Can I use ESR meter on a big capacitor with a bleed resistor? - Page 1
Forum section: electronics / testgear

author: MikeyFresh
author_rank: Newbie
timestamp: May 07, 2018, 09:54:56 pm
content: Hi all, I'm hoping you can help me with my ESR meter.I'm a sunbed technician and recently bought an MESR-100 meter to compliment my DMM and diagnose capacitor issues.Will the ESR meter work correctly on a big cap that has a bleed resistor across the two terminals?The cap is 50uF. It says 400V 50/60Hz. It operates at 240VAC. See attachment below.Thanks in advance for your help!Mike

author: MikeyFresh
author_rank: Newbie
timestamp: May 07, 2018, 09:59:22 pm
content: Here's what the ESR measured. It says the cap is ok, but that's also based on table for a 25V cap. If anyone could explain the results that would be helpful!

author: ogden
author_rank: Super Contributor
timestamp: May 07, 2018, 10:04:15 pm
content: Quote from: MikeyFresh on May 07, 2018, 09:54:56 pmIndeed it will work correctly. Bleeder resistors usually have high resistance, many, many orders of magnitude bigger than ESR, thus will not change measurement. Things will change with series resistor

author: pigrew
author_rank: Frequent Contributor
timestamp: May 07, 2018, 10:19:05 pm
content: Based on a quick online search, it looks like the meter (not sure if the video I saw is version 1 or 2, it had a small label for the chart of good cap values) alternates applying a positive and negative current on the meter. It determines the ESR by looking at the voltage step as it switches polarity. The test signal is on the order of many kilohertz. However, the frequency is variable, and inversely proportional to the capacitance.My intuition is that if the measurement period is much shorter than the RC time constant of your capacitor in parallel with the bleeder resistor, the ESR will be accurate. I'd suggest playing with a spice simulation to confirm.

author: oldway
author_rank: Super Contributor
timestamp: May 08, 2018, 06:23:08 am
content: First of all, welcome to this forum.An important thing in life is to understand what you are doing.This does not seem to be your case.This capacitor is not an electrolytic capacitor, so what is the matter of measuring the ESR of this capacitor?The ESR of an electrolytic capacitor is measured because it can change over the course of its life but the ESR of a non-electrolytic capacitor does not change.So what is this measure for? nothing useful.EDIT: NB: to check the capacitor, measuring his capacitance with your DMM is enough.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 08, 2018, 07:36:22 am
content: Quote from: MikeyFresh on May 07, 2018, 09:54:56 pmIt looks like the capacitor in this picture is still installed in a sunbed. What reading do you get if you measure a known good capacitor?How have you detected failures of this capacitor before you got your new meter?

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2018, 08:00:09 pm
content: With AC mains film capacitors like that, the failure mode is usually reduction in capacitance due self-healing after transients. An ESR test isn't going to show this up (unless it's a really extreme case).You really want to do a capacitance value measurement instead - for that, you would need to disconnect the bleeder resistor to get an accurate reading.

author: MikeyFresh
author_rank: Newbie
timestamp: May 08, 2018, 09:01:23 pm
content: Quote from: oldway on May 08, 2018, 06:23:08 amMaking sure that it is indeed an electrolytic cap would be a good first step. Thanks for clarification! I understand now that electrolytic caps are designed for DC usage and are polarized. Mine is not, therefore, an ESR measurement is useless. Is that correct?I'll continue to check with my DMM which generally reads 54uF for a good cap. Does the bleed resistor affect this measurement, as Gyro mentioned?I was hoping the ESR meter would have helped me better troubleshoot issues when a high-pressure sunbed lamp turns on slowly or not at all. I thought maybe there were capacitor issues that I was unable to detect.

author: Gyro
author_rank: Super Contributor
timestamp: May 08, 2018, 09:10:49 pm
content: It looks as if the bleeder resistor value is high enough not to be a problem in this case then. I would expect it to [Edit: cause it to] read a bit low, but at 54uF it is within the high end of the +/-10% tolerance range.P.S. Where does the capacitor connect in the circuit? If it's straight across the incoming mains, then it would only be a Power Factor correction capacitor and unlikely to be involved in your starting problems.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 08, 2018, 09:29:14 pm
content: Quote from: MikeyFresh on May 08, 2018, 09:01:23 pmTo determine if it's a bad capacitor that causes slow turn-on (or not at all) can't you just replace the capacitor with a new one? If replacing the capacitor fixes the problem, then you know you have a bad capacitor. Knowing that, then you can try making measurements on the capacitor to see if you can tell what's wrong with the bad capacitor.