What Is ESR? Why Does It Matter in a Capacitor?
ESR (Equivalent Series Resistance) is the non-ideal, small internal series resistance inside a capacitor. In an ideal capacitor it would be zero; in reality every capacitor has some ESR.
What Is ESR?
When current flows through the capacitor, a small voltage drops across this internal resistance and power turns into heat. The lower the ESR, the closer the capacitor is to "ideal".
Why Does It Matter?
High ESR causes these problems:
- Heating: ripple current produces heat in the ESR.
- Poor filtering: high ESR can't suppress noise well.
- Low efficiency: loss rises in power supplies.
ESR by Type
| Type | ESR |
|---|---|
| Ceramic (MLCC) | Very low |
| Polymer | Very low |
| Tantalum | Low-medium |
| Electrolytic (liquid) | High |
How Is ESR Lowered?
There are two common ways: choosing a low-ESR type (ceramic, polymer) or connecting several capacitors in parallel — the ESRs of parallel capacitors decrease like parallel resistors. That's why at power-supply outputs a large electrolytic and a small ceramic are often used together: one provides the capacitance, the other the low ESR.
Common Mistakes
- Looking only at capacitance: in power circuits, a capacitor with high ESR at the same µF heats up.
- Old/worn capacitor: as an electrolytic ages its ESR rises and it fails.
- Choosing the wrong type: using a liquid electrolytic where low ESR is needed.
Frequently Asked Questions
Where is low ESR needed?+
How is ESR measured?+
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