LCR Meters and Component Testers
A multimeter measures resistance well, and many can measure capacitance roughly, but it tells you nothing about how a component behaves with alternating current: the inductance of a coil, the equivalent series resistance (ESR) of an electrolytic capacitor, the quality factor of a filter inductor, or how a ceramic capacitor's value collapses with applied voltage. Those properties decide whether a power supply runs cool, a filter has the right cutoff, or an oscillator starts. LCR meters, ESR meters and component testers measure them.
Three ways to measure components: a bench LCR meter with four-wire Kelvin clips, an ESR meter for checking electrolytic capacitors in circuit, and a low-cost component tester that identifies parts and their pinouts.
View LargerHow an LCR Meter Works
An LCR meter applies a small sine-wave test signal to the component at a chosen frequency, measures the voltage across it and the current through it, and measures the phase angle between them. From those three numbers it calculates the complex impedance:
For an inductor X = 2πfL; for a capacitor X = −1/(2πfC)
Because it knows the test frequency, the meter converts the reactance into inductance or capacitance and reports the resistive part as series resistance, ESR, Q or dissipation factor. Bench LCR meters let you choose the frequency (commonly 100 Hz, 120 Hz, 1 kHz, 10 kHz and 100 kHz) and the test voltage; handheld meters usually offer a few fixed frequencies.
Series and Parallel Models, Q and D
A real component is never a pure L or C. The meter represents it as an ideal component with a resistor either in series or in parallel, and you choose the model that matches the physics:
- Series model (Ls–Rs, Cs–Rs): best for low impedances — inductors of a few millihenries or less, and large capacitors (above about 10 µF), where the series resistance of wires and plates dominates.
- Parallel model (Lp–Rp, Cp–Rp): best for high impedances — large inductors and small capacitors (below about 10 nF), where leakage or core loss dominates.
The quality of the component is expressed as Q (for inductors, higher is better) or its reciprocal, the dissipation factor D (for capacitors, lower is better):
A 100 µH power inductor has a winding resistance of 0.5 Ω.
At 1 kHz: X = 2π × 1,000 × 100 × 10⁻⁶ = 0.63 Ω, so Q = 0.63 / 0.5 = 1.3 — the meter is mostly measuring the wire resistance, and the inductance reading is imprecise.
At 100 kHz: X = 62.8 Ω. If the resistance (now including core loss and skin effect) has risen to 1.2 Ω, Q = 62.8 / 1.2 = 52 — a much more meaningful measurement, made close to the frequency the inductor actually works at in a switching converter.
Test Frequency and Test Level
A component's measured value depends on how it is measured, which is why datasheets always state the test conditions:
- Electrolytic capacitors are specified at 120 Hz (the ripple frequency of a full-wave rectifier on 60 Hz mains) for capacitance and D, and at 100 kHz for ESR.
- Ceramic and film capacitors are measured at 1 kHz (larger values) or 1 MHz (small values).
- Inductors should be measured near their working frequency: 1 kHz for audio chokes, 100 kHz for switching-supply inductors, much higher for RF coils.
- Class 2 ceramic capacitors (X7R, X5R, Y5V) lose capacitance with applied DC voltage and change with the AC test level. A 10 µF 6.3 V X5R capacitor in a small package can measure under 4 µF with 5 V of DC bias. Bench LCR meters with a DC bias function show this directly; a meter without one reports only the zero-bias value.
- Iron- and ferrite-cored inductors change with current: an inductor near saturation reads its full value on an LCR meter (which uses a tiny test current) but much less in the real circuit.
Connections and Compensation
At low impedances the test leads themselves matter. A good LCR meter uses four-terminal (Kelvin) connections: one pair of wires carries the test current and a separate pair senses the voltage right at the component, so lead resistance is excluded. Kelvin clips and tweezers for surface-mount parts keep this four-wire arrangement up to the jaws.
Before measuring small or low-value parts, perform open and short compensation: with the fixture open, the meter measures and remembers the stray capacitance; with the fixture shorted, it measures the lead resistance and inductance. Both are then subtracted from every reading. Without it, a 10 pF capacitor or a 100 nH inductor can read badly wrong.
ESR Meters
The commonest failure in old equipment — especially switching power supplies and monitors — is an electrolytic capacitor whose electrolyte has dried out. Its capacitance may still read close to normal, but its ESR has risen from a fraction of an ohm to several ohms, so it can no longer filter ripple and it heats up. An ESR meter measures with a small 100 kHz signal of only a few hundred millivolts, too low to turn on semiconductor junctions, so it can usually test capacitors in circuit without desoldering them.
| Electrolytic capacitor | Typical good ESR at 100 kHz | Suspect above |
|---|---|---|
| 10 µF, 25 V general purpose | 1–3 Ω | 5 Ω |
| 100 µF, 25 V general purpose | 0.2–0.5 Ω | 1 Ω |
| 1,000 µF, 16 V low-ESR | 0.02–0.05 Ω | 0.2 Ω |
| 470 µF, 10 V polymer | 0.005–0.015 Ω | 0.05 Ω |
A switching supply's output capacitor carries 2 A RMS of ripple current. When new, its ESR is 0.03 Ω: P = I²R = 2² × 0.03 = 0.12 W — negligible. After years at high temperature its ESR has risen to 0.5 Ω: P = 2² × 0.5 = 2 W, and the ripple voltage is 17 times higher. The capacitor heats, dries out faster, and eventually bulges or vents.
Component Testers
Low-cost “transistor testers” built around a small microcontroller have become popular bench tools. Connect an unknown part to any of their three test points and they identify it — resistor, capacitor, inductor, diode, BJT, MOSFET or JFET — and display its pinout and key values: resistance, capacitance with ESR, diode forward voltage, transistor hFE and VBE, and MOSFET gate threshold. They are excellent for sorting unmarked parts and checking salvaged components.
Their limits come from the very low test voltages and currents they use: they cannot test high-voltage behavior or power transistors at real currents, inductance readings are rough, and capacitors must be discharged before testing — a charged capacitor can destroy the tester. Treat their readings as identification and a quick check, not precision measurement.
Beyond the LCR Meter
An LCR meter measures at one frequency at a time. Real components change with frequency: every capacitor has series inductance and becomes inductive above its self-resonant frequency, and every inductor has winding capacitance and becomes capacitive above its own. To see that behavior, or to measure components at radio frequencies, you need an impedance analyzer or a vector network analyzer (VNA), which sweeps across a range of frequencies and plots impedance against frequency. Low-cost VNAs such as the NanoVNA have made this accessible on a hobby budget; they are covered in the Test Equipment: Advanced module.
Q, D and Reactance Calculator
Q, Dissipation Factor and Reactance Calculator
Enter an inductance or capacitance, its series resistance (ESR or winding resistance), and the test frequency. The calculator gives the reactance, impedance, phase angle, Q and dissipation factor.
Frequently Asked Questions
Why does my capacitor read a different value at 1 kHz and at 100 kHz?
Because real capacitors are not ideal. The series inductance of the leads and internal structure partly cancels the capacitive reactance as frequency rises, so the meter's apparent capacitance increases toward the self-resonant frequency; and the dielectric itself (especially in electrolytics and class 2 ceramics) has a value that genuinely falls with frequency. Always compare readings at the frequency the datasheet specifies, and measure at the frequency the part will work at if you want to know how it will behave.
Can I test components without removing them from the board?
Sometimes. An ESR meter is designed for it: its low test voltage does not turn on diodes and transistors, so it reads the capacitor's ESR even with other parts connected. An ordinary LCR meter or component tester, however, measures everything connected in parallel with the part — resistors, other capacitors, the transistors' junctions — so its readings in circuit can be badly wrong. If a reading in circuit looks suspicious, lift one leg of the component and measure again. Always switch the equipment off and discharge large capacitors first.
My 10 µF ceramic capacitor works badly in the circuit, but it measures 10 µF. Why?
The meter measured it with no DC voltage across it. In the circuit it may have several volts of bias, and a class 2 ceramic (X5R, X7R, and especially Y5V) loses a large part of its capacitance under DC bias — a small 6.3 V-rated part can lose more than half at its rated voltage. Check the manufacturer's capacitance-versus-DC-bias curve, choose a higher voltage rating or a larger package, or use a meter with a DC bias function to measure it under realistic conditions.
Should I use series or parallel mode?
Use series mode for low impedances (below about 100 Ω at the test frequency) and parallel mode for high impedances (above about 10 kΩ); in between, either works. In practice: series mode for large capacitors, small inductors and ESR; parallel mode for small capacitors and large inductors. Many meters select automatically. The inductance or capacitance reading is nearly the same in both modes when Q is high; it only differs significantly for lossy components.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.