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nunu
Joined: 07 May 2009 Posts: 1 Location: FL
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Posted: Thu May 07, 2009 11:43 am Post subject: Digital multimeter capacitance limitations!? |
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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: 2476 Location: Far, Far Away
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Posted: Thu May 07, 2009 2:00 pm Post subject: |
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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
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Posted: Tue May 12, 2009 12:58 pm Post subject: |
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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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