Electronics > Beginners
Safety Caps and ESR
overlordManny:
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.
wraper:
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.
overlordManny:
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?
wraper:
--- Quote from: overlordManny 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?
--- End quote ---
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.
overlordManny:
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.
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