Measuring an aluminum electrolytic capacitor's ESR

RE: Measuring an aluminum electrolytic capacitor's ESR

mszil — Wed, 24 May 2023 06:04:55 GMT

I came across this when trying to decide if some ancient 4700uF/63V caps are still good, and decided to tweak it for the tools on hand. I used a portable digital scope, a power supply, and power resistors. It worked quite well so I thought it might be of use to someone else with a similar setup:

First make sure the capacitors are safe (google how electrolytics heal for more info), along with safety considerations for the voltages you're working at - I kept mine in a box as a precaution. Also start with lower voltages and check the general discharge curve shape to make sure your capacitor is still in reasonable shape before trying to measure ESR.

I charged my caps to 60V thru 510Ohms with the resistor on the positive end (negative connected directly to power supply). I chose a 12Ohm power resistor (60V/12Ohm=5A peak). If possible, check ripple current limits on your caps - don't go crazy - but bigger currents increase the ESR signal and should give you some indication of capacitor condition and a few discharges at or somewhat above the service limits shouldn't cause trouble.

Next set up the scope (be careful: consider what is grounded and avoid shorting the probe ground/shield). Connect the scope probe shield to the power supply positive output, and the tip to the capacitor positive voltage - it should read ~ 0V once charged. Next setup the scope to capture a single pass, falling slope - say 200mV, 200uS/div - trigger level around -1 to -2V.

Connect one end of the power resistor to the negative supply (so the resistor, the power supply and the capacitor negative terminals are all connected throughout).

Leave the power supply running and disconnect the cap from its charging resistor and connect it to the other end of the power resistor - discharging the cap - you may get a small spark.

The scope should capture the start of the discharge curve and you should see a well pronounced, near vertical step down at the start, followed by the normal discharge slope. That correlates to ESR. In my case it was about 400mV - divided by the initial 5A discharge current = 0.08Ohm. You can repeat easy enough and play with the scope settings to improve accuracy. Wiring also matters: minimize high currents in the voltage measurement circuit, ideally use 2 sets of connections on the cap, one set to the power resistor and separate connections to the probe tip and power supply negative.

Overall it worked pretty well the first time and wasn't too hard to setup - and the caps look pretty good too. Hope it's pretty clear.

RE: Measuring an aluminum electrolytic capacitor's ESR

mmichael — Fri, 14 Jan 2022 19:20:13 GMT

Great tutorial! Lacking a function generator, could one use the oscilloscope's "Probe Adjust" (typically 500mV P-P, 1kHz) instead, even for a qualitative assessment?

RE: Measuring an aluminum electrolytic capacitor's ESR

koudelad — Wed, 26 Jun 2019 18:41:13 GMT

One more question for [mention:57a79c8ceae84c06b59d42e67a0ded50:e9ed411860ed4f2ba0265705b8793d05] :

If an old piece of electronics gets to you, like this 35 years old computer, do you automatically replace the electrolytic capacitors (and any other components)?

RE: Measuring an aluminum electrolytic capacitor's ESR

14rhb — Thu, 20 Jun 2019 09:49:42 GMT

A better overview and explanation than I could find anywhere on the internet (although of course your post is now on there [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e355dfcd-7f34-435d-a434-55770e4c5e53/contentimage_5F00_938.png:16:16]). Thank you.

Rod

RE: Measuring an aluminum electrolytic capacitor's ESR

koudelad — Thu, 20 Jun 2019 05:26:07 GMT

Very well explained, thank you, James.

RE: Measuring an aluminum electrolytic capacitor's ESR

Andrew J — Wed, 19 Jun 2019 22:21:00 GMT

Farnell have a kit for the LCR40 and DCA55 plus case for £115 at the moment. https://uk.farnell.com/peak/lcr40-dca55-atc55/meter-analyser-passive-semiconductor/dp/7830270?st=tandtexcess&krypto=bVNX…

RE: Measuring an aluminum electrolytic capacitor's ESR

luislabmo — Wed, 19 Jun 2019 20:17:11 GMT

Nice tutorial, very clear and easy to understand.

Luis

RE: Measuring an aluminum electrolytic capacitor's ESR

DAB — Wed, 19 Jun 2019 19:28:33 GMT

Nice overview of ESR and measurement techniques.

DAB

RE: Measuring an aluminum electrolytic capacitor's ESR

Jan Cumps — Wed, 19 Jun 2019 12:15:17 GMT

That power supply <3. I have the ...2a version. Very good programming capabilities, this family.