Hazel,

ESR stands for "Effective Series Resistance."

All real capacitors cannot charge (or discharge) infinitely fast.

For now, let's leave superconductivity, inductance and distributed-parameter effects out of the discussion.

Any real circuit with a capacitor contains at least a little resistance, from the capacitor's leads and the capacitor plates (as well as some related delays from the plate/dielectric interfaces/interactions), even if the other wires and the power-supply are "perfect" (That is, those other components have resistances that are way, way smaller than those "hiding" in the capacitor.)

These resistances prevent infinite currents (from finite voltages), and thus slow down the charging or discharging of a capacitor. So, a key parameter that limits the speed of charging/discharging is the capacitor's ESR, measured in ohms, just like any other resistance.

The voltage on a capacitor (of capacitance C), being charged from a voltage V0 (starting at t=0), with a total resistance in the circuit, R is:

V(t) = V0 * (1 - exp( -t / RC) )

In this case, R = R_voltage_source + R_rest_of_wires + ESR_capacitor.

So, for a given capacitance, a lower resistance leads to faster charging; and ditto discharging. (You can also see from this that large capacitors act like charge reservoirs, and "try" to keep voltages from changing quickly.

That's the typical use of electrolytic caps like these.)

The ESR is the best-fit single resistance value that models the capacitor's charge/discharge speed, lumping all the resistance and (resistance-like) terms together in a single value.

A low ESR is generally desirable, because it means the capacitor can charge or discharge faster.

FYI, there's another, different spec for capacitors, the leakage resistance, which you can think of as an, "effective parallel resistance." The leakage resistance governs how fast the cap would discharge even if it's disconnected from a circuit, after being charged up. A high leakage resistance is generally desirable. Electrolytics typically have worse (lower) leakage than other caps, but they also have higher capacitance/size (or cost.) For non-electrolytic caps, the leakage resistance is so high that it's not really a factor, so often not even specified.

To get links (URLs) to show up in the forums nicely, try including the "http://" with the rest of the link, with no added spaces -- that works for me.

HTH,

Edited 3 time(s). Last edit at 06/22/2009 02:03PM by Larry Pfeffer.

Larry Pfeffer,

My blog about building repstrap Cerberus:

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