Dan;

There are ads for an ESR meter in hobby magazines.  They go stealth by
the reference of capacitor testers.

If you intend to measure yourself, there is one way I have used.  The
method assumes you are using a buck switching regulator, where most of
the current from the inductor is going to the capacitor.  The current
through the inductor is sawing up and down.  It may be difficult, but
you must measure the current through the inductor on the scope.  You
want peak to peak current.  I find those hall effect transducers work
nice.  Then you look at the peak to peak voltage across the capacitor.
Using ohms law,..  But then, I suspect that the voltage on the capacitor
is moving up and down with the sawing current.  So this ohms value would
be on the conservative side.

Then you know ohms at your test frequency, which is always different
than the datasheet at their test frequency.  BUT don't forget in a
switcher to look at the max RMS current through the capacitor, and see
if you are exceeding it.  This is the parameter that toasts capacitors
in switchers.  Of course, do this test at max Iload.

Hope that helps.
Chris Eddy

Dan Michaels wrote:

> Does someone know if you measure the ESR of an electrolytic cap
> simply by taking the minimum impedance vs frequency, and comparing
> against Xc = 1/(2*pi*F*C), or does it require a more complicated
> measurement procedure?
>
> Also, from a practical viewpoint, what is considered to be a
> bad ESR value? Maybe, > 1 ohm?
>
> Any definitive documents?
>
> - Dan Michaels
> Oricom Technologies
> http://www.sni.net/~oricom
> ==========================