty cycle. Let's user the well-worn
> formula dV=I x t/C. Using t=20mS , C=1uF and I=0.25 we get (lo and
> behold!) 5kV. What? The supply is only 5 kV. So that seems to point to
> the fact that the capacitor is giving most of its charge to the
> magnetron. That means that the relation between small changes in C and
> changes in then mean magnetron voltage is 1:1 or, in terms of power, 2:1
> (since power is proportional to voltage squared). 2:1 is a big
> difference from the repair brief that suggests 1:4 , therefore this
> would indeed explain the apparently small differences in values between
> the capacitors being offered as spares. A 10% difference in C would make
> 20% difference in the time to cook a chicken, and I would not be
> surprised if my wife noticed this!!
>
>
>
>
>
Sam Goldwasser
that 2:1 based only on the capacitor is TOO sensitive to capacitor value.
Is anyone in a position to do an accurate test with, say 3 caps: .9, 1, 1.1 uF
and an accurate microwave power measurement?
By your argument, an infinite size cap would result in infinite power - clearly
obsurd. I don't think the statement can be made that the magnetron voltage
is proportional to the uF value directly. On the other hand, power IS a non-
linear function of peak voltage so that DOES result in small increases in peak
voltage making somewhat larger increases in power. As you note, this is a
highly non-linear process (even accepting possible current limiting of the
power transformer). Forget SPICE, actual tests are needed! :)
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Mirror Info: http://www.repairfaq.org/REPAIR/F_mirror.html
| Sam's Laser FAQ: http://www.misty.com/~don/lasersam.html
John Stagg
The argument I put forward does not at all postulate infinite power with an
infinite cap - maybe I passed over this a bit quickly but I did say we were looking
at small changes ("....relation between small changes in C....") and not an overall
relation.
After reading Roger愀 contribution I would guess the truth lies somewhere between
the two extremes, but at the moment I haven't got enough chickens to do any
empirical work ;-)
Best regards,
John
Michael Caplan
>news:3707CDEA...@tt-tech.se...
>> With
>> help from the brief, I could rapidly locate the fault which was a short
>> in the HV diode (unfortunately located _inside_ the capacitor).
Is this correct? Do any microwave ps capacitors have built-in HV
diodes?
John Stagg
Yes, some manufacturers have done this, presumably to save a couple of cents
in production. But when I ordered a replacement for my Whirlpool oven they
actually supplied a "capacitor kit" consisting of 2 discrete parts (diode+cap)
and a DIY instruction how to install the thing. So I guess there may have been
a backlash from customers/serviceguys who are tired of paying 50$ when it´s a
5$ diode that´s broken....
Regards,
John
Sam Goldwasser
> >John Stagg <jo...@tt-tech.se> wrote in message
> >news:3707CDEA...@tt-tech.se...
> >> With
> >> help from the brief, I could rapidly locate the fault which was a short
> >> in the HV diode (unfortunately located _inside_ the capacitor).
> Is this correct? Do any microwave ps capacitors have built-in HV
> diodes?
Yes, some at least. This is discussed in the FAQ. I don't know how common
this is nowadays.
Sam Goldwasser
I agree. Thus the request for someone to do the tests. I would do them but
don't have a variety of caps to try (or a clean workbench). :)
And chickens are always in short supply! ;-)
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Mirror Info: http://www.repairfaq.org/REPAIR/F_mirror.html
| Sam's Laser FAQ: http://www.misty.com/~don/lasersam.html
In article <3708AE13...@tt-tech.se> John Stagg <jo...@tt-tech.se> writes:
The argument I put forward does not at all postulate infinite power with an
infinite cap - maybe I passed over this a bit quickly but I did say we were
looking at small changes ("....relation between small changes in C....")
and not an overall relation.
After reading Roger´s contribution I would guess the truth lies