Capacitance Measurement

156

01:55:14

Circuit + Stop watch + Math = Capacitance
Vin = 5
Vc(t) = 5 - 5e^(-t/RC)
Solve for C
C = t/((-ln((Vc(t) - 5)/-5))*R)
R = 1Meg
Vc(t) = Vin/2 = 2.5v (LED turns On) 
Estimated time ~ 7 seconds (stop watch)
C ~ 10.1uF after calculation (1.443*7)uF

Simplified..
C = -t/ln(0.5) uF
C = t/ln(2) uF
C = t/0.693 uF
C = 1.443*(time) uF
Only useful for approximations and values greater than 1uF. 
**caution** shorting large C caps might decrease the life of your button.

published 10 years ago

hurz

10 years ago

Nice, and working as you mention for 1uF caps. And bigger ones then 100uF are probably also not possible, cuz of impractical long tau and leakage current which has to taken also into account. Anyway, this circuit can be used easily to put a an arduino and is a very good start!!!!

hurz

10 years ago

How was the movie?

angel17666666766

10 years ago

Gooooooooooooooooooooooooooooooooooooooooooooooooooooooooooood

GingerKing

10 years ago

I was expecting more criticism hurz. The movie was good.

GingerKing

10 years ago

Solve for time in my second to last equation. Time = 0.693*C, when C = 100uF, time = 69.3 seconds. Yeah not practical to wait for it. Switch out the resistor so that it takes less time to charge and the cap will leak less.

hurz

10 years ago

Yes, we can increase the range by having a separate switch to lower charge resistor. But i would like to see for this method a range of capacitance you can measure practical and when this kind of measurement went into its limits. E.g. think about gold caps in Farad range? How little can the resistor be? Dont forget the high ESR of gold caps. Measuring leakage and ESR needs a different way. I still think about a universal cap-meter 1p till 1F :-) 1e-12F to 1F is a dynamic range of 240dB!!!

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