Electrolytic capacitors degrade over time, leading to an increase in equivalent series resistance (ESR). This is an inherent resistance that can be thought of as being in series with the capacitor. This value is normally less than 3 ohms. When ESR increases, the capacitor does not function as designed and the electronic equipment it is installed in often fails or performs incorrectly. You can use an ESR meter to determine the ESR of a capacitor.
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Things you need
- ESR meter
- Discharge resistor
- Insulated test leads
Separate the test leads of the ESR meter so that the probes are not touching. Turn the meter on. If the meter has more than one mode, ensure that is set to ESR mode. The meter may perform an automatic calibration routine when powered on. If so, wait until it indicates it is ready to use. This may be indicated by an LED or an audible tone.
If the capacitor to be measured is in-circuit, ensure that the circuit is not energised. Unplug the electronic equipment the capacitor is installed in, if possible.
Discharge the capacitor before testing if the ESR meter does not do this automatically. Choose a high-wattage resistor with a resistance of between 5 and 50 times the rated voltage of the capacitor. Connect a test lead to each lead of the resistor. Carefully touch the other ends of the test leads to the leads of the capacitor and hold them there for a few seconds. Use a voltmeter to confirm that the capacitor is not charged. The voltmeter should show a value of zero.
Touch the ESR meter test probes to the leads of the capacitor. Read the ESR of the capacitor from the meter. It may be displayed as a deflection on an analogue scale, a level on an LED bar scale or a digital readout. Some ESR meters also provide an audible tone indicating the range of the ESR.
Determine if the ESR is acceptable by comparing the measured ESR value with the guidelines provided with the meter.
Tips and warnings
- Many ESR testers use a 9-volt battery. Make sure the battery is good before testing.
- Do not use an ESR meter on a live circuit.
- Large capacitors can store potentially lethal charges. Do not touch an in-circuit capacitor by hand without discharging it or measuring it for a voltage.
- Never work on electrical equipment alone.
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