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measuring volts



TWO QUESTIONS ABOUT TWO INSTRUMENTS
Question 1
I am using the MBL volt probe (Mac and ULI).
It shows 4.55 V for three D batteries in series, as it should. I charge the
2 nC capacitor to this voltage and connect it to the probe. The purpose is
to see the rate of discharing in order to determine the input impedance of
the probe. For about 40 seconds the voltage decreases monotonically. Then
it settles at about 1.55 V. It looks like the voltmeter (ULI) is preventing
the d.o.p. from going below this voltage. Another indication of this is that
the d.o.p. on the same C was growing toward 1.55V when the initial voltage
was 1.

Can somebody who knows the electronics of ULI explain this? By the way I
did subtract 1.55 volts from all readings and found that R*C is 46 seconds.
This means that the input impedance R is equal or larger that 23 giga-ohms.
Does this make sense, electronically? A fantastic voltmeter! But only up
to + or - 10 volts.

Question 2
I just learned how to use the CBL unit without the calculator. They call it
the "multimeter mode". The CBL voltage probe, attached to CH1 of the unit,
shows the same battery voltage as ULI. I wanted to find its impedance in
the same way as above but this was not possible. Onece the battery is
disconnected from the capacitor the CBL reading is a constant value of
2 volts. The capacitor may be charged to 4.5 volts or discharged to zero
but the reading is 2 volts. What am I doing wrong?. The step-by-step
procedure is as follows.

1) The voltage probe is attached to CH1 of CBL
2) The power is turned on CBL
3) The MODE button is pressed to enter the multimeter mode. Leads are
not connected to anything (in air) but the display shows 2 volts.
4) Leads are connected to each other and the dispay shows -12.3 mV.
This is nearly zero, as it should.
5) Leads are connected to the battery and the d.o.p. is correct.
6) Leads are connected to the discharged capacitor and the d.o.p is 2 V.
7) Leads are connected to a charged capacitor and the d.o.p is 2 V.

What is this 2.00 V? Can somebody confirm these observations? By the way
the electrolytic capacitor (whose leakage resistance is certainly much
smaller than for C=2 nF) behaves "normally"; I can see how it discharges
(mostly through its own R) after being disconnected from the battery.
It looks like CBL does not behave unless the R it sees is below some
value. Does this make any sense, electronically?

Ludwik Kowalski