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I have a CBB61 capacitor from a fan that I believe might be faulty (the fan motor won't turn), but I get a reading of 0.966 μF with my multimeter. (The capacitor is rated 1 μF ±5.)

This suggests to me that the capacitor is fine, but I also get an open circuit resistance reading across it. Can both readings be true at the same time?

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    \$\begingroup\$ A cap at dc behaves as an open circuit. Once the little current the multimeter injects to measure the resistance charges up the capacitor, no more current can flow and hence, in the ohmmeter setting, it will read open circuit. \$\endgroup\$
    – Big6
    Aug 5 at 17:10
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    \$\begingroup\$ Note that the capacitance measurement doesn’t tell the whole story about how good the capacitor is. You need an ESR meter to determine the health of a capacitor. I’ve had plenty of capacitors that measure ok in capacitance but are through the roof in ESR. If in doubt, replace the capacitor as they are a common failure item. A 1uF motor start cap should only be a few $$$. \$\endgroup\$
    – Kartman
    Aug 5 at 23:57

3 Answers 3

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The schematic symbols give a good clue.

schematic

simulate this circuit – Schematic created using CircuitLab

  • The resistor symbol represents a length of resistance wire. Current will flow through a wire when voltage is applied across it.
  • The inductor symbol represents a coil of wire. When a voltage is applied across its terminals a current will rise from zero to a maximum value determined by the coil's resistance.
  • The capacitor symbol represents two metal plates separated by an insulator (air or something else). If a voltage is applied across its terminals a current will flow until the device is charged to the applied voltage. Then no current will flow.

If you use a multimeter's ohm-meter on the capacitor you might see a flicker while it charges up and then get the open-circuit indication. If you now switch to DC volts you should be able to measure the capacitor voltage and this will tell you the test voltage applied by the resistance test.

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A capacitor is open circuit at DC.

If you look at the formula for impedance of a capacitor, and if you plug in a frequency of zero because it means DC, you get denominator of zero, which means the capacitor has infinite impedance, so it has no impedance at DC.

When charged up to some voltage the multimeter tests resistance, there is no current, and multimeter shows open circuit.

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Normally a larger capacitor with a DMM across it will show increasing resistance and eventually open circuit because the capacitor charges to the DMM resistance test voltage.

How quickly this happens depends on the capacitance and the DMM probes voltage in resistance mode. With small caps, they'll charge faster than the DMM display update rate, which is often 1/3 sec or so.

If you put, say 100 uF across the DMM probes you should observe this.

For your small 1 uF, try connecting a 100K resistor in series with the capacitor and you will see the resistance increase from 100K to open circuit. Make sure you short the cap on its own first to discharge it.

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