To test a capacitor, follow these steps using a digital multimeter with capacitance measurement capability:
Step-by-Step Guide to Test a Capacitor
1. Power Off and Safety
- Ensure the power to the circuit is completely off.
- Discharge the capacitor safely by connecting a 20,000 Ω, 5-watt resistor across its terminals for about 5 seconds. Confirm it is fully discharged with your multimeter
2. Remove the Capacitor
- Detach the capacitor from the circuit to avoid inaccurate readings caused by other components
3. Set Up the Multimeter
- Turn the multimeter dial to the capacitance measurement mode (often marked with a capacitor symbol). Some multimeters require pressing a function button to activate this mode
4. Connect the Test Leads
- Connect the multimeter probes to the capacitor terminals: red probe to the positive terminal (usually the longer lead) and black probe to the negative terminal (shorter lead)
5. Read the Measurement
- Allow the multimeter a few seconds to stabilize and display the capacitance value.
- Compare the measured value with the capacitor’s rated capacitance printed on its case. A reading within about 10-20% tolerance is generally acceptable
- If the multimeter shows "OL" or a value far from the rated capacitance, the capacitor may be faulty
Additional Testing Methods
- Resistance Test: Set the multimeter to measure resistance (ohms). A good capacitor will show a low resistance initially, then the reading will rise towards infinity as the capacitor charges. A constant low or zero resistance indicates a shorted capacitor
- Voltage Test: Charge the capacitor with a known voltage source and measure if it holds the voltage. If the voltage drops quickly, the capacitor may be bad
Summary
- Always discharge the capacitor before testing.
- Remove it from the circuit for accurate measurement.
- Use the capacitance mode on a digital multimeter to measure its value.
- Compare the measured value with the rated capacitance to determine if the capacitor is good or faulty
This method is reliable for electrolytic and many other types of capacitors commonly found in electronic circuits.
