Safety Caps and ESR - Page 1
Forum section: electronics / beginners

author: overlordManny
author_rank: Newbie
timestamp: February 14, 2019, 03:10:50 pm
content: In troubleshooting a SMPS I found that the transformer isn't being turned on, on the primary side. The rectifier has 172vdc and all of the passive components and diodes look good on the primary side. A ceramic safety cap coupling, what looks like maybe, a feedback signal ( I don't have schematics so I may be wrong), has appropriate capacitance ~47nF but infinite ESR. Normally I'd say it's definitely bad, but I'm not sure how safety caps work in regard to an ESR meter. Should it look the same as a normal cap? I don't have any to swap it out with and wanted to know if anyone could enlighten me before I place an order for parts.

author: wraper
author_rank: Unknown
timestamp: February 14, 2019, 03:32:05 pm
content: ESR meters are basically useless for caps below 1uF. Not to say, when testing components during repair, ESR measurement is useful for electrolytic capacitors only.

author: overlordManny
author_rank: Newbie
timestamp: February 14, 2019, 03:47:42 pm
content: I understand that I shouldn't expect accurate measurements with such a small capacitance, but shouldn't it read 0 or close to 0 instead of open?

author: wraper
author_rank: Unknown
timestamp: February 14, 2019, 04:15:45 pm
content: Quote from: overlordManny on February 14, 2019, 03:47:42 pmESR meters usually don't measure actual ESR but total impedance. And capacitor you mentioned likely is 4.7n, not 47n. Most of ESR meters would show even 47n out of range, not to say 4.7n.

author: overlordManny
author_rank: Newbie
timestamp: February 14, 2019, 04:29:17 pm
content: The things you learn... Thanks. And yeah, it may be 4.7nF. I checked a lot of things with my DMM yesterday so I'm not sure what's what without my notebook in front of me.

author: kripton2035
author_rank: Super Contributor
timestamp: February 14, 2019, 04:43:19 pm
content: Quote from: overlordManny on February 14, 2019, 03:47:42 pmyes the esr of a small capacitor should be around zerodesolder the capacitor and measure it outside of the circuit to be sure.

author: wraper
author_rank: Unknown
timestamp: February 14, 2019, 04:51:11 pm
content: Quote from: kripton2035 on February 14, 2019, 04:43:19 pmHigher capacitance = Lower ESR (in general). ESR is usually measured at 100kHz. @ 100kHz 47n cap already has 34 ohm impedance, 4.7n - 340 OHm. This is besides any ESR. Measuring ESR of ceramic/film caps as a method of checking them is nonsense.

author: kripton2035
author_rank: Super Contributor
timestamp: February 14, 2019, 05:40:00 pm
content: Quote from: wraper on February 14, 2019, 04:51:11 pmyes you're right.but it should not be open for a 47nF

author: wraper
author_rank: Unknown
timestamp: February 14, 2019, 05:44:11 pm
content: Quote from: kripton2035 on February 14, 2019, 05:40:00 pmMany ESR meters don't go above 20 ohm or so. Also as I've said, IMO it's not 47n. As per OP's description, it sounds like Y capacitor.

author: dmills
author_rank: Super Contributor
timestamp: February 14, 2019, 10:34:43 pm
content: Quote from: wraper on February 14, 2019, 05:44:11 pmYou need better gear!The venerable HP 4192A never mind its more modern equivalents from HP/Agilesight, Boonton and the rest will all measure ESR to many k ohms. Also, most of the old school manual bridges would get it done, this ESR meter with naff all range is a very modern thing.This is an excuse to get your gear buying hat on!Regards, Dan.

author: wraper
author_rank: Unknown
timestamp: February 14, 2019, 10:39:23 pm
content: Quote from: dmills on February 14, 2019, 10:34:43 pmBut it's completely unsuitable tool to use for everyday repair. And you need to pray god you won't accidentally stumble across some charged cap. As for actual repair, ESR meter usually is better than proper LCR meter. Works faster and cares less about components connected in parallel.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2019, 12:59:20 am
content: Infinite ESR would read zero capacitance. Think about it.Safety (Y type) caps are usually employed between mains, and ground or output. They are rarely bigger than 4.7n, due to ground leakage current.Tim

author: Cliff Matthews
author_rank: Unknown
timestamp: February 15, 2019, 02:48:58 pm
content: Welcome! Feedback is often via optocoupler (like PC817) or inductor straddling over the PCB isolation area. Since you don't have schematics, can you post a picture and/or some part numbers? and also what tests you've done on the secondary side.