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Old 02-06-2007, 08:52 PM
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Some DMMs have modes for capacitor testing. These work fairly well to
determine approximate uF rating. However, for most applications, they do
not test at anywhere near the normal working voltage or test for leakage.
However, a VOM or DMM without capacitance ranges can make certain types of tests. For small caps (like .01 uf or less), about all you can really test is for
shorts or leakage. (However, on an analog multimeter on the high ohms scale
you may see a momentary deflection when you touch the probes to the
capacitor or reverse them. A DMM may not provide any indication at all.)
Any capacitor that measures a few ohms or less is bad. Most should test
infinite even on the highest resistance range. For electrolytics in the uF range or above, you should be able to seethe cap charge when you use a high ohms scale with the proper polarity - theresistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with VOMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode
test) indicates that the positive lead is on the anode (triangle) and
negative lead is on the cathode (bar). If the resistance never goes very high, the capacitor is leaky. The best way to really test a capacitor is to substitute a known good one. A VOM or DMM will not test the cap under normal operating conditions or at itsfull rated voltage. However, it is a quick way of finding major faults.A simple way of determining the capacitance fairly accurately is to build
a 555 oscillator. Substitute the cap in the circuit and then calculate
the C value from the frequency. With a few resistor values, this will
work over quite a wide range.
Alternatively, using a DC power supply and series resistor, capacitance
can be calculated by measuring the rise time to 63% of the power supply
voltage from T=RC or C=T/R.
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