How to Test a Capacitor With a Multimeter
One of the most commonly blamed parts in a non-working circuit is the capacitor; swollen or dried-out electrolytic capacitors, especially in power supplies, are the hidden cause of many faults. Fortunately, you can largely tell whether a capacitor is good with a simple multimeter. In this article we cover the most practical test methods.
Safety first: discharge the capacitor
Before testing, always remove the capacitor from the circuit (at least unsolder one leg) and discharge it. Large capacitors, especially in power supplies, can hold a high voltage even when off. Discharge them safely by shorting the legs through a resistor (for example a few kΩ). A charged capacitor can both shock you and damage the multimeter.
Method 1: Capacitance mode (the most accurate)
If your multimeter has a capacitance mode (usually an 'F' or µF symbol), this is the most precise method. Select the mode, connect the capacitor to the probes and compare the reading to the nominal value on the capacitor. If the measured value is within about 10–20% tolerance of the nominal, the capacitor is healthy. If the value is much lower or reads nothing at all, the capacitor is dried out/degraded.
Method 2: Resistance (ohm) mode
If there's no capacitance mode, you can use resistance mode. A healthy capacitor shows a low resistance at first when the probes are connected (as it charges) and then the resistance gradually rises (ideally to infinity). If the multimeter shows a constant very low resistance (short), the capacitor has broken down; if it shows infinity constantly and doesn't move at all, it may have gone open-circuit. This method doesn't give an exact value but usually makes the "dead or alive" distinction.
Method 3: Visual and physical inspection
Sometimes the fault is visible. On electrolytic capacitors, a bulging (domed) top surface, leakage, rust or a burst appearance are definite signs of failure. A swollen capacitor should be replaced without even needing a multimeter test. That's why starting the test with a visual inspection always saves time.
ESR: the hidden fault
A capacitor can pass the capacitance test and still cause problems in the circuit; because in aging electrolytic capacitors the equivalent series resistance (ESR) rises. A standard multimeter can't measure ESR; you need a separate ESR meter for that. High ESR is a common cause of unexplained faults in power supplies.
Which capacitors fail most?
Not all capacitor types fail at the same rate. Electrolytic capacitors, which contain a liquid electrolyte — especially those working near heat (power supplies, motherboards, LED drivers) — dry out and lose value over time and are the type that fails most often. Ceramic capacitors, on the other hand, are far more durable; they generally don't fail except from a mechanical crack or overvoltage. That's why, when a device fails after years, the first suspect is heat-exposed electrolytic capacitors.
Choosing the right replacement
When replacing a faulty capacitor, match three values: capacitance (µF), voltage (V) and, if possible, the temperature grade. You can choose the voltage rating equal or higher, but choosing lower is dangerous. In power supplies, prefer a low-ESR capacitor. To find a suitable equivalent, check the replacement selection guide.
Common mistakes
- Not discharging: a charged capacitor is both dangerous and skews the measurement.
- Measuring in-circuit: parallel components mislead the reading; unsolder at least one leg.
- Confusing polarity: mind the + and − terminals on an electrolytic capacitor.
- Forgetting ESR: even if the value reads correct, high ESR can cause a fault.
Frequently asked questions
How do I test a small ceramic capacitor?
For small-value ceramics, capacitance mode is best; resistance mode doesn't give a meaningful result at such small values. Often a visual and short-circuit check is enough.
The capacitor tests good but the circuit still doesn't work?
It could be high ESR or another component fault; check with an ESR meter and test the other parts in the circuit too.
Do I need to test a swollen capacitor?
No; physical swelling is a definite sign of failure, so replace it directly with a new one of the same value and voltage.
Conclusion
Testing a capacitor is a simple routine that starts with a visual inspection and confirms with capacitance or resistance mode. You can eliminate a suspect part in a few minutes and avoid unnecessary part swaps. When looking for a new one, you can use search and the comparison tool to find an equivalent of the right value and voltage.