Accurately measure C of electrolytic capacitors ?

Is there anyway to accurately measure the capacitance of an electrolytic capacitor in the range of 10uf to 100uf.

These capacitors are to be used at frequencies under 1 Hz.

Everytime I try to use my meter to measure, the capacitance will change from one range scale to the next, I guess because the frequency changes too. When I use a typical non-electrolytic capacitor these changes in readings are not there.

I have been thinking of making a simple RC inverter oscillator and using the resulting frequency to determine the true capacitance in the small signal application I want to apply them to.

Maybe there are non-electrolytic capacitors in the 10uf to 100uf range but if so I have never heard of them before.

I do not know if any of you have ever seen the SCI FI movie THIS ISLAND EARTH but I am in need of several of those fantastic capacitors.

I am trying to get a reading within 1% of reality.

Help here is appreciated.

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forget about the RC oscillator, build a simple series RC circuit and conect the input to a function generator producing 1hz output.

Find a proper value of R so the capacitor will just have enough time to charge with the incomming square wave.

monitor the charging of the capacitor using an oscilloscope, you can use the exponential equation for the charging of a capacitor in a series RC circuit and solve it for C.

For more precise results, you can monitor the capacitor charging process with a digital oscilloscope and hook up a computer to it using GPIB (IEEE-488, HPIB)

Write a program to do a "data dump" from the oscilloscope to the computer carefully look at your data and determine where 1 time constant is.

In an exponential curve, voltage changes quickly at the begining of the charge curve and then things slow down.

TAKE your measurement at the begining (t,v) if you wait until the cap is almost fully charged, a very small error in Vc can result in the wrong value of t (curve is almost horizontal)

Option 2 don't use function generator, use a constant current source instead!!

IF you charge a capacitor with a constant current source, the result is a linear ramp, and the capacitance can be obtained from simple arithmetic!!!

V = (1/C) * I * t

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Julio Rodriguez

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