Example Data Table
| Case | Method | Inputs | ESR Result | Use |
|---|---|---|---|---|
| Output capacitor | Ripple voltage/current | 120 mV, 2.4 A | 0.05 ohm | Power supply repair |
| Electrolytic test | Dissipation factor | D 0.12, 470 uF, 100 kHz | 0.000406 ohm | Datasheet comparison |
| Lab analyzer | Impedance phase | |Z| 0.051 ohm, angle -11.3 | 0.050 ohm | Advanced impedance check |
Formula Used
Ripple method: ESR = Vripple / Iripple.
Dissipation factor method: ESR = D / (2πfC).
Q method: ESR = 1 / (2πfCQ).
Power method: ESR = P / I².
Impedance phase method: ESR = |Z| × cos(|θ|).
Reactance: Xc = 1 / (2πfC). Estimated |Z| = √(ESR² + Xc²).
Heating: P = I² × ESR. Voltage drop = I × ESR.
How to Use This Calculator
- Select the method that matches your available measurement data.
- Enter capacitance and test frequency for reactance results.
- Enter ripple voltage and current for the direct method.
- Use dissipation factor or Q when using datasheet values.
- Add allowed ESR and rated ripple current for comparison.
- Press the calculate button and review the result above the form.
- Use CSV or PDF download buttons for reports.
Understanding Capacitor ESR
Equivalent series resistance is the small internal resistance that sits in series with an ideal capacitor. It is not printed clearly on every part, yet it affects real circuits every day. High ESR wastes power, warms the case, lowers ripple filtering, and can make regulators unstable. Old electrolytic capacitors often fail by rising ESR before capacitance looks very wrong.
Why ESR Matters
A capacitor with low ESR can handle ripple current with less heat. Power loss follows I squared times ESR, so a small resistance can still create serious heating when current is high. In switch mode supplies, motor drives, audio crossovers, and LED drivers, ESR also changes damping. Too little ESR may even cause ringing in some networks, so the right value depends on design.
Measurement Choices
This calculator supports common lab methods. The ripple method divides measured AC voltage by measured AC current. The dissipation factor method uses ESR equals D divided by two pi frequency capacitance. The Q method uses the inverse of that relationship. A power loss method is useful when a thermal or wattage reading is available. A magnitude and phase method helps when an impedance analyzer gives polar data.
Using Results Safely
Always measure at a useful frequency. Many ESR meters use high frequency, while datasheets may specify one hundred hertz, one kilohertz, ten kilohertz, or one hundred kilohertz. Temperature also matters. Electrolytic ESR usually rises in cold conditions and changes with age. Compare the result with the same frequency, temperature, and capacitor type whenever possible.
Design Interpretation
The result table gives ESR, capacitive reactance, impedance, ripple power, voltage drop, loss tangent, Q factor, and time constant. Use ripple power to estimate heating risk. Use voltage drop to judge output ripple. Use Q and loss tangent to compare capacitor quality. For repairs, a very high ESR compared with similar parts usually points to drying, leakage damage, or poor internal contact.
Practical Notes
Discharge capacitors before testing. Lift one lead when nearby parts can affect the reading. Use short test leads, clean probes, and steady frequency. For very low ESR parts, lead resistance can be larger than the capacitor ESR, so subtract fixture resistance or use a Kelvin connection for best accuracy.
FAQs
What is capacitor ESR?
ESR is the equivalent series resistance inside a real capacitor. It represents internal loss. It affects heating, ripple voltage, damping, and power supply stability.
Which method should I choose?
Use ripple voltage divided by ripple current for direct bench measurements. Use dissipation factor, Q, or impedance phase when those values come from a meter or datasheet.
Does ESR change with frequency?
Yes. ESR can change strongly with frequency. Always compare results at the same frequency used by your datasheet, meter, or circuit condition.
Why does high ESR damage circuits?
High ESR increases heat and ripple voltage. It can reduce filtering, stress switching regulators, lower efficiency, and cause unstable or noisy operation.
Can ESR be lower than expected?
Yes. Some circuits need damping. Very low ESR may cause ringing or regulator instability when compensation was designed for higher capacitor resistance.
Should I test capacitors in circuit?
In-circuit testing is convenient but can be misleading. Parallel parts and board paths may lower the reading. Lift one lead when accuracy matters.
What is a good ESR value?
A good ESR depends on capacitance, voltage rating, capacitor type, frequency, and temperature. Compare against a datasheet or a known good part.
Does ESR create heat?
Yes. Ripple current creates heat through ESR. The common heating formula is P = I² × ESR. Higher ripple current increases heating quickly.