Re: Measuring MMCs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bart Anderson" <tesla111-at-sbcglobal-dot-net> 

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" <youngsters-at-konnections-dot-net>
>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 totals 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.
>
>