Jim said:
Wouldn't that mean you need DF given at a specific frequency so that
you can work back to ESR (which should(?) be fairly constant)?
At 0Hz, the ESR of a capacitor will be darn high. It drops as one goes
up in frequency until inductance takes over.
When you do find specs for ESR in capacitor data sheets, they specify at
which frequency (usually 120Hz in the US) they are providing the ESR for.
According to
http://www.cooltron.com/component/technical/library_of_capacitor.shtml
7. Equivalent Series Resistance (ESR)
It's the sum of all the internal resistances of a capacitor measured in
Ohms. It includes:
- Resistance due to aluminum oxide thickness
- Resistance due to electrolyte / spacer combination
- Resistance due to materials (Foil length; Tabbing; Lead wires; Ohmic
contact resistance)
The lower the ESR the higher the current carrying ability the capacitor
will have. The amount of heat generated by ripple current depends upon
the ESR of the capacitor.
ESR is both frequency and temperature dependent, increasing either will
cause a reduction in ESR. The ESR is an important parameter in
calculating life expectancy as the power dissipation (internally
generated heat) is directly proportional to its value.