Equivalent Series Resistance or ESR is a resistance value representing the distributed resistances associated with a real capacitor (as opposed to an ideal capacitor). Real capacitors do not merely have a capacitance associated with them, but also a resistance and an inductance (See ESL). ESR is an important factor to take into account when designing circuits as the ESR of a capacitor determines the maximum rate of current flow into and out of the capacitor. This is especially important in power supply design when the current draw of the load can vary quickly and by large amounts. In general higher quality (read more expensive capacitors will have a lower ESR. The ESR value of capacitors is usually available from the manufacturer.

The effects of ESR can be reduced by using multiple capacitors in series. Effectively reducing the current draw from each capacitor.Ignoring the ESR of a capacitor can be dangerous. Since ESR is a resistance any current flowing across it will incur a power loss, in other words heat will be generated. This can destroy capacitors, especially aluminum electrolytic types which are do not stand up well to increased temperatures.

The equivalent circuit for a capacitor can be represented by the schematic in figure 1. This is not entirely accurate but is a good approximation at low frequencies.

              ESR      ESL       | |
        -----/\/\/\---UUUUU------| |------
                                 | |

        Figure 1: Schematic representation of a real capacitor

Straw, R.D. Ed. The ARRL Handbook for Radio Amateurs. Newington, CT: ARRL, 1999