Capacitors fail more often than most people think. In power supplies, they dry out and bulge. In coupling circuits, they drift out of spec and distort signals. In motor-start applications, they can go open, causing the motor to hum but not spin. When a circuit stops working, technicians often check the capacitor first, and for good reason. It's one of the few parts that wears out predictably during normal use.

However, testing a capacitor is not always easy. A capacitor that looks fine might measure 10 times its rated capacitance, or it might show the correct value on one meter but fail on another if the test frequency is different. A capacitor that reads open on a simple meter might test fine on an ESR meter. Some problems, such as high equivalent series resistance (ESR), low insulation resistance, or occasional internal breaks, cannot be detected with basic capacitance tests.

This guide explains five practical ways to test capacitors using a digital multimeter, which most technicians and hobbyists already have. The methods range from quick checks anyone can do to more advanced techniques that reveal subtle problems. We explain what each method can and cannot detect, and when you might need a dedicated capacitor tester or ESR meter.

Before touching any capacitor with test leads, discharge it. Capacitors store energy, and even a small capacitor charged to a high voltage can deliver a painful shock. Large electrolytics in power supplies can hold lethal charge for minutes after the power is disconnected.

To safely discharge a capacitor, use a discharge resistor—not a screwdriver across the terminals. A screwdriver causes a short circuit, creating a bright spark and loud noise that can damage the capacitor, the circuit board, and your hearing. A proper discharge resistor limits the current to a safe level. A 1 kω to 10 kω resistor rated at least 2 watts works in most cases. Connect the resistor across the capacitor's terminals and wait for the voltage to drop to near zero. For high-voltage capacitors in power supplies, use a special discharge tool with insulated probes and a built-in current-limiting resistor.

After discharging, verify with your multimeter in DC voltage mode that the capacitor voltage is below 1 volt before handling. For small capacitors in low-voltage circuits, you can short the leads with a low-value resistor for a few seconds, but never short high-voltage electrolytics directly.

The easiest way to test a capacitor is to measure its capacitance. Most modern digital multimeters (DMMs) have a capacitance measurement feature, usually shown by a capacitor symbol (two parallel lines) or the letter "F" for farads. The meter sends a small AC test signal, usually a square or sine wave at a set frequency, often around 100 Hz or 1 kHz, and measures the resulting voltage drop to calculate the capacitor's capacitance, if possible. Testing the capacitor while it is still in the circuit is unreliable because other components, such as resistors, other capacitors, or IC pins, create additional paths that affect the reading. If you cannot remove the capacitor, at least disconnect one leg from the circuit to break these paths. For surface-mount capacitors, this usually means melting one solder joint and slightly lifting the pad. For through-hole capacitors, clip one lead or bend it away from the circuit board.

Set your multimeter to the capacitance mode. If the meter has manual range selection, start with the range that matches your capacitor's expected value. For example, a 100 µF capacitor should be measured on the 200 µF or higher range. Some meters auto-range, which is more convenient but sometimes slower.

Connect the test leads to the capacitor terminals. For polarized capacitors such as electrolytics and tantalums, make sure to observe the polarity if your meter requires it. Many multimeters do not need polarity for capacitance mode because the test signal is AC, but some less expensive meters may give incorrect readings if the leads are reversed. For non-polarized capacitors, such as ceramic and film capacitors, orientation does not matter.

What the Reading Means

A capacitor in good condition should measure within its rated tolerance. For electrolytics, the tolerance is typically ±20 %. A 100 µF capacitor measuring between 80 µF and 120 µF is usually acceptable. For ceramic capacitors, tolerances vary by dielectric type: C0G/NP0 capacitors are typically ±5 percent, while X7R and X5R are ±10 or ±15 percent. Film capacitors are typically within ±5% or ±10%.

If the measured capacitance is much lower than the rated value, for example, if a 100 µF capacitor measures only 40 µF, the capacitor is degraded. In electrolytics, this usually means the electrolyte has dried out, or the oxide layer is partly damaged. In ceramics, it could be a manufacturing defect or physical damage, but ceramics rarely fail this way unless they are cracked by mechanical stress or thermal shock.

If the measured capacitance is much higher than the rated value, the capacitor is not necessarily bad. Some electrolytics, especially older ones, can absorb moisture and show higher capacitance. More often, if you are testing in-circuit, parallel capacitors or other circuit elements can make the reading higher. Always check by removing the capacitor from the board.

If the meter shows "OL" (overload) or "1---" on auto-ranging meters, the capacitance is either too high for the selected range or the capacitor is shorted. Try switching to a higher range if your meter allows it. If the meter still shows overload, the capacitor is probably shorted or has very high leakage.

If the meter reads zero or near-zero capacitance, the capacitor is open. This means the internal connection is broken, the plates are no longer connected, or, for electrolytics, the electrolyte has completely evaporated. This is a common failure in old electrolytics or in capacitors that have been exposed to too much ripple current or overvoltage.

Limitations of Capacitance Mode

Capacitance mode shows the basic value but does not tell you how the capacitor works in real circuits. A capacitor can measure exactly 100 µF on a meter but still have ESR so high that it does not work well in a switch-mode power supply. The meter uses a small test signal at a single frequency, which does not impose the same stress on the capacitor as real circuit conditions do. Capacitance mode will not find high ESR, high leakage current, or occasional internal breaks. It is a good first test but not the final answer on capacitor health.

Also, remember that multimeters measure capacitance at one test frequency. A capacitor's effective capacitance can change with frequency due to small, unwanted inductance and the material's frequency response. A meter reading at 1 kHz might be a little different from one at 100 Hz. For most uses, this difference does not matter, but for precise or high-frequency circuits, it can be important.

If your multimeter does not have a capacitance mode, or if you want a quick check without changing settings, the resistance-mode test is a classic method that works with any digital or analog multimeter. It is especially useful for electrolytic capacitors because it shows how the capacitor charges using the meter's built-in test voltage.

How to Perform the Test

Set your multimeter to resistance (ohms) mode, usually marked with the omega symbol. Use a high range, such as 200 kΩ or 2 MΩ, because this test relies on watching the resistance reading change over time as the capacitor charges.

Discharge the capacitor completely before connecting the meter. If the capacitor holds any charge, the meter will initially show a very low resistance, which is normal for a charged capacitor, but it will not provide the useful charging curve you are looking for.

Connect the red lead to the positive terminal of the capacitor and the black lead to the negative terminal. For non-polarized capacitors, orientation does not matter. For polarized capacitors, make sure to observe polarity, because the meter's test voltage can apply reverse bias if connected backward, which may damage some electrolytics if left that way.

Watch the display. A good capacitor will show a low resistance value that slowly increases over several seconds, eventually reaching or approaching the overload reading (often shown as "1---" or "OL"). This happens because the multimeter sends a small DC voltage through an internal resistor. The capacitor draws current as it charges, so the meter initially shows low resistance. 

As the capacitor charges, the current drops and the resistance reading goes up. Once fully charged, the capacitor draws only a tiny leakage current, so the meter shows almost infinite resistance.

What the Reading Means

If the resistance starts low and climbs steadily to near-infinity, the capacitor is charging normally. This is the expected behavior for a good capacitor. The time it takes to reach near-infinity depends on the capacitance and the meter's internal resistance. A 100 µF electrolytic capacitor might take 5 to 15 seconds to read on a typical meter. A 1,000 µF capacitor might take 30 seconds or more to charge. A 10 nF ceramic charges almost instantly, so the meter will show infinite resistance immediately or within a fraction of a second.

If the resistance stays low and does not increase, the capacitor is shorted inside. The meter sees a direct connection between the terminals. This is a definite failure, so replace the capacitor right away. Shorted capacitors are common in power supplies that have been exposed to overvoltage or heat stress.

If the resistance starts at or near infinity and never changes, the capacitor is open. No current flows, which means the internal plates are disconnected, or the dielectric is completely damaged. This is common in very old electrolytics that have dried out, or in capacitors that have been exposed to mechanical shock that broke the internal connections.

If the resistance goes up but then stops at a certain value instead of reaching infinity, for example, 50 kΩ or 500 kΩ, the capacitor has too much leakage current. Some leakage is normal, especially for electrolytics, but the meter should eventually show overload or very high resistance. If the resistance stays in the middle range, the insulating material inside is damaged. The capacitor might still work in some circuits, but it will waste power as heat and could fail soon. In precise circuits, this capacitor is already considered bad.

Limitations of Resistance Mode

Resistance mode is a basic test, not a precise one. It tells you whether the capacitor is charged, shorted, or open, but it does not give you the capacitance value. You cannot tell a 100 µF capacitor from a 1,000 µF capacitor with this test alone unless you time the charging curve with a stopwatch and know the meter's internal resistance, which is not practical.

Resistance mode also cannot detect high ESR or subtle problems. A capacitor with dried-out electrolyte might still charge to infinity in resistance mode because the DC test voltage is low and the current is very small. But in a real circuit operating at 100 kHz with high ripple current, the same capacitor would overheat and fail because its ESR has increased significantly. Resistance mode is a good basic test for obvious failures, but it does not show how the capacitor performs under real conditions.

The DC voltage-mode test is less well known but is very useful for detecting leakage current in electrolytic capacitors. You charge the capacitor to a known voltage and then measure how fast the voltage drops. A good capacitor holds its charge for a long time, while a leaky one loses voltage quickly.

How to Perform the Test

Set your multimeter to DC voltage mode. Select a range that covers the voltage you plan to apply. For a 25-volt-rated capacitor, the 20-volt or 50-volt range is appropriate.

Use a DC power supply, a battery, or even the meter's resistance mode to charge the capacitor to a known voltage. The safest way is to use a current-limited bench power supply set to about 50-70% of the capacitor's rated voltage. For example, charge a 25-volt capacitor to 12 or 15 volts. Never exceed the rated voltage.

Charge the capacitor for a few seconds, then disconnect the charging source. Immediately connect the multimeter across the capacitor terminals in voltage mode. Record the initial voltage, then monitor the reading for 30 seconds to 5 minutes, depending on the capacitor size.

What the Reading Means

A good electrolytic capacitor should hold its voltage with only a small drop. A slight drop of a few percent over a minute is normal because the meter's internal resistance loads the capacitor. For example, a 100 µF capacitor charged to 12 volts might drop to 11.8 volts after 60 seconds when measured with a meter with a 10 MΩ input impedance. This is acceptable.

If the voltage drops quickly, for example, from 12 volts to 8 volts in 30 seconds, the capacitor has high leakage current. The dielectric is damaged, or the electrolyte has degraded to the point that it no longer insulates properly. This capacitor is failing and should be replaced.

If the voltage drops to zero almost right away, the capacitor is either shorted or has a severe leakage path. Disconnect the meter and check with resistance mode to see if it is a short or just extreme leakage.

For comparison, a good ceramic or film capacitor should hold its charge for hours or even days when disconnected, because its leakage current is much lower than that of electrolytics. If you are testing a non-polarized capacitor and the voltage drops noticeably in a few minutes, it is defective.

Limitations of DC Voltage Mode

This test needs a separate voltage source or a creative workaround using the meter's resistance mode to charge the capacitor. It also takes time, especially for large capacitors. It is not a quick field test, but it is one of the most reliable ways to find leakage current without a special insulation resistance tester. This method is especially useful for power supply troubleshooting, where leaky capacitors can cause high standby current and overheating even if they measure the correct capacitance on a meter.

The continuity or diode test mode on a multimeter is not meant for capacitors, but it can give a quick functional check that many technicians use in the field. Continuity mode applies a small voltage and beeps if the resistance between the probes is below a certain threshold, typically 20-50 ohms. When you connect it to a capacitor, the meter will beep briefly as the capacitor charges, then stop once it is fully charged and the current drops to nearly zero.

How to Perform the Test

Touch the probes to the capacitor terminals. Listen for the beep and watch the display.

What the Reading Means

A good capacitor will make the meter beep briefly, usually less than a second for small capacitors and one to two seconds for larger electrolytics. The display might show a rapidly rising resistance value or a brief low-ohm reading before indicating overload. This happens as the capacitor charges from the meter's test voltage.

If the meter beeps continuously, the capacitor is shorted. The resistance between the terminals is below the continuity threshold, which means the plates are directly connected. Replace the capacitor.

If the meter does not beep at all and immediately shows overload, the capacitor is open. No current is flowing, which means the internal connection is broken. Replace the capacitor.

If the meter beeps longer than expected but eventually stops, the capacitor is charging slowly. This could mean the capacitance is much larger than you thought, or the ESR is high enough to slow the charging. Without knowing the exact capacitance, this result is unclear. It is not a definite failure, but it means you should run a capacitance or ESR test to be sure.

Limitations of Continuity Mode

Continuity mode is the simplest of all capacitor tests. It only tells you three things: short, open, or maybe okay. It cannot measure capacitance, ESR, or leakage. It is useful as a quick check when you are testing many capacitors on a bench or want to find obviously failed parts before doing more detailed tests. Do not rely on continuity mode alone to decide if a capacitor is good. It is a quick filter, not a full diagnostic tool.

Also, some multimeters have reverse-bias protection in continuity mode, which can stop the test voltage from fully charging large electrolytics. If you get odd results with a large capacitor, try resistance or capacitance mode instead.

A dedicated ESR meter is the best tool for checking capacitor health, especially for electrolytics in power supplies. ESR, or equivalent series resistance, includes all the resistive losses inside the capacitor: the resistance of the foil plates, the electrolyte or polymer, the terminal connections, and the internal wiring. As an electrolytic capacitor ages, its ESR goes up. In switch-mode power supplies, high ESR causes the capacitor to heat up under ripple current, which accelerates aging. Eventually, the capacitor cannot filter ripple well, and the power supply output becomes noisy or unstable.

A real ESR meter uses a 100 kHz test signal to measure the resistive component of the impedance. Most multimeters cannot do this directly. However, you can do an indirect ESR test using your multimeter's AC voltage mode along with a function generator or even a known ripple source in a live circuit.

How to Perform the Test (Indirect Method)

This method requires a function generator or a circuit that generates a known AC voltage at a relevant frequency. For switch-mode power supply capacitors, 100 kHz is the standard ESR test frequency. For audio circuits, 1 kHz might be more relevant. For mains filtering, 100 or 120 Hz is the right frequency.

Set up the function generator to output a sine wave at your test frequency with an amplitude of 1 volt peak-to-peak or less. Connect the capacitor in series with a known resistor—typically 10 ohms or 100 ohms, depending on the expected ESR range. Apply the test signal across the series combination.

Use your multimeter in AC voltage mode to measure the voltage across the capacitor and across the known resistor. The current in the circuit is I = V_resistor / R_known. The impedance of the capacitor is Z = V_capacitor / I. At frequencies where the capacitive reactance is much smaller than the ESR, which is true for large electrolytics at 100 kHz, the measured impedance is about the same as the ESR.

For example, if you apply a 100 kHz signal and measure 0.1 volts across the capacitor and 0.9 volts across a 10-ohm resistor, the current is 0.09 amperes. The capacitor's impedance is 0.1 divided by 0.09, which equals 1.11 ohms. For a typical 100 µF electrolytic at 100 kHz, the capacitive reactance is about 0.016 ohms, which is very small. So the measured 1.11 ohms is basically the ESR.

Compare this to the manufacturer's ESR specification. A good low-ESR aluminum electrolytic might have an ESR of 0.1 to 0.5 ohms at 100 kHz. A general-purpose electrolytic might be 1-3 ohms. If your measured ESR exceeds the datasheet value by more than double, or exceeds 5 ohms for a low-ESR type, the capacitor is degraded.

Simpler Field Alternative: The Ripple Voltage Method

If you do not have a function generator, you can measure ESR indirectly in a live switch-mode power supply. With the power supply running, use your multimeter in AC voltage mode to measure the ripple voltage across the output capacitor. Then use a clamp-on current probe or a small current-sense resistor in series with the capacitor to measure the ripple current through it. The ESR is approximately V_ripple / I_ripple. This is a rough estimate because the ripple contains multiple harmonics, but it is good enough for field troubleshooting. If the capacitor is running hot and the ripple voltage is high for the measured current, the ESR is elevated.

What Elevated ESR Means

High ESR is the primary failure mode of aging electrolytic capacitors. The electrolyte dries out, the internal connections oxidize, and the resistive losses increase. A capacitor with high ESR might still measure the correct capacitance on a multimeter because the nominal capacitance depends on the dielectric geometry, not the series resistance. But in the circuit, it cannot effectively absorb ripple current. It heats up, the electrolyte dries faster, and the failure accelerates. High ESR is also undetectable in resistance- and capacitance-mode tests, which is why dedicated ESR meters are essential for power supply repair and preventive maintenance.

If you do not have an ESR meter and cannot set up the indirect test, the best alternative is to replace the capacitor with a known-good part of the same value and voltage rating, preferably with a lower ESR specification. If the circuit behavior improves, the original capacitor was degraded. This is the oldest and most reliable test in electronics: substitution.

Choosing the right test method depends on what you are trying to learn, what equipment you have, and how much time you can spend.

If you need a quick yes-or-no answer on a pile of suspect capacitors, use the continuity beep test. It takes 5 seconds per part and identifies all shorts and opens. It will miss high ESR and subtle degradation, but it will catch the obvious failures.

If you have a few minutes and want to know whether the capacitor is anywhere near its rated value, use capacitance mode. This is the standard first test for most technicians. It will catch opens, severe degradation, and capacitance drift. It will not catch high ESR or moderate leakage, but it will give you a numerical answer you can compare to the datasheet.

If you do not have a capacitance mode on your meter, or if you want a more intuitive view of the capacitor's charging behavior, use resistance mode. Watch the numbers climb. This is the classic test that old-school technicians still prefer because it reveals the capacitor's dynamic response without requiring a specialized function.

If you are troubleshooting a power supply with high standby current, or if you suspect a capacitor is leaking but it measures fine for capacitance, use the DC voltage-mode leakage test. This is the only multimeter method that reveals excessive leakage current, which is a common failure mode in old electrolytics and in capacitors that have been thermally stressed.

If you are repairing switch-mode power supplies, LCD monitors, or motherboards, and the circuit shows ripple, instability, or overheating capacitors, you need to know the ESR. A multimeter cannot measure ESR directly. The indirect function-generator method works but is cumbersome. For serious repair work, invest in a dedicated ESR meter. They are affordable—many good models cost less than fifty dollars—and they will save you hours of guesswork and prevent rework from replacing capacitors that test fine for capacitance but fail in the circuit due to high ESR.

For technicians building or restocking a bench, here are a few considerations.

A basic digital multimeter with capacitance mode is sufficient for 80 percent of capacitor testing needs. Look for a meter that measures capacitance up to at least 100 µF, preferably 10,000 µF or more for power supply work. The ANENG, UNI-T, and Mastech brands offer affordable meters with capacitance ranges up to 200 µF for under 30 dollars. For professional use, the Fluke 87V or 117 measures capacitance to 10,000 µF and is built to survive industrial environments, though it costs significantly more.

For ESR testing, a dedicated ESR meter is the best investment. The Peak Atlas ESR70 is a popular handheld model that measures ESR and capacitance simultaneously, displays the result in milliseconds, and includes a table of typical ESR values for common capacitors. It is priced in the mid-range and is widely used by repair technicians. Lower-cost alternatives from brands like Signstek and Micsoa are available online and work well for hobbyists and occasional repair work.

An LCR meter is the professional-grade tool for capacitor characterization. It measures inductance, capacitance, resistance, and often ESR and dissipation factor (DF) at multiple frequencies. The DER EE DE-5000 and the Keysight U1733C are examples at different price points. An LCR meter is overkill for basic troubleshooting but essential for component-level design, incoming inspection, and failure analysis.

Sometimes the capacitor tests fine on the bench, but the circuit still does not work. Before you blame the capacitor, consider these measurement pitfalls.

In-circuit testing is the most common source of false readings. If you measure capacitance with the capacitor still soldered to the board, parallel components can add or subtract from the reading. A parallel resistor will discharge the capacitor during the test, making it look leaky. A parallel inductor or transformer winding can create resonant effects that confuse the meter. A parallel diode can clamp the meter's test voltage. Always remove one lead if possible, or at least verify suspicious readings by lifting the capacitor.

Temperature matters. A cold capacitor has different characteristics than a warm one. Electrolytics, in particular, show higher ESR at low temperatures. If you are testing a capacitor that just came out of a freezer or was shipped in winter, let it warm to room temperature before judging the results.

Test frequency matters for some dielectrics. Class II ceramic capacitors have frequency-dependent capacitance due to dielectric relaxation. A meter reading at 100 Hz might differ from one at 1 kHz. This is normal and not a failure. For precision applications, note the test frequency and compare it to the datasheet conditions.

Polarized capacitors can be damaged by reverse test voltage. If your multimeter's capacitance mode applies a DC-biased AC signal, or if you use resistance mode with the leads reversed, you might stress the capacitor. Always observe polarity for electrolytics and tantalums, and if you are unsure about the meter's test signal, consult the manual.

Finally, remember that some capacitors may require reconditioning after long storage. A capacitor that has been on a shelf for five years might initially show high leakage and low capacitance, but after being powered in-circuit for a few hours, the oxide layer reforms and the parameters improve. This is particularly true for large aluminum electrolytics. If you are testing old stock, apply the rated voltage through a current-limiting resistor for an hour before final testing.

Q1: Can I test a capacitor without removing it from the circuit? 

You can, but the results are often unreliable. Parallel components create alternate current paths that distort the reading. A parallel resistor makes the capacitor look leaky. A parallel capacitor adds to the measured capacitance. Active components, such as ICs, can clamp the meter's test voltage. For accurate testing, disconnect at least one lead of the capacitor from the circuit. If you must test in-circuit, use the continuity or resistance mode as a quick screen, but verify any suspicious results with the capacitor removed.

Q2: Why does my multimeter show a different capacitance value than the capacitor's rating? 

Several factors can cause this. First, the capacitor's tolerance: a 100 µF capacitor rated at ±20 percent could legitimately measure anywhere from 80 to 120 µF. Second, the test frequency: different meters test at different frequencies, and capacitance varies with frequency for some dielectrics. Third, DC bias effect: for Class II ceramic capacitors, the meter's test voltage can reduce the apparent capacitance. Fourth, temperature: capacitance changes with temperature, especially for Class II ceramics. Fifth, the capacitor might actually be degraded. Always compare the reading to the tolerance range on the datasheet.

Q3: What does it mean if a capacitor reads infinite resistance in resistance mode? 

It means no current is flowing between the terminals. For a small capacitor, this could be normal because it charges instantly and then draws no current. For a large electrolytic, it suggests the capacitor is open—the internal plates are disconnected, the electrolyte has completely evaporated, or a lead has detached internally. Compare the behavior to a known-good capacitor of the same value. If the good capacitor shows a charging curve and the suspect shows infinity immediately, the suspect is open.

Q4: Can a capacitor test good on a multimeter but still fail in the circuit? 

Yes, absolutely. This is the most important limitation of multimeter testing. A capacitor can measure the correct capacitance and show normal charging behavior but still have high ESR, high leakage under load, or intermittent internal connections that only appear under real operating conditions. High ESR is the most common hidden failure in power supply capacitors. It does not affect the capacitance measurement, but it makes the capacitor ineffective at filtering ripple current. If you suspect a capacitor is bad but the multimeter tests are inconclusive, substitute it with a known-good part or test it with an ESR meter.

Q5: How do I test a surface-mount capacitor? 

Surface-mount capacitors are more challenging because of their small size. For 0603, 0805, and larger packages, you can touch the multimeter probes directly to the capacitor pads if they are accessible. For smaller packages like 0402 or 0201, you may need fine-tipped probes or needle probes. Be careful not to short adjacent pads with the probe tips. For in-circuit testing, a pair of SMD tweezers with meter connections is helpful. If you need to remove the capacitor for testing, use a hot air station or two soldering irons to melt both pads simultaneously. Never pry an SMD capacitor off with force, as you can crack the ceramic or damage the PCB pads.

Q6: What is the best way to test a capacitor in a high-voltage circuit?

Safety is the first priority. Ensure the circuit is fully de-energized and the capacitor is discharged before testing. For large high-voltage capacitors, such as those in power supplies, inverters, or motor drives, use a discharge resistor or a dedicated discharge tool. Never short the terminals with a screwdriver. After discharging, verify with a voltmeter that the voltage is below 1 volt. Then test with capacitance mode or resistance mode as described. For high-voltage film or ceramic capacitors, the same methods apply, but be extra cautious with the voltage rating and residual charge.

Q7: How do I test a capacitor for leakage current? 

The DC voltage-mode test described in Method 3 is the best way to measure leakage with a multimeter. Charge the capacitor to a known voltage, disconnect the source, and measure the voltage drop over time. A good capacitor should hold its voltage with minimal drop. A leaky capacitor will lose voltage quickly. For a more precise measurement, you can calculate the leakage current from the rate of change of voltage: I_leak = C × (ΔV / Δt). For example, a 100 µF capacitor that drops from 10 volts to 9 volts in 60 seconds has a leakage current of 100 × 10⁻⁶ × (1/60) = 1.67 µA. Compare this to the datasheet leakage specification, which is typically given as I_leak = k × C × V, where k is a constant like 0.01 to 0.03.

Q8: Why do some old capacitors measure higher capacitance than rated? 

Old aluminum electrolytics can absorb moisture over time, especially if the seal is compromised. This increases the effective dielectric constant and raises the capacitance. It is usually a sign of degradation rather than improvement. The capacitor might also have high leakage and elevated ESR. Do not rely on a high capacitance reading as a sign of health. Replace old electrolytics that show unusual readings, especially if they are in critical circuits.

Q9: What is the difference between ESR and resistance? 

Resistance is a pure DC property: it opposes current flow regardless of frequency. ESR is the resistive component of a capacitor's impedance at a specific AC frequency. It includes the resistance of the plates, the electrolyte, the terminals, and any internal connections. A capacitor can have very high DC resistance (good, meaning low leakage) but also high ESR (bad, meaning it cannot handle ripple current). That is why a capacitor can measure fine in resistance mode but fail in an ESR test. ESR is frequency-dependent and typically specified at 100 kHz for electrolytics and 1 kHz for general-purpose capacitors.

Q10: Where can I buy a good multimeter for capacitor testing? 

Basic digital multimeters with capacitance measurement are available from all major electronics distributors, including DigiKey, Mouser, and Amazon. For professional repair work, consider the Fluke 87V or 117 for durability and accuracy. For dedicated ESR testing, the Peak Atlas ESR70 is widely recommended by repair technicians. For a full LCR meter with ESR and dissipation factor measurement, the DER EE DE-5000 offers excellent value. For competitive pricing and assistance with test equipment selection, contact Welllinkchips directly.