Mr RB
Well-Known Member
Perhaps I misunderstood you, but there seems to be a flaw in what you said.
How does constant current negate ESR? The voltage across a cap will be Vc = Integral(i/C, dt) + i.R, where R is the ESR of the cap.
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MrAl beat me to it. My original point was that the rate of change of the voltage will be based on the current, and not affected by ESR. However my "quickie implementation" of starting from zero volts was flawed as you rightly noticed.
Ok, in the interests of simplicity, why not use a schmidt trigger oscillator of one resistor and one cap. The peak to peak voltage of the cap waveform can be seen on the 'scope, and the resistor can be measured with the multimeter.
Then it can be solved for "true capacitance" based on Vwave, Vdd, and period.
My first pico cap meter was a schmidt trigger inverter I installed in my frequency meter and all I had to do was calibrate the one resistor with a trimpot and click the meter onto "period". It worked like a charm.