ESR (equivalent series resistance) is the small internal resistance inside every capacitor, and you can measure it with a dedicated ESR meter, an LCR meter, or a function generator paired with an oscilloscope. The method you choose depends on the tools you have and whether the capacitor is still soldered to a circuit board. Most dedicated ESR meters give you a reading in seconds, while the oscilloscope method takes more setup but works with equipment many electronics hobbyists already own.
What ESR Actually Tells You
Every capacitor has some resistance baked into its construction, from the leads, internal connections, and the dielectric material itself. This resistance is tiny in a healthy capacitor, often well under 1 ohm for larger electrolytics. But as capacitors age or overheat, ESR climbs. That extra resistance wastes energy as heat and degrades performance long before the capacitor loses significant capacitance.
In power supply circuits, rising ESR increases output voltage ripple, which causes instability in sensitive components like microcontrollers and amplifiers. In audio circuits, high ESR introduces noise and distortion. A capacitor can still measure close to its rated capacitance on a standard multimeter and yet have ESR high enough to cause real problems, which is why measuring ESR specifically matters.
Typical ESR Values for Healthy Capacitors
ESR varies widely depending on capacitance and voltage rating. Having a rough sense of normal values helps you interpret your measurements. These figures come from a DigiKey reference chart for aluminum electrolytic capacitors:
- 4.7 µF: 25 to 62 ohms, depending on voltage rating
- 1,000 µF: 0.29 to 0.74 ohms
- 22,000 µF: around 0.04 ohms
The pattern is straightforward: larger capacitance means lower ESR. If your 1,000 µF electrolytic reads several ohms, it’s failing. Most capacitor datasheets list a maximum ESR value, and that’s the number to compare against.
Method 1: Using a Dedicated ESR Meter
A dedicated ESR meter is the fastest and easiest option. These handheld devices apply a small AC signal at a fixed frequency, typically 100 kHz, and display the resistance directly in ohms. You simply touch the probes to the capacitor’s terminals and read the number. Good ESR meters cost between $20 and $100, and popular models include the Peak Atlas ESR, the Bob Parker design clones, and various branded units from component suppliers.
The reason these meters work well is that at 100 kHz, the capacitor’s reactance (its opposition to AC based on capacitance) drops low enough that the resistive component dominates the measurement. For a 100 µF capacitor at 100 kHz, the reactive impedance is only about 0.016 ohms, so practically everything the meter reads is ESR. At lower frequencies, the capacitive reactance would swamp the resistance and make ESR harder to isolate.
Method 2: Using an LCR Meter
An LCR meter measures inductance, capacitance, and resistance, and most models can display ESR directly or report the dissipation factor (tan δ), which lets you calculate ESR. If your meter shows dissipation factor instead of ESR, the conversion is simple:
ESR = tan δ ÷ (2 × π × frequency × capacitance)
Set the measurement frequency to 100 kHz for electrolytics when possible. Some bench LCR meters offer multiple test frequencies (100 Hz, 1 kHz, 10 kHz, and 100 kHz), and your reading will change depending on which one you select. Industry convention for ESR testing uses 100 kHz, so that’s the frequency to choose if you want results comparable to datasheet specs. For very high capacitance values above a few thousand microfarads, 1 kHz or 10 kHz sometimes gives more stable readings.
Method 3: Function Generator and Oscilloscope
If you already have a function generator and a two-channel oscilloscope, you can measure ESR without buying additional equipment. This approach uses what’s called the I-V method: you pass a known AC current through the capacitor and measure the voltage across it to calculate impedance.
Setting Up the Circuit
Connect a precision resistor (10 to 100 ohms works for most electrolytics) in series with the capacitor under test. Feed a sine wave from the function generator into this series circuit. Set the frequency to 100 kHz and the amplitude to a few hundred millivolts. The function generator’s 50-ohm output impedance is part of the circuit, so keep that in mind.
Connect oscilloscope channel 1 across the capacitor and channel 2 across the precision resistor. The voltage across the resistor tells you the current flowing through the circuit (since you know the resistor’s value), and the voltage across the capacitor gives you the impedance.
Reading the Results
Measure the peak voltage on each channel and note the time difference between the two waveforms. The impedance magnitude equals the capacitor voltage divided by the current (which is the resistor voltage divided by the resistor value). The phase angle between the two waveforms lets you separate the resistive part from the capacitive part. The resistive component is your ESR.
In practice, if you’re testing at 100 kHz and the capacitor is reasonably large (100 µF or more), the phase angle will be close to zero degrees because the capacitive reactance is negligible. That means the voltage reading across the capacitor divided by the current is approximately equal to the ESR, simplifying the math considerably.
Testing Capacitors While Still on the Board
Dedicated ESR meters use very low test voltages, typically under 200 millivolts. This is low enough that semiconductors on the board won’t turn on, so diodes and transistors stay out of the measurement. That makes in-circuit ESR testing practical for troubleshooting without desoldering every suspect capacitor.
There are limits, though. If other capacitors of similar size sit in parallel with the one you’re testing, the meter reads their combined ESR, which will be lower than any individual component. Nearby low-value resistors can also skew results. In-circuit readings are best treated as a screening tool: if the ESR reads high, the capacitor is almost certainly bad. If it reads normal, the capacitor is probably fine, but parallel components could be masking a problem. When results are ambiguous, desolder one leg of the capacitor and test it out of circuit for a definitive answer.
Discharge the Capacitor First
Before connecting any meter to a capacitor, make sure it’s fully discharged. Even small capacitors in switch-mode power supplies can hold enough charge to damage your test equipment or give a false reading. For capacitors rated below 50 volts, touching a resistor (anywhere from 100 to 1,000 ohms) across the terminals for a few seconds is sufficient. For higher-voltage capacitors, use a bleeder resistor with a higher value to slow the discharge and prevent sparking. Connect one end of the resistor to each terminal and wait until a voltmeter across the cap reads zero.
A practical trick for larger capacitors: wire a small incandescent bulb in series with a resistor across the terminals. The bulb lights up during discharge, giving you a visual confirmation that current is flowing, then goes dark when the capacitor is safe to handle.
Why Your Measurement Frequency Matters
ESR is not a fixed number. It changes with frequency, and the value you measure at 1 kHz will differ from the value at 100 kHz. At lower frequencies, dielectric losses in the capacitor contribute more to the apparent resistance, so ESR readings tend to be higher. At 100 kHz, those losses diminish and you’re mostly measuring the resistance of the physical materials: the foil, the electrolyte, the terminal connections.
Capacitor manufacturers specify ESR at 100 kHz for aluminum electrolytics and tantalum types. If you’re comparing your measurement to a datasheet value, make sure you’re testing at the same frequency. Film capacitors and ceramics sometimes use 1 kHz or 1 MHz as the reference, so check the specific datasheet. Testing at the wrong frequency doesn’t mean your reading is useless, but it won’t match published specs, and you might flag a healthy capacitor as defective or miss one that’s failing.
Signs a Capacitor Has High ESR
Knowing when to pull out the ESR meter saves time. In power supplies, the most common symptom is increased ripple on the DC output, which shows up as instability, random resets in digital circuits, or visible flickering in LED drivers. Capacitors near heat sources (voltage regulators, power transistors) degrade fastest because heat accelerates electrolyte evaporation in aluminum electrolytics.
Visually, bulging tops or leaking electrolyte are obvious signs, but many capacitors with high ESR look perfectly normal from the outside. If a circuit worked fine for years and gradually developed problems, testing the electrolytic capacitors for elevated ESR is one of the most productive first steps in troubleshooting. A single capacitor with ESR two to three times its rated maximum can cause symptoms that seem unrelated to a simple passive component.