Octavio,
Good Question on ESR, it has bothered engineers forever!

Here is my method:

Beg some friend that works at a company that has an impedance analyser
(or is rich and has his own (a "Rico").  You might have to buy your
friend a lunch or dinner, but it is worth it!  HP makes some units for
lots of $!  With the shortest leads you can manage, connect and scan the
Xc over the frequency of interest.  You will see the impedance come down
(as Xc goes down with frequency, Xc=1/2pi x f x c) and then turn
upward.  At that transition, the impedance is resistive.  It reads
directly out in miliohms on the HPs.  It might be as low as several
miliohms and at a high frequency (as high as 20 mHz), depending on the
capacitor.  By the way, at that point the capacitor is a series resonant
circuit. No Xl or Xc, just ESR.
To avoid problems with this resonance point, you will see circuits
parallel another capacitor to "take" over the job at higher frequencies.

OR

Try to make a circuit that utilizes resonance with an inductor to look
at the same things as mentioned above.  Maybe a bridge circuit?  The
problem is usually the ESR is really low!  It is hard to measure.  Since
the ESR effects the Q of a resonant circuit, bandwidth or "ringing"
could also be used to compare capacitors in a given circuit.  You need a
good scope and RF generator along with a lot of luck.

Mike Hagen


 to
At lower frequencies you may try to make a bridge and try to null out
the ESR against a low resistance OR try to make a resonate circuit and
measure the current through the , it never gets better than that