Measuring the ESR value of a 120uf 400volt cap - Page 1
Forum section: electronics / beginners

author: Tomvv
author_rank: Contributor
timestamp: August 09, 2015, 09:24:36 am
content: Hi,I've got a Readynas Duo with a defective power supply which was given to me by a friend.After opening the power supply I located a big cap which was kind of bulging so I wanted to measure the ESR.My problem is that when connecting the leads my ESR meter ( EVB ) does not return any values.Would desoldering the cap make my ESR meter be able to measure the ESR of the cap?

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2015, 09:42:28 am
content: yesbut a 120uF cap should not make reading problems on many esr meters ?the cap can be shorted, or totally dry inside so the no value ?

author: Tomvv
author_rank: Contributor
timestamp: August 09, 2015, 09:47:43 am
content: Thank for the reply.I will desolder the cap and try to measure again.

author: wraper
author_rank: Unknown
timestamp: August 09, 2015, 09:48:02 am
content: According the principle EVB how meter seem to work, it should measure in circuit. Very likely that capacitor is just faulty (open). Not so rare thing to happen if there is PFC in this power supply.

author: Retep
author_rank: Regular Contributor
timestamp: August 09, 2015, 09:53:56 am
content: The EVB (Bob Parker) ESR meter can only measure up to 99 ohms, above that it only displays -. If it doesn't measure anything in-circuit that means that the cap is open (> 99 ohms) and needs to be replaced. Taking the cap out of the circuit wouldn't make a difference in this case because in-circuit ESR measurement can cause the indicated ESR being lower than it actually is (due to components in parallel) but not higher. I.e. if the in-circuit ESR measurement indicates that the ESR is too high you can be sure that the cap is bad, however if the in-circuit ESR measurement indicates that the ESR is acceptable it still might mean that the cap is bad and you might need measure it out of the circuit after all.

author: Tomvv
author_rank: Contributor
timestamp: August 09, 2015, 10:14:35 am
content: Woahhh. So many replies in so little time.This is awesome :- ).I've just tested it again and still no reading ( - ) so it must be kaput.I've decided to buy a new 12V 5A power supply to replace my defective one.Does anyone have a suggestion for a replacement ( can I use any generic 12V 5A supply)?

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2015, 10:22:25 am
content: it will be cheaper to buy a new capacitor than a new power supply ...

author: Tomvv
author_rank: Contributor
timestamp: August 09, 2015, 12:04:17 pm
content: I understand it will be cheaper put the power supply had some sort of copper protective film all around which I had to remove.Would it be safe ( and wise ) to replace the cap and just put it back in the plastic enclosure without the copper film?

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2015, 12:09:59 pm
content: it is to protect the radio devices (and presumabely human beeings too) from radio electric interferencesif it is completely destroyed, then you can't put it back and probalby should buy a new power supplyoften it's easy to reinstall it after opening the box but hey...

author: Tomvv
author_rank: Contributor
timestamp: August 23, 2015, 11:09:30 am
content: One last update so that this post can be closed.I received my replacement PSU and the NAS is now running without any issues.Thank you all for responding.