Measuring Internal Resistance in Large Electrolytic Capacitors

[oz2cpu]

i have been working quite a while with this topic,
I try to explain a method i came up with, where it is possible to measure very low mili ohms
in large caps.
I explain it in detail here

https://youtu.be/M4eMLfFe_0w

i hope you like it, maybe you can use this information,
you are also very welcome to comment, if you found a better, easier, cheaper, more accurate way.

[jwet]

This is well thought out and interesting.  There may be one missing element though- capacitor ESR is a frequency dependent parameter.  Your slow rise and fall narrow pulse measurement scheme is getting the ESR at a frequency related to your rise time and your pulse rate- a pretty low frequency. You might consider using a strong drive for the switched FET and drive it at frequencies varying from 100Hz to 1 Mhz.  This is really the important ESR measurement for capacitors used in switching power supplies etc.  If your interest is just in bulk ESR at 120 Hz like in a standard transformer 60 Hz supply with full wave bridge, your method will work fine.  Often I'm looking for the ESR at 100 Khz or higher for example.  There are some high value capacitors that really excel at this like Panasonic OS-Con's- 10 of milliohms,  HF electrolytics 100 milliohms and finally regular aluminum electrolytics at fraction of ohms.  These days, its common to parallel a decent HF electrolytic with a high value ceramic of perhaps 10% of the value to get a good HF cap. Old electrolytics had very poor ESR at any frequency above 120 Hz.