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Re: MMC dissipation factor measurement

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Jim,

At 05:37 PM 5/20/2002 -0700, you wrote:
>Could one measure dissipation with a calorimeter?

This is how "I" do it.  I run 5 amps at 350kHz through it with a thermal
couple attached.  However, we were wondering if a far more common scope and
signal generator could be set up to do it.  Thus you would not need the
fancy high power signal but just common test equipment.

>And, there's a technique where you measure the Q of a tuned circuit with a 
>swept sine wave...

That has possibilities!  We are looking for a loss of about 0.05 ohms of
resistance in a typical coil MMC.

>I think, having looked over the various literature on the RLC meters, that 
>the best approach will be some form of precision voltage measurement. 
>Getting 22-24 bit measurements (6-7 digits) is plausible.  Using a scope, 
>with its 10-12 bit A/D, just isn't going to hack it (although, the decrement 
>measuring approach given in NBS circular is applicable to the scope.. and 
>there, you are combining 10,000 measurements, so the accuracy should be on 
>the order of sqrt(10,000)=100 times better than the raw measurement (i.e. 1 
>part in 409600... )

Paul's TCMA program's method may overcome this since it calculates Q from
all 10000 data points.  Small digitizing errors get "cleaned" up by a few
orders of magnitude.

Paul writes:
>Interesting.  And you'd want it to be the dissipation at the
>TC operating frequency...

Yes, dissipation factor varies much with frequency for polypropylene.

>Resonate each with a large inductor to the TC frequency
>and obtain the two Q values with the pinger and tcma.  Use the
>difference in Q factor to calculate the extra ESR introduced by the
>MMC.  This method has two problems: you have to assume the air spaced
>cap has negligible loss, and the result depends on measuring the small
>difference between two Q factors. 

This is like measuring Q mentioned above.  I wonder if TCMA could measure
the Q to a far higher precision than "hand" methods?

>And a third method that you could try is to measure that phase angle.
>Capture the V and I of the test cap at the TC frequency in some
>convenient setup and measure the time delay between the zero crossing
>points of the two waveforms.  Calibrate out the phase error of the
>scope by repeating the measurement again with the roles of the two
>Y amplifiers exchanged.  This last I think would be the least
>accurate with only 10,000 points per sweep.

I still have not figured out just how little the phase would be.  It is
complicated by inductances in the test setup wiring.  I note the $10k
meters are all four wire probed...  That may be a critical thing...