ESR meter readings describe how much a capacitor behaves like a resistor, which is a separate question from how much capacitance it has. A part can measure close to its marked value and still be useless in a switching supply because its losses have risen. Knowing which property you are testing determines whether the result means anything.
What equivalent series resistance represents
A capacitor’s equivalent series resistance is a single number standing in for all the resistive losses in the part: the leads and terminations, the foil or plate structure, the electrolyte in a wet electrolytic, and the dielectric’s own losses. It is called equivalent because it is a model, not a physical resistor you could find inside. When alternating current flows through the capacitor, that resistance dissipates power and produces a voltage drop in phase with the current. In a power supply output filter, this is why a capacitor with high ESR fails to smooth ripple even though its capacitance reads fine: the ripple current develops a voltage across the resistance that appears directly on the rail. It also explains why such a capacitor gets warm in service, which accelerates its decline.
Why an ESR meter uses a high test frequency
Measuring resistance in the presence of a large capacitive reactance is difficult, because the reactance swamps the resistance at low frequencies. Raising the test frequency shrinks the reactance until the resistive component is a measurable fraction of the total impedance. That is why ESR meters test at frequencies well above the power line rate rather than at DC. It also means the number depends on the frequency used, and different instruments testing at different frequencies will report different values for the same capacitor. When comparing against a datasheet figure, check what frequency that figure applies to. Comparing a reading from one meter against a reading from another is only meaningful if both test similarly, which is why many technicians compare against a known-good part of the same type instead.
Temperature changes the reading substantially
Wet electrolytic capacitors rely on an electrolyte whose conductivity falls sharply as it gets cold, so ESR rises as temperature drops and can be several times higher near freezing than at room temperature. A supply that starts badly on a cold morning and works fine once warm is a classic symptom. The reverse matters for measurement: testing a capacitor immediately after removing it from warm equipment gives a lower reading than testing the same part once it has cooled to ambient. If you are comparing readings over time to track a part’s condition, take them at a consistent temperature and note that temperature. Datasheet ESR values are quoted at a stated temperature for the same reason, and manufacturers frequently include a multiplier for low-temperature operation.
Testing in circuit and what limits it
One reason ESR meters are popular is that many can test a capacitor without removing it from the board. They achieve this by using a test signal small enough that it does not turn on semiconductor junctions, so the surrounding circuit does not conduct. That works well when the capacitor is on a node with nothing else low-impedance across it. It fails when other capacitors sit in parallel on the same rail, because the meter then reads the combination, which is lower than any individual part. It also fails where an inductor or a low-value resistor bridges the measurement. The reading is still useful as a screening step: a good result on a node with several parallel capacitors does not clear any of them, but a bad result on an isolated one is strong evidence. Always discharge the circuit before connecting any meter.
Reading the result honestly
There is no universal threshold that separates good from bad. Acceptable ESR depends on the capacitance, the voltage rating, the physical size, the capacitor family and the application, and small capacitors legitimately have much higher ESR than large ones. Charts circulating online give rough expectations by value and voltage, but the manufacturer’s data for the specific series is the only authoritative source. The most reliable practical test is comparison: measure several capacitors of the same type and rating from the same circuit and look for the outlier. A part reading several times higher than its neighbors is suspect regardless of what any chart says. Combine ESR with a capacitance reading and a visual inspection for bulging or leakage before condemning a component.
What to check before you act
- Discharge the circuit fully before attaching any meter to a capacitor.
- Note the meter’s test frequency when comparing against datasheet values.
- Measure at a consistent temperature if you are tracking change over time.
- Compare against identical parts in the same circuit rather than a generic chart.
Common questions
Can a capacitor have good capacitance and bad ESR?
Yes, and that combination is one of the most common failure patterns in switching power supplies. Capacitance and loss are independent properties of the same part.
Does a low ESR reading mean the capacitor is fine?
Not by itself. Check capacitance as well, and remember that an in-circuit reading may be showing several parallel parts rather than the one you touched.
The practical takeaway
Use an ESR meter to find the failure a capacitance test cannot see, and interpret its number in context rather than against a memorized limit. Discharge first, keep temperature consistent, compare like against like in the same circuit, and confirm a suspect part out of circuit before you replace anything around it.
References and further reading
- CDC/NIOSH: electrical hazards and safe work practices
- CPSC: home electrical safety guidance
- NFPA: electrical safety resources and code background
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
