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I am trying to automate a method of testing the ESR of many super capacitors simultaneously. I have read that a good method of measuring the ESR of capacitors is to apply a square wave across the cap and look at the instantaneous drop/rise levels with an oscilloscope. With this measurement it is possible to calculate the esr. I have a few concerns in potentially using this method for my case. I do not have access to a programmable O-scope, but I DO have access to a programmable DMM that would allow automation of the test. This DMM is coupled so that any DC offest present in the measurement would be killed of. The model of programmable DMM I have reads a true RMS value. My concerns/questions that I wasn't able to find information on are below:

  • Will this RMS value from a DMM be of any use in reliably calculating esr? The data sheet for the component, unfortunately, lists only that the esr "shall not exceed 35 ohms".. Pretty ambiguous for a data sheet if you ask me. I do not have a control to compare the results I get to.

  • I have tried to run this myself a few times. I have a handheld DMM that is coupled in the same fashion and reads true RMS as well. But the two dmms output different values. The programmable one reads about 155 mV while the handheld reads about 135 mV for the particular set up I used (a 1 Vpp square wave with 0.5 V offset at 1kHz across a 1.0 F, 5.5 V rated super Cap.) Is it possible that the handheld is reading a lower value due to the sampling rate? The output resistance on my function generator is set to 50 ohms.

**If anyone has an idea on how to accurately measure esr in a completely different way, I would love to hear that as well. The limiting variable has been the inability to access a programmable O-scope. Thanks!

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  • \$\begingroup\$ Note that for super caps not only the ESR will vary but also the actual capacity. They're often specified as xx Farad -5 ... +20%. \$\endgroup\$ – JimmyB Jul 19 '16 at 15:56
  • \$\begingroup\$ Why you don't buy a LCR meter ? \$\endgroup\$ – Marko Buršič Jul 19 '16 at 16:05
  • \$\begingroup\$ I am trying to keep the automated format while avoiding a purchase. If I need to purchase something for this project, a programmable scope would prove more useful in the future than an LCR meter. But a handheld LCR meter for manual testing is not a bad idea. Thanks! \$\endgroup\$ – Audio Guy Jul 19 '16 at 16:24
  • \$\begingroup\$ @JimmyB There is a separate portion of the circuit designated to checking both charge and discharge capacitance. Thanks! \$\endgroup\$ – Audio Guy Jul 19 '16 at 16:26
  • \$\begingroup\$ You may get there with one of those cheap PC-based USB scopes (around US$ 20 or so). - I mentioned the capacity because when you know the capacity you can basically time the discharge through a known resistance to calculate/estimate the ESR. \$\endgroup\$ – JimmyB Jul 19 '16 at 18:36

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