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Re: MMC dissipation factor measurement
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
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.
>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...