Comparing Electrolytic Capacitor ESR and Impedance at the Relevant Frequency

An electrolytic capacitor’s impedance and equivalent series resistance are related but different quantities. Comparing a catalog impedance value with an ESR-meter reading at another frequency can produce a misleading conclusion. Define which parameter the circuit requires and how it was measured.

Read the parameter label

Nichicon’s aluminum-electrolytic technical description presents an equivalent circuit and explains the frequency dependence of capacitance and impedance. Use the selected series’ actual limits and test conditions when evaluating a part rather than treating the general model as a guaranteed curve.

Record whether a table specifies ESR, impedance magnitude, dissipation factor or capacitance. Include frequency, temperature and the relevant tolerance or maximum limit. These labels should remain attached to values when they are copied into a comparison worksheet.

Relate the parameter to the application

Identify the frequencies and current components that matter to the circuit. A bulk energy-storage requirement, a regulator stability requirement and high-frequency ripple suppression can depend on different parts of the capacitor’s behavior. Do not choose one catalog number as a substitute for all three checks.

For illustration, a low-frequency measurement can be dominated by capacitive reactance, while an impedance minimum near resonance can be closer to the resistive contribution. The exact transition depends on the component and mounting, so use data applicable to the selected part.

Compare candidates under common conditions

Request or measure impedance and ESR over the relevant range when the available single-frequency values do not support the comparison. Include cold operation where required by the product. Keep measured typical behavior separate from supplier-guaranteed limits.

Review the connected circuit before making an in-circuit measurement. Parallel capacitors and other branches can cause an instrument to report the network’s impedance rather than that of the component under investigation. Record whether a result comes from a removed part or the assembled board.

Check the consequence of substitution

Verify the regulator’s documented capacitor requirements and observe ripple, load-step response and temperature with the candidate fitted. A lower ESR value is not automatically an improvement for every control loop. Retain the exact test load and operating conditions.

Keep parameter definitions, curves and bench results with the approved replacement record. Revisit the comparison when the switching frequency, temperature range or capacitor arrangement changes. This prevents an apparently superior number measured under one condition from becoming the sole reason to approve a part whose behavior differs in the frequency region that actually controls the product’s performance and reliability.