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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.
 >
 >