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Computer Components

How to Test a Capacitor Using a Multimeter

Before You Start

  • Discharge the Capacitor: Safety first. Capacitors sometimes hold significant charge even after a circuit is disconnected. Always discharge a capacitor by shorting its leads together with a resistor (appropriate resistance depends on the capacitor).
  • Remove from Circuit: It’s a good idea to remove the capacitor from the circuit for accurate testing. In circuit testing can be misleading because of the other components.

Methods

1. Capacitance Mode (if available)

  • Safest and most direct method.
  • Set Multimeter: Select the capacitance mode which is usually marked with a capacitor symbol. Choose a range slightly higher than the expected capacitance of the capacitor.
  • Connect Probes: Connect the multimeter probes to the capacitor terminals. Polarity matters for polarized capacitors, positive to positive, negative to negative.
  • Reading: The multimeter will display the capacitance value. Compare this to the value marked on the capacitor itself. A tolerance of 10-20% is usually acceptable.

2. Resistance Mode (Ohmmeter)

Convenient even without a capacitance mode.

  • Set Multimeter: Set your multimeter to resistance mode (ohms, Ω). Choose a high resistance range (1kΩ or higher)
  • Connect Probes: Connect probes to the capacitor terminals, polarity doesn’t matter here.
  • Observe:
  • The resistance should start low, then gradually increase towards infinity (open circuit) as the capacitor charges.
  • If the resistance stays low, the capacitor is likely shorted.
  • If the resistance stays at infinity, the capacitor is likely open.

3. Continuity Mode

Quick check for major problems, not precise capacitance measurements.

  • Set Multimeter: Choose the continuity mode indicated by a diode and/or sound wave symbol.
  • Connect Probes: Connect the probes to the capacitor terminals.
  • Observe:
  • A short beep that dies out as the capacitor charges means the capacitor is probably functional.
  • Continuous beep usually indicates a short.
  • No beep at all usually indicates an open circuit.

Important Things

  • Polarity: For polarized capacitors (electrolytic), make sure you connect the positive probe to the positive terminal of the capacitor, marked with a “+” sign.
  • Tolerance: Most capacitors have a tolerance range. A measured value within 10-20% of the marked value is usually good.
  • Special Cases: Some high capacity capacitors or specific types require more specialized testing.

Also look for physical signs like bulging, leaking or discoloration which can tell you it’s a failed capacitor even if it tests within range.

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