Measuring MMC's
Date : Sun, 08 Aug 2004 11:53:32 -0600
Subject : Measuring MMCs
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 MMC's. 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
MMC's? 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.