Re: Measuring MMCs

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

Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> 


We never use bleeders.  I've never found an MMC that has held AC charge.  I
know Terry has proved it's true, but we've probably built over 30 small
coils with MMCs and never used any bleeder resistors.

Dr. Resonance

 >
 > 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.
 >
 >
 >