Is there any reason for this type of inconsistency? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: September 08, 2014, 11:54:11 pm
content: Hi,So I bought some LED's:http://www.osram-os.com/Graphics/XPic9/00078860_0.pdfThen I bought them in a different color:http://www.osram-os.com/Graphics/XPic8/00078869_0.pdfOne thing that makes no sense to me is that they use the same marking and on one LED is means one thing and on the other it is different! One calls the mark the "anode mark" and the other calls it the "cathode mark" - the same mark!!!Why would they do this???Thanks,Alan

author: Rufus
author_rank: Super Contributor
timestamp: September 09, 2014, 01:28:40 am
content: Quote from: alank2 on September 08, 2014, 11:54:11 pmBecause the design and processing of some LED die results in the light coming out of the cathode side and others the anode side. Mounting die in the same package results in the pins/pads swapping.You used to be able to look inside 5mm LEDs and the the lump of metal the die bonded to was always the cathode. That changed when they came out with fancy colors and high efficiency LEDs.

author: hlokk
author_rank: Contributor
timestamp: September 09, 2014, 04:51:26 am
content: Stupidity? The die can be either way around but you think the outer package marking would be consistentTheres also the leds with the marking underneath, and the 06 sizes that are so small you cant tell either. I use some smd tweezers and see which way it lights up.5mm leds seem to have the flat side on inconsistent sides for some of my leds (pretty sure itshouldbe universally on the cathode). However, I have some with the flat on the side

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2014, 07:22:01 am
content: Know what's really awful? LEDs without a flat at all -- completely round!

author: Precipice
author_rank: Frequent Contributor
timestamp: September 09, 2014, 07:38:42 am
content: At least then you know you don't know...Assembling a run of boards and then having to swap all the 0603 LEDs round is quite dispiriting.

author: Psi
author_rank: Super Contributor
timestamp: September 09, 2014, 09:03:50 am
content: Quote from: Precipice on September 09, 2014, 07:38:42 amdepends on the run size i guess10100100010000

author: Precipice
author_rank: Frequent Contributor
timestamp: September 09, 2014, 09:37:28 am
content: It was a run of 40 boards, 18 LEDs each, so an annoying afternoon with 2 fine-tip soldering irons.(It's always a risk, though - even when you're sure about a design, sometimes reels will turn up with components in surprising orientations. Sometimes it's obvious (a backwards SOT-89 stands out like a sore thumb), but a backwards 0603 LED, not so much. Square QFNs with ambiguous markings (oh, pin one is the corner _without_ a mould mark. Fine) are my least favourite.If Altium had a plugin that would let me dump all used parts in a line of parts on the PCB panel border, so I could do a single test placement run, then check those parts obsessively, it'd be quite reassuring - but never quite reassuring enough to bother!

author: janekm
author_rank: Unknown
timestamp: September 09, 2014, 10:35:56 am
content: Diode polarity markings are one of the banes of my life, truly. (I have also had runs of PCBs assembled with LEDs the wrong way round). There's not even a universally understood standard for marking the polarity on the PCB! (Bar/dot on cathode side is common but can mean anode to some). I have taken to placing a "k" (for kathode) symbol at the cathode end, rotated so it approximates a little diode symbol (might be better to draw a proper diode symbol, actually).Those Osram markings are terrifying! They should be ashamed of themselves, I've lost some respect for Osram today.

author: schopi68
author_rank: Regular Contributor
timestamp: September 09, 2014, 11:53:44 am
content: On page 13 you can see that both types are taped in the same direction - So i'd assume the "mark" is more a substrate (and cooling) mark.

author: alank2
author_rank: Super Contributor
timestamp: September 09, 2014, 12:13:23 pm
content: Quote from: janekm on September 09, 2014, 10:35:56 amThis is what I think. These are practically the same thing except the color, completely backwards from each other. Why make something more complicated than you need to? Why do something so stupid as to create failure or confusion? It makes no sense.

author: Rufus
author_rank: Super Contributor
timestamp: September 09, 2014, 01:57:38 pm
content: Quote from: alank2 on September 09, 2014, 12:13:23 pmBecause the markings are on the ceramic (or whatever) substrate and they would need two versions of the substrate which makes it more complicated for them and the cost of that complication would be passed to you.Osram are not the only ones. Kingbright KP-1608 series 0603 LEDs have Super Bright Red one way round and the remaining 4 colors the other way round.I assume you got bitten, but, you should have known to be careful and now you do. I have been aware of the inconsistency of which end of an LED die the light comes out of since the first super-bright LEDs came out.

author: alank2
author_rank: Super Contributor
timestamp: September 09, 2014, 05:10:19 pm
content: Quote from: Rufus on September 09, 2014, 01:57:38 pmI actually decided to test it beforehand, but there was a moment where I though, "I've already done these, I know how they go.".Consistent marking just makes sense...