Experimentally determine capacitor polarity - Page 1
Forum section: electronics / projects

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 06:06:06 pm
content: All,When working on a recent project, I was stupid and clipped the leads off my polarized electrolytic capacitors before discovering that the packages were not marked, and relied on the lead length to determine polarity.After a few moments of disbelief, I figured there must be a way to experimentally determine capacitor polarity.I did some googling, but the methods I found seemed hokey at best (measuring can voltage when reverse biased), and manufacturer dependent at worst. Any suggestions?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: February 29, 2020, 07:01:57 pm
content: I don't think I've ever seen a polarised capacitor, without a marking to determine the polarity. Please post a picture.Electrolytic capacitors behave like poor diodes and will conduct, if reverse biased. Ensure it's fully discharged Connect it to 5V, via a 1k resistor. If it charges up normally, as expected, then the polarity is correct. If it sits at under a volt or so and never charges, then it will be reverse biased.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 29, 2020, 07:07:56 pm
content: Yep, and if you're potentially reverse biasing a polarized cap, do limit the current to a small value, or the cap may just explode in your hands in a matter of a couple seconds. Yes it happened to me once and I still remember it. Not fun.

author: duak
author_rank: Super Contributor
timestamp: February 29, 2020, 07:10:17 pm
content: Try measuring the voltage with a sensitive voltmeter - I'm assuming a DVM with a 200 mV range with a 10 M input impedance. I just tried this on new and unused caps from 100 to 1000 uF and saw at least 50 mVDC on most of them.I would then try charging them from 10 V (or less if the cap's rated voltage is lower) through a 1M0 resistor and observing the voltage across the cap. You should see the voltage reach 9 V or so within five time constants (t = RC). If the voltage goes part way and sticks there, try the reverse connection and see if it improves.

author: wraper
author_rank: Unknown
timestamp: February 29, 2020, 07:24:30 pm
content: Quote from: jcosper on February 29, 2020, 06:06:06 pmSounds like nonsense. The only electrolytic caps without polarity marking I've ever seen were non-polar type. Even if there is no polarity marking, sleeve likely has certain orientation which you can compare with capacitors which have full length leads.

author: ozcar
author_rank: Frequent Contributor
timestamp: February 29, 2020, 07:36:48 pm
content: Quote from: wraper on February 29, 2020, 07:24:30 pmPerhaps there is no specific polarity marking, but it is implied in some way - like the old colour coded tantalum capacitors, where you had to know "positive lead on the right when the spot is facing you, and the leads are pointing down".

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 09:04:25 pm
content: There is no standard ---- marking like other caps I've dealt with... that is why they used the leads to show polarity.You are correct. I could compare against another cap with the lead... if I had one.

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 09:06:13 pm
content: They are nichicon 1uF 50V ultra-miniture. I think this is the link:https://www.nichicon.co.jp/english/products/alm_mini/index.html

author: Zero999
author_rank: Super Contributor
timestamp: February 29, 2020, 09:37:56 pm
content: Quote from: jcosper on February 29, 2020, 09:04:25 pmOh there is a standard. The negative is the side of the can with a stripe vertically running down it.Quote from: jcosper on February 29, 2020, 09:06:13 pmThere are lots of different capacitors on that page, including those which are bipolarised, meaning they have no polarity and can be connected either way, thus will have no polarity marking.https://www.nichicon.co.jp/english/products/pdfs/e-ump.pdfhttps://www.nichicon.co.jp/english/products/pdfs/e-uvp.pdf

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 09:45:09 pm
content: Thanks for the advice. I will try some of the charging experiments.Here is a photo:

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 09:47:32 pm
content: Quote from: Zero999 on February 29, 2020, 09:37:56 pmCrap, just noticed that. Its marked with BP. So I imagine that is it. Thanks.

author: jcosper
author_rank: Contributor
timestamp: February 29, 2020, 10:34:19 pm
content: Quote from: Zero999 on February 29, 2020, 09:37:56 pmAll I meant was that there was no standard stripe on the package. It should have been a clue to me.