"Does the black band mean positive or negative? Depending on the marking con..." - Page 1
Forum section: electronics / beginners

author: cdev
author_rank: Super Contributor
timestamp: January 12, 2017, 11:35:40 pm
content: Preventing Incorrect Installation of Polarized Capacitors"Does the black band mean positive or negative? Depending on the marking convention, it could be either!"https://www.nasa.gov/sites/default/files/atoms/files/nesc_tb_15-01_preventing_incorrect_installation_of_polarized_capacitors.pdf

author: Benta
author_rank: Super Contributor
timestamp: January 13, 2017, 12:02:49 am
content: cdev, where do you find this stuff?Reading through it, I thought about April 1st. But as it seems to be real, it's tragic.

author: hamster_nz
author_rank: Super Contributor
timestamp: January 13, 2017, 12:08:31 am
content: Is it just me, or is the marking on SMD LEDs just as inconsistent?

author: jpanhalt
author_rank: Super Contributor
timestamp: January 13, 2017, 12:13:28 am
content: This is one case where the datasheet is important. Wikipedia gives a nice review in its section on electrolytic capacitors. It is near the end. See snipping below. It adds a little to the NASA comment.JohnEdit:https://en.wikipedia.org/wiki/Electrolytic_capacitor

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 13, 2017, 12:23:44 am
content: I would say perhaps 99% of capacitor failure incidents are with good reversed polarity mounted tantalum capacitors. Even NASA falls victim to that.Makes us wonder why the engineer who was tasked to mark the first batch had chosen to mark the POSITIVE end with a band.No one had ever explained to me why it is better to mark the capacitor this way than the conventional method already in use widely.

author: cdev
author_rank: Super Contributor
timestamp: January 13, 2017, 12:35:40 am
content: This ambiguity situation seems like it really has to be fixed, soon!

author: Brumby
author_rank: Unknown
timestamp: January 13, 2017, 02:01:51 am
content: From the bulletin:"In another recent instance (different hardware), an on-orbit failure was experienced due to a reverse polarity solid tantalum capacitor."A bit hard to get out the soldering iron for an on-site fix.I am curious, though, how a reverse connected solid tantalum cap would have survived testing. I've not had a lot of experience with them - but I've always understood tantalums to be very unforgiving.

author: hamster_nz
author_rank: Super Contributor
timestamp: January 13, 2017, 02:37:11 am
content: Quote from: Brumby on January 13, 2017, 02:01:51 amAccording to the linked PDF, such caps are heavily derated for Space use, so tend to not fail during the test cycles.

author: Brumby
author_rank: Unknown
timestamp: January 13, 2017, 03:08:04 am
content: Even with reverse polarity?

author: hamster_nz
author_rank: Super Contributor
timestamp: January 13, 2017, 03:38:37 am
content: Quote from: Brumby on January 13, 2017, 03:08:04 amSo they say...from the original linked document...Quote

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 13, 2017, 05:31:13 am
content: Quote from: eblc1388 on January 13, 2017, 12:23:44 amAs far as I know electrolytic capacitors were the first two terminal devices which had a polarity. About eighty years ago. And the oldest ones in my possession used a black band to indicate the positive lead. These are definitely at least sixty years old. So what is the conventional method already in wide use that preceded that?I always found it strange that the smaller aluminum electrolytics that became widely available in the 1960s switched to marking the negative terminal - with a distinctive band, not always black. At least these were clearly marked with minus signs, indicating to me that their designers knew that there was potential confusion. That worked out fairly well until package sizes shrank to make this clarity impractical.

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 13, 2017, 06:10:07 am
content: Quote from: CatalinaWOW on January 13, 2017, 05:31:13 amIsn't it the case when SMD tantalum capacitors appears on the market, all electrolytic capacitors were already had their negative polarity marked with a band?Or someone suddenly realised that the markings were all wrong and decided to put it right again?

author: Ian.M
author_rank: Super Contributor
timestamp: January 13, 2017, 07:23:26 am
content: The earliest commercially available 'wet' electrolytics used the Aluminium can as the negative electrode. There was already a convention for tubular wound foil non-polarised capacitors, of marking the end connected to the outer foil with a black band so that it could be connected to the lower impedance or 'earthy' node to reduce the effects of stray capacitance. Its not too speculative to assume that the marking band convention for Aluminium electrolytics followed on from that.I don't know how the convention for Tantalum electrolytics got reversed. Unless there is an expert on the construction of early silver cased wet Tantalum caps, that could tell us whether the case was connected to the anode or cathode foil or neither, I doubt we'll get an explanation . . .

author: BMack
author_rank: Regular Contributor
timestamp: January 13, 2017, 07:50:37 am
content: I'll tell you that I almost screwed up by not paying close attention to the cap and started soldering it in backwards until I noticed the size of (what I thought was the positive end) pad. This was in the output section of an old console stereo system. It almost ruined my perfect record in replacing caps.