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Re: Measuring MMCs
Original poster: "John" <fireba8104-at-yahoo-dot-com>
Hello Steve and Bart,
Same situation here. 50nF calculated and 50nF measured. I have 10 meg ohm
across each cap(also carbon).
Cheers,
John
P.S Bart, why are your caps yellow? Camera? U.V. damage?
Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Bart Anderson"
Hi Steve,
Interesting observation. As to your question, I have "not" noticed a
change. One of my MMC banks consist of 3 strings of 18 caps per string of
the (0.15uF CD942's). It calculates to .025uF and measures .025uF exactly.
Each cap includes a 10M ohm resistor (carbon) soldered across each cap.
http://www.classictesla-dot-com/photos/mmc/mmc.html
Seems something is different between your MMC and my own. Possibly cap type
or resistor type?
Take care,
Bart
Tesla list wrote:
>Original poster: "S&JY"
>To all MMC users,
>
>I always wondered why calculated and measured MMC capacitance differs more
>than expected. So I did some experiments that proved that bleeder resistors
>are the culprit.
>
>Using a capacitance meter good for 1% (5,000 count), I measured a typical
>tank cap value with various bleeder resistance values across it, and got
>these results (R in megohms, C in nanofarads):
>
> R C
>infinite 29.3 (the actual value)
>40.2 32.1
>30.1 33.1
>20.3 35.4
>10.1 45.28
>
>So you can see, bleeders can cause serious errors when measuring MMCs. No
>doubt, the results will vary depending on the type of capacitance meter.
>
>So how does one get rid of the effects of the bleeder resistors when
>measuring MMCs? It appears that the error is caused by DC current flowing
>through the bleeders. So the cure is to put another good (low leakage)
>capacitor in series to block DC. Then calculate the value of the MMC.
>
>As an example, My MMC is 15 paralleled strings of 12 22 nF caps, which
>should be 27.5 nF. My bleeder resistance tota! ls 24.3 meg. Direct
>measurement indicated 30.8 nF, which is wrong (15% high!) because of the
>bleeders.
>
>I put a 331 nF cap in series with the MMC, and that combination measured
>24.6 nF. Calculating the MMC capacitance gives 26.7 nF, which is about 3%
>low but within the 5% capacitor tolerance and believeable.
>
>(I tried using a 45 mF PFC cap in series with the MMC to avoid the need to
>calculate results. But it didn't work with my meter because the meter
>current is a tenth of a microamp and it would take almost forever for the
>two caps in series to reach equilibrium).
>
>Bottom line - don't trust capacitance measurements of MMCs that have
>bleeders, unless you add a cap in series and calculate the MMC value.
>
>Have others noticed this effect, and how did you solve it?
>
>--Steve Y.
>
>