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Digital multimeter capacitance limitations!?

 
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nunu



Joined: 07 May 2009
Posts: 1
Location: FL

PostPosted: Thu May 07, 2009 11:43 am    Post subject: Digital multimeter capacitance limitations!? Reply with quote

I purchased a Digital multimeter for $5 in a garage sale no name on it
It has a capacitance option, how do I find out the maximum capacitance it can measure?
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jts1957



Joined: 24 Nov 2008
Posts: 2475
Location: Far, Far Away

PostPosted: Thu May 07, 2009 2:00 pm    Post subject: Reply with quote

Punch in its alpha-numeric model number into your favorite search engine. Even without a "Brand" something should be found.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Tue May 12, 2009 12:58 pm    Post subject: Reply with quote

Please note that simple multimeter with a capacitance range often are useless for measuring large electrolytic capacitors, as the losses that always are present in a electrolytic capacitor causes the multimeter to give unreliable results. It is very common that it shows a much higher value than the capacitors real capacitance if the losses are high.

A better method, if you can't afford hundreds or thousands of $ for a proper RCL meter, is to measure capacitors by an oscilloscope and lissajous methods.

Connect the unknown capacitor in series with a resistor of, for example, 330 ohms. Supply this circuit with 5-10 V of mains frequency AC via a small transformer from a wall plug, if you have much distortion on the mains waveform, maybe you have to add a low pass filter with a resistor and a bipolar electrolytic. Connect the node between the capacitor and resistor to the oscilloscope's ground measure the voltage across the capacitor with the X input and the voltage across the resistor with the Y input of the scope.
Adjust the X and Y gain so you get an ellipse on the screen, by measuring the X and Y p-p voltages and doing some math, you can calculate the capacitance. Or, maybe yet simple, compare the results with a known capacitor.
If you don't get a good ellipse with its main axis in X and Y direction, but an inclined ellipse or line, then the capacitor has very high losses and should be regarded as defective.
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