How to test if a capacitor is faulty - Page 1
Forum section: electronics / beginners

author: Cricri
author_rank: Contributor
timestamp: December 24, 2021, 01:53:11 pm
content: Hello all,I have scavenged components from a dead PCB, and testing them with a LCR tester.A capacitor's capacity is reported correctly, the capacitor is not bulging, but the tester says it has a whopping ESR of 30 to 10 ohms, and a V loss of 7-9%.Des that mean that, although a pure capacitance test would indicate it's fine, it is in fact faulty?Thanks!

author: Peabody
author_rank: Super Contributor
timestamp: December 24, 2021, 03:48:33 pm
content: I think it does. Particularly if you have other similar capacitors that don't produce such a reading. Typically you would also have some reduction in capacitance with high ESR, but not always. You can also test for ESR using a function generator and a scope.

author: Shock
author_rank: Super Contributor
timestamp: December 24, 2021, 04:30:54 pm
content: Capacitance, esr and leakage and if low resistance dc short accounts for almost all scenarios of physical or age related failures in that order (unless there is some high frequency issue).Capacitance and esr will expose the majority of them.

author: GerryR
author_rank: Frequent Contributor
timestamp: December 24, 2021, 05:18:29 pm
content: This chart might help:

author: Cricri
author_rank: Contributor
timestamp: December 24, 2021, 05:27:32 pm
content: Thanks all!In my OP, I meant 30 to 40 ohms.Sicne it's a 4.7uF, 50V, then according to the chart provided by GerryR it could actually be ok.In which case, I have no idea which component was bad since all the ones I tested seem to be ok. This was a PCB from a LED bulb by the way, and the bulb didn't light fully on.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 24, 2021, 05:31:45 pm
content: Which meter do you have? I think I need such a meter. Measuring capacitance with a DMM is not a sure thing to check the capacitors.

author: Cricri
author_rank: Contributor
timestamp: December 24, 2021, 07:01:32 pm
content: Quote from: BeBuLamar on December 24, 2021, 05:31:45 pmI've got this model:https://image.pushauction.com/0/0/fc79e315-f446-4e91-bb17-802a12b02f6e/2cf1394d-9c53-42d9-953a-0c450593a902.jpg

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 24, 2021, 07:30:19 pm
content: In order to test a cap for leakage, you need to provide the correct amounts of volts.This is a problem with filter caps. and a big cause of failure in power supplies.Most cap testers do not do this function. If they do it is at low voltage not operating voltage.You can put the cap across the plus and minus of a adjustable power supply and see if it draws any current., it should not draw current.

author: wizard69
author_rank: Super Contributor
timestamp: December 24, 2021, 10:51:46 pm
content: Quote from: Cricri on December 24, 2021, 01:53:11 pmMost likely yes. You could find the manufactures data sheet and see if it has anything to say about ESR. By the way "Mr. Carlsons Lab" on youtube has a lot of videos on capacitors and ways to check them.Now as an aside; I'm someone that literally got his start in electronics with salvaged junk. For the most part I'd have to say that it is a waste of time to salvage capacitors, but especially electrolytics. I fully understand not having cash but if you have an ESR meter then you have cash to buy new caps. Even caps that are not electrolytics are easily damaged in the salvage process. So in all honesty I'd go after the high value parts and ignore delicate parts or those subject to aging. There is some reasoning behind the practice of recapping instruments and audio equipment and in part that is driven by history and known failure points.

author: Oilngas
author_rank: Contributor
timestamp: December 25, 2021, 01:38:59 am
content: You may find this video useful to check for leakage...

author: Shock
author_rank: Super Contributor
timestamp: December 25, 2021, 02:25:46 am
content: That blue esr type chart sucks, I'd go off the original Bob Parker charts which put the ESR of a 4.7uF 50V at around 4 ohms and then later revised at 2 ohms on the MKII. Note that was about 20 years ago and caps have only become smaller and lower in esr since.The chart for the ESR Meter MKII can be found here:http://bobparker.net.au/esr_meter/k7214.pdf

author: rsjsouza
author_rank: Super Contributor
timestamp: December 25, 2021, 03:02:02 am
content: I have salvaged many capacitors in my life and I tend to be extra safe with capacitors that come from well worn equipment or that was subjected to extremes (temperature, humidity, etc.). On the other hand, this is not applicable to all scenarios. I have a pair of 250V Jamicon branded electrolytic capacitors used in an electronic ballast for more than ten years that still test very well in various frequencies and have very low leakage. Would I use them on mission critical? Obviously not, but otherwise can still be useful in experiments, for example.One of the issues in testing "ESR" with these testers is to be absolutely sure contacts are well made - it is easy to get false readings due to this. Also, the tested frequency may give distorted and these generic tables can give a false impression given there are many types of capacitors for many different applications. I would always try to consult the dstasheet of the manufacturer to get either the ESR or the tan delta (dissipation factor).

author: Cricri
author_rank: Contributor
timestamp: December 25, 2021, 08:43:26 pm
content: Thanks all.To be clear, I'm not a pro: all I want is when discarding a PCB, salvage the components that could be useful to fix something else down the lane. I'm not too bothered about certifying that a component is meeting 100% of its specifications; all I need to decide is whether the salvaged component will get something which has the same component (but dead) running again or not.Also, as an electronics dummy, I probably won't have the same level of sophistication in my toolkit. If I didn't have the ESR tester, I'd just have relied on my Fluke to test a resistor/cap by just measuring their R/C value, and use the diode "beep" test for a diode. The real reason I bought the ESR tester is for semi-conductors, and I don't have to worry about connecting the component legs into a specific order (and also it was dirt cheap).

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 25, 2021, 09:15:46 pm
content: I find it's best to keep salvaged PCBs intact in the junk box, this gives you an idea of the specifications/application of the part in question and saves the work of breaking boards down for parts you may never need/want.

author: Cricri
author_rank: Contributor
timestamp: December 25, 2021, 09:22:25 pm
content: Thanks, but I just don't have the space to keep a bunch of PCBs. I don't even have a separate room dedicated as a workshop.