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Topic: LED tutorial: "shorter anode pin should be connected to positive". Eh?! WTF? (Read 1 time) previous topic - next topic

lardconcepts

So, I'm following this Cambridge University GPIO tutorial and there's the following text and image:

Quote
Connect the LED along with the 270? resistor and wire them to the 3.3V rail according to the circuit diagram. From the datasheet, the shorter pin is labelled 'anode' and should be connected to the positive rail. The datasheets are usually found on the product page for electronic components.


I don't think I have ever come across an LED or project before where they used a "reverse bias" (what's the right term?!) LED. And I've never seen a tutorial this way round, either. They've always like this.

Now, bearing in mind this is Cambridge University, where a tutorial for beginners would carry some authority, why have they chosen something which would probably just confuse a newbie when they picked up any other "normal" LED / tutorial and found it didn't light? Am I being massively thick and missing something here?

elac

Following the led link in the tutorial and viewing the datasheet, the anode is shorter.
A common cathode led has a longer "-" leg, but this one only has two leads.
Different.
It's all about the skills

Runaway Pancake

I've seen them all ways, whether comparing lead-lengths, or vs the flat, or looking at the internals ("small part" vs "big part").
Just rig up a low current (5V, 2K) test and find out yourself.
Assume nothing.
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

CrossRoads

Boffin bought a load of "reverse polarity" LEDs a while back, got a good price from the factory as I recall. Pretty sure it was a manufacturing error.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

lardconcepts


I've seen them all ways, whether comparing lead-lengths, or vs the flat, or looking at the internals ("small part" vs "big part").
Just rig up a low current (5V, 2K) test and find out yourself.
Assume nothing.
Clearly. It's just that as I've never ever come across an LED, single (or more usually in a value pack) the "long negative" way round, I was just thinking it must be a bit confusing to have a newbie tutorial using parts with opposite spec to what they're likely to come across in beginners/hobby bundles and packs.

James C4S

The real crime is how many tutorials rely on a convention, not a standard.

I teach electronics classes at my local TechShop.  Before each class, I purposely cut the anodes to be shorter than the cathodes, then while teaching how to use a multimeter, I ask the students to identify the anode and cathode...  Very few realize they should use the multimeter, not their eyes.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

Runaway Pancake

Yes, my "tutorial" would trouble the student to test his LEDs - in the manner I described.
I realise that, to many, that'd be "discouraging", the equivalent of getting them to clean the Augean stables, vexing them with "bits" and triviality, but so be it.
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

cjdelphi


The real crime is how many tutorials rely on a convention, not a standard.

I teach electronics classes at my local TechShop.  Before each class, I purposely cut the anodes to be shorter than the cathodes, then while teaching how to use a multimeter, I ask the students to identify the anode and cathode...  Very few realize they should use the multimeter, not their eyes.




I've always looked at the led itself look carefully and you see both the anode and cathode and the bonding wires, the little segment bonding wire larger segment, all of the 5mm / 3mm leds i've ever had has worked using this method the small block is positive, negative from the larger segment.


MarkT


Boffin bought a load of "reverse polarity" LEDs a while back, got a good price from the factory as I recall. Pretty sure it was a manufacturing error.


Lead length is manufacturer-dependent and not standard! The reliable indication is the flat or
notch on the plastic body which is always the cathode side.  All diodes have a cathode mark in fact.
[ I won't respond to messages, use the forum please ]

Marvin Martian


I've always looked at the led itself look carefully and you see both the anode and cathode and the bonding wires, the little segment bonding wire larger segment, all of the 5mm / 3mm leds i've ever had has worked using this method the small block is positive, negative from the larger segment.


This is my rule of thumb too, but it's NOT foolproof. I have an 8mm red LED on my table right now that breaks the rule...

polymorph

Quote
I've always looked at the led itself look carefully and you see both the anode and cathode and the bonding wires, the little segment bonding wire larger segment, all of the 5mm / 3mm leds i've ever had has worked using this method the small block is positive, negative from the larger segment.


That's funny, because all LEDs used to have the large internal lead (that the die is bonded to) as the cathode as you say, until superbright LEDs came along, then almost all of them had the large lead as the anode. And of course now it is all over the place.

Steve Greenfield AE7HD
Nick Gammon on multitasking Arduinos:
http://gammon.com.au/blink
http://gammon.com.au/serial
http://gammon.com.au/interrupts

Boffin1

Quote
The reliable indication is the flat or
notch on the plastic body which is always the cathode side. 


Not on the " reverse polarity" ones I bought from China Mark.  The flat and the short leg were on the anode.

I always check before assembling now, my old meter has a transistor tester and I just plug the LED into the C and E of the socket ( with the meter switched to npn )

If I want to check the approx forward voltage, I use the continuity test position.

polymorph

I had the same problem years ago with a pack of random surplus LEDs, so I've never quite trusted an LED since then. It used to be that the lead that cupped the die inside was always the cathode, then some superbright LEDs came out that reversed even that.
Steve Greenfield AE7HD
Nick Gammon on multitasking Arduinos:
http://gammon.com.au/blink
http://gammon.com.au/serial
http://gammon.com.au/interrupts

Nick Gammon


The real crime is how many tutorials rely on a convention, not a standard.

I teach electronics classes at my local TechShop.  Before each class, I purposely cut the anodes to be shorter than the cathodes, then while teaching how to use a multimeter, I ask the students to identify the anode and cathode...  Very few realize they should use the multimeter, not their eyes.


And then you rewire the multimeter so that the red terminal is negative and this is doubly confusing ... or does it just cancel out?
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

Nick Gammon

I was working with some LEDs today, and the annoying thing is when you've cut the legs to be the same length so they go into the breadboard straight. And your eyes aren't up to spotting the flat bit on the plastic. So you measure it with the tester, and then as you go to plug it in you think "did I just rotate that 180ยบ before putting it in?".
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

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