Author Topic: Safety Caps and ESR  (Read 2188 times)

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Offline overlordMannyTopic starter

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Safety Caps and ESR
« on: February 14, 2019, 03:10:50 pm »
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.
 

Online wraper

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Re: Safety Caps and ESR
« Reply #1 on: February 14, 2019, 03:32:05 pm »
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.
 

Offline overlordMannyTopic starter

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Re: Safety Caps and ESR
« Reply #2 on: February 14, 2019, 03:47:42 pm »
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?
 

Online wraper

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Re: Safety Caps and ESR
« Reply #3 on: February 14, 2019, 04:15:45 pm »
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?
ESR 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.
 
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Offline overlordMannyTopic starter

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Re: Safety Caps and ESR
« Reply #4 on: February 14, 2019, 04:29:17 pm »
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.
« Last Edit: February 14, 2019, 04:31:50 pm by overlordManny »
 

Offline kripton2035

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Re: Safety Caps and ESR
« Reply #5 on: February 14, 2019, 04:43:19 pm »
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?
yes the esr of a small capacitor should be around zero
desolder the capacitor and measure it outside of the circuit to be sure.
 

Online wraper

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Re: Safety Caps and ESR
« Reply #6 on: February 14, 2019, 04:51:11 pm »
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?
yes the esr of a small capacitor should be around zero
desolder the capacitor and measure it outside of the circuit to be sure.
Higher 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.
 

Offline kripton2035

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Re: Safety Caps and ESR
« Reply #7 on: February 14, 2019, 05:40:00 pm »
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?
yes the esr of a small capacitor should be around zero
desolder the capacitor and measure it outside of the circuit to be sure.
Higher 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.
yes you're right.
but it should not be open for a 47nF
 

Online wraper

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Re: Safety Caps and ESR
« Reply #8 on: February 14, 2019, 05:44:11 pm »
but it should not be open for a 47nF
Many 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.
 

Offline dmills

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Re: Safety Caps and ESR
« Reply #9 on: February 14, 2019, 10:34:43 pm »
Many ESR meters don't go above 20 ohm or so.

You 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.   
 

Online wraper

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Re: Safety Caps and ESR
« Reply #10 on: February 14, 2019, 10:39:23 pm »
Many ESR meters don't go above 20 ohm or so.

You 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.
But 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.
 

Offline T3sl4co1l

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Re: Safety Caps and ESR
« Reply #11 on: February 15, 2019, 12:59:20 am »
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
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Cliff Matthews

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Re: Safety Caps and ESR
« Reply #12 on: February 15, 2019, 02:48:58 pm »
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.
 


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