How to measure capacity

[hpolkerman]

Hello,

I'va got a filter from a friend, but the capacitance values are scratched out of the capacitors, is there a (cheap) way to measure them? I don't have a capacitor multimeter,just a normal multimeter.

Thanks, Henri

[richie00boy]

You would need a signal generator at the least. It would probably be easier to just buy a new meter with capacitance on, they don't cost much these days.

[satterfi]

You probably have everything you need except a resistor. I use a 10 ohm, 2 watt, 0.1% resistor.

Set up the simple circuit and vary the signal until the voltage drop across the resistor and the voltage drop across the cap are the same. Then calculate the capacitance. You can do inductors this way too.

You can easily get better than 1% accuracy. Hand held meters usually are 5%.

An impedance bridge would probably be faster but cost an extra dollar.

[richie00boy]

Be wary of running in the 20Hz region this way though as most PC soundcards have roll offs around there. Also check your soundcard can drive a low load.

[hpolkerman]

So I can connect the cicuit directly to my soundcard an try different frequency's, and if the Volts across the cap and the resistor are equal, then i can measure the capacitance?

But how can I calculate the capacitance?
if the volts are the same, should i use the following:

Capacitance = ( volts across resistor / 10 (ohm) ) / ( 2 x pi x frequency in herz x volts across capacitor)

And: can i just play a sine of a value of Herz bij a program on my computer?

Grtz. Henri

[satterfi]

Capacitance = 1 / ( 2*Pi*f*Z)

Z = 10 ohm when you use a 10 ohm resistor.

I don’t drive the circuit with a sound card. I use the signal from a sound card and use that signal as the input to an amplifier. My amplifier has output meters so I can see how much power is flowing through the circuit.

I use this function generator

http://www.marchandelec.com/ftp/fg_lite.exe

It is very simple but as richie00boy has pointed out it's not perfect. You will not be able to measure a 0.1 uF capacitor with any accuracy because your sound card can't see it. You can't hear it either so I wouldn't be to concerned about it.

For audio frequencies, and the values of capacitors you will find in almost all speakers, this method will work just fine.

If you need a better explanation of what I do, email me.

Jon