Re: Measuring MMCs

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Steve,

Some capacitance meters will indeed be thrown off by the bleeder 
resistors.  The small DC current is enough to cause a reading change.  This 
will very greatly from meter to meter.  The more sensitive the meter, the 
more it will be affected.  Some of the nice ones that do dissipation factor 
should be able to read both the capacitance and the DC resistance well.

Good idea you have about using a series cap!

Cheers,

         Terry


At 07:42 PM 8/7/2004, you wrote:
>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.