A capacitor can look perfect and still be the reason a power supply hums, a fan runs slow or a PLC card resets. Here are four simple bench methods, a results table and a calculator that tell you in minutes whether the part is good.
Most capacitor faults can be found with a normal digital multimeter and a little method. This guide shows how to test a capacitor safely, read the result correctly and confirm doubtful parts with an ESR meter.
How to Test a Capacitor?
Knowing how to test a capacitor means checking three things: its capacitance against the marked value, its internal series resistance and whether it is shorted, open or leaky. The quickest route is to discharge it, take it out of the circuit and read it on the capacitance range of a digital multimeter, then confirm a doubtful part with an ESR meter.
- Switch off, then discharge the capacitor through a resistor and confirm zero volts.
- Remove at least one leg from the board for an accurate reading.
- Use the capacitance mode and compare the reading with the marked value and tolerance.
- Check ESR for electrolytics, because a part with correct capacitance can still be bad.
- Reject parts that are shorted, open, bulging, leaking or far outside tolerance.
A capacitor stores charge between two plates separated by a dielectric. It fails when the dielectric breaks down, the electrolyte dries out or the internal connections open, and each failure leaves a different signature on the meter.
Fluke describes a good capacitor as one that can stay energized after power is removed, so every test starts with a discharge. Electrolytic, ceramic, film and motor run parts all follow this order, though their failure modes differ.
Safety First: Discharge Before Any Test
Fluke advises confirming that power is off with the multimeter, then holding a 20000 Ω, 5 W resistor across the capacitor terminals for five seconds. After that, measure DC voltage across the part again to prove it is really discharged.
Large capacitors need longer, because discharge follows the RC time constant. After five time constants the voltage falls below 1 percent of its starting value, which is a safe working rule.
Safe discharge time ≈ 5 × τ
Example: R = 20 kΩ, C = 100 µF
τ = 20000 × 0.0001 = 2 s
5τ = 10 s on the resistor
For 470 µF on the same resistor:
τ = 9.4 s, so wait about 47 s
Keep a 20 kΩ, 5 W resistor with two insulated crocodile clips in your tool bag as a discharge tool. Clip it across the capacitor, wait, then verify zero volts on the DC range before you touch anything.
How to Test a Capacitor with a Multimeter
Fluke explains that a multimeter measures capacitance by charging the capacitor with a known current, measuring the resulting voltage and calculating the value.
If the display shows OL, Fluke notes that either the capacitance is above the meter range or the capacitor is faulty. For very small values, use the Relative mode to subtract the capacitance of the test leads before you connect the part.
Most handheld meters measure from a few nanofarads up to a few thousand microfarads, so check the range in your manual. Very small ceramic and film capacitors below about 1 nF are better checked on an LCR meter.
| Capacitor Type | Typical Tolerance | Example Good Range |
|---|---|---|
| Aluminium electrolytic | ±20 percent | 470 µF marked: 376 to 564 µF |
| Ceramic, general purpose | ±10 or ±20 percent | 100 nF marked: 80 to 120 nF |
| Film, polyester or polypropylene | ±5 or ±10 percent | 1 µF marked: 0.95 to 1.05 µF |
| Ceiling fan motor capacitor | ±5 percent | 2.5 µF marked: 2.38 to 2.63 µF |
| Tantalum | ±10 or ±20 percent | 10 µF marked: 9 to 11 µF |
A digital multimeter capacitance reading is taken with a small DC charging signal, so it says nothing about ripple heating or internal resistance. That is why a capacitor can pass the multimeter test and still fail in a switching power supply.
Testing a Capacitor Without a Capacitance Range
Older or low cost meters have no capacitance range, but the ohm range still gives a useful go or no go test. The meter pushes a small current through the part, so a healthy capacitor charges and the reading climbs, exactly as Ohm law and the charging curve predict.
A reading that stays near zero ohms means a shorted dielectric. A reading that jumps straight to OL on a large electrolytic means an open lead or a dried out part, while a reading that stops at a steady low value points to high leakage.
ESR Test: Why Capacitance Is Not Enough
The equivalent series resistance of an electrolytic rises as the electrolyte dries. The Peak Electronic Design guide for its Atlas ESR meter notes that ESR tends to rise with age, abuse and temperature cycling, and that a lower reading is generally better for the same rating.
The same guide explains that manufacturers usually quote ESR at 100 kHz, which is also its test frequency. Its test voltage stays below a few tens of millivolts at full scale, low enough that semiconductor junctions on the board stay off.
| Capacitance | Typical ESR at 10 V Rating | Typical ESR at 250 V Rating |
|---|---|---|
| 10 µF | 20 Ω | 6.3 Ω |
| 100 µF | 2.0 Ω | 0.63 Ω |
| 1000 µF | 0.20 Ω | 0.06 Ω |
| 10000 µF | 0.02 Ω | 0.01 Ω |
These rough figures from the Peak guide apply to standard grade electrolytics at room temperature. The guide adds that good quality parts often read well below these values.
The Peak guide warns that in circuit ESR testing can give less accurate readings, and parallel capacitors show a combined ESR. Its built in discharge also works only for charges below 50 V, so higher voltage parts must still be discharged by hand.
RC Time Method With a Solved Example
When you need a value but have no capacitance range, time the charge through a known resistor. In one time constant, a capacitor charging from a steady DC source reaches 63.2 percent of the source voltage, so C equals the measured time divided by R.
t = time to reach 63.2 % of supply
Solved example: R = 10 kΩ, 9 V battery
63.2 % of 9 V = 5.69 V, reached in t = 4.1 s
C = 4.1 ÷ 10000 = 0.00041 F = 410 µF
Marked 470 µF, tolerance ±20 %
Deviation = (410 minus 470) ÷ 470 = minus 12.8 %
Result: within tolerance, capacitor is good
Use a supply well below the voltage rating, such as a 9 V battery on a 25 V part. Pick R so the time constant is several seconds, so reaction time adds little error.
Capacitor Test Calculator
Now you know how to test a capacitor even without a capacitance range, using only a resistor, a battery and a timer. Enter your own readings to check any part against its tolerance.
In Circuit vs Out of Circuit Testing
In circuit readings are tempting, but other parts in parallel, such as resistors, rectifier diodes and other capacitors, change the result. So use an ESR meter for a fast in circuit screen, then lift one leg of any suspect part and confirm it out of circuit.
Ceiling Fan and AC Motor Capacitors
In India, the most common job is checking the run capacitor of a ceiling fan, cooler or single phase pump. A typical ceiling fan uses a non polar capacitor of about 2.5 µF rated around 440 V AC, and a weak one makes the fan run slow or hum.
Discharge it, disconnect both wires and read it in capacitance mode, then compare against the printed value and its ±5 percent tolerance. The same method suits AC compressor dual capacitors and the power factor correction capacitors in panels, which need extra care because of the high voltage.
A fan capacitor that reads 1.8 µF instead of 2.5 µF still lets the fan start but cuts its speed noticeably. Replace it with the same value and an equal or higher voltage rating, never a smaller one.
Capacitor Test Results Interpretation Table
| Reading or Sign | Meaning | Action |
|---|---|---|
| Within tolerance, normal ESR | Good capacitor | Refit and use |
| Well below marked value | Dried electrolyte or ageing | Replace |
| Near zero ohms on ohm range | Shorted dielectric | Replace and find cause |
| Instant OL on a large part | Open lead or dried out | Replace |
| Correct value, high ESR | Aged electrolytic | Replace in power circuits |
| Bulging vent or brown residue | Internal gas or leakage | Replace immediately |
Treat a shorted capacitor as a symptom as well as a fault, because something drove it there. Check for overvoltage, reversed polarity, excessive ripple current or heat, especially in ageing power supplies inside hot control panels.
Capacitor Testing Myths
Capacitor Test Checklist
- Power off and voltage verified with the meter.
- Capacitor discharged through a resistor and zero volts confirmed.
- At least one leg lifted from the board.
- Correct range selected and test leads nulled for small values.
- Polarity observed on electrolytic and tantalum parts.
- Reading compared with marked value and tolerance.
- ESR checked on electrolytics in power circuits.
- Replacement has equal or higher voltage and temperature rating.
Use this checklist with an oscilloscope when you need to see the real effect in the circuit. Excess ripple on a DC rail, before and after the replacement, is the clearest proof that you fixed the right part.
- Uses a meter every technician already has.
- Fast, safe and repeatable when done out of circuit.
- Finds shorted, open and badly drifted parts.
- Works for fan, motor and electronic capacitors.
- Does not measure ESR or ripple capability.
- Inaccurate in circuit because of parallel parts.
- Limited range for very small values.
- Does not show leakage at rated voltage.
Atlas ESR Meter User Guide PDF
Video on Capacitor Testing With a Multimeter
How to Test a Capacitor FAQ
A bad capacitor reads well outside its tolerance, shows a short or open on the meter, or has high internal resistance. Visible signs include a bulging top, leaking electrolyte or brown residue on the board.
In the circuit, you may see extra ripple, humming, slow fans or random resets. Confirm the fault with a meter before you replace the part.
Yes, most digital meters have a capacitance range that reads the value directly after the part is discharged. Remove it from the board for an accurate result and compare it with the marked tolerance.
Meters without that range can still use the ohm method to spot shorted or open parts. Neither method checks internal resistance, so add an ESR test for electrolytics.
A good capacitor reads within its printed tolerance, which is often plus or minus 20 percent for electrolytics. For example, a 470 µF part should read anywhere from about 376 to 564 microfarads.
Film and fan capacitors usually have tighter tolerances of 5 or 10 percent. Its ESR should also be close to or below the typical value for its size.
Use the high ohm range and watch the reading rise from low to open as the part charges. A steady low reading means a short, and an instant open reading on a large part means it is dead.
For a number, charge it through a known resistor and time the rise to 63.2 percent of the supply. Divide the time by the resistance to get the capacitance in farads.
Yes, this is common with aluminium electrolytics in switching power supplies. Their capacitance can still read close to normal while the internal series resistance has risen several times.
The high resistance causes internal heating and extra ripple, so the circuit misbehaves even though the value looks fine. An ESR meter test at 100 kHz finds these hidden failures quickly.
An ESR meter works fairly well in circuit because its test voltage is too low to switch on diodes and transistors. A capacitance or ohm reading in circuit is usually affected by parallel parts.
Use the in circuit test as a quick screen only. Lift one leg of any doubtful capacitor and repeat the test out of circuit before you decide.
Switch off the supply and prove zero volts with the meter first. Then connect a resistor of about 20 kΩ and 5 W across the terminals using insulated clips.
Wait at least five time constants for large parts, then measure again to confirm the voltage is close to zero. Never short the terminals with a screwdriver or bare wire.
Related Articles
- Capacitor ESR Equivalent Series Resistance
- Capacitor Types Explained
- Electrolytic vs Ceramic vs Tantalum Capacitor Failure Modes
- Capacitor Working Principle
- RC and RL Time Constant Explained
External References
- Atlas ESR Meter User Guide, Peak Electronic Design
- How to Measure Capacitance with a Digital Multimeter, Fluke
- Equivalent Series Resistance, Wikipedia
What We Learn Today
- Learning how to test a capacitor starts with a safe resistor discharge and a zero volt check before the meter is set to capacitance.
- A good capacitor reads within its printed tolerance when tested out of circuit, while shorts, opens and large drifts all mean the part must be replaced.
- Electrolytics can show correct capacitance yet fail through high ESR, so an ESR meter at 100 kHz completes the test.
