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capacitor



SEE MY QUESTION AT THE END
Surprised by the 1 mega-ohms impedance of the CBL probe, as reported last
week, I tested my ULI probe from Vernier and found that the lower limit
of its impedance is 20000 mega-ohms. What a difference! The operational
range of that ULI probe, however, is only 0 to 5 volts.

The method used was as follows. A capacitor of C=0.05 microfarads was
charged to 4.5 V and then discharged through the ULI probe. The data
are shown below.

time(s) 0 200 400 600 800 1000 etc.
V(volts) 4.51 3.89 3.63 3.46 3.35 ... ...

As you can verify, the initial slope gives R*C of about 1000 seconds.
This means that R is 20000 mega-ohms. I call it "the lower limit"
because the leakage resistance of my C is in parallel with the probe.
The input impedence may actually be much higher than R.

Note that I am showing only the beginning of the discharge curve. The
semi-log plot of all data is not a straight line indicating that R
grows rapidly as the voltage is lowered. The capacitor may or may not
be responsible. Does it make sense to think that the input impedence
itself is voltage-dependent?
Ludwik Kowalski