Through hole LEDs are so confusing

I don't use LEDs much a a rule but I have been using them recently whilst developing an LED VU meter. however I find the coding of the lead lengths confusion. I always remember the ways electrolytics caps are coded with the longer lead being the plus side of the cap and I remember it because  a plus sign needs more length to create it than a minus sign. So I am expecting LEDs to be the same. To me the cathode is the plus side of a diode so I expect the cathode leg to be the longer of the two wires of a LED. But  it is the other way round, the cathode is the short lead. I have lost count of the number of times I have connected a LED the wrong way round.

Confused

Ian

!!!  Strange.

I always remembered by saying to myself, anode in edona punctured by a long syringe hits the red artery...    :

I always try and spot the larger element inside the plastic , thats always going to be the cathode , then even if the legs have been snipped you know your right .

Why do you consider cathode to be plus side? The anode should be plus.

Tubetec said:

I always try and spot the larger element inside the plastic , thats always going to be the cathode , then even if the legs have been snipped you know your right .

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+1, when in doubt look inside... you can usually see the small wire attached to the middle of the LED die.  IIRC the cathode is the back of the die... so + wire coming from anode...  or not.  8)

Of course try one to confirm.

Impossible to see with tiny SMD LEDs but they usually mark the + end.

JR

john12ax7 said:

Why do you consider cathode to be plus side? The anode should be plus.

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No, cathode has always been plus, it is the end that becomes +ve when you use it as a rectifier. I remember it even used to be marked on schematics that way many decades ago

Cheers

Ian

ruffrecords said:

No, cathode has always been plus, it is the end that becomes +ve when you use it as a rectifier. I remember it even used to be marked on schematics that way many decades ago

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I would be curious to see that,  maybe conventions have changed.  Current flows from + to -. In a diode current flows from anode to cathode. So it makes sense for anode to be +, as in the picture Tubetec posted. I also did a search,  and any online image I could find has anode labeled + and cathode -.

I have always always considered the Anode to be the + side, the long leg of the LED.

Which usually has the small terminal inside the plastic lens. That was always my fail safe, the small bit is +!

Boy was I wrong..... Had a whole batch where the small internal part was actually the cathode. The lead length was correct though.

Now I measure with a meter to check.

Peter

When I have one I'm not sure about I whack the leads against my bench power supply terminals set at 5v.  A quick blip the wrong way won't hurt.

Some rectifiers were marked so the B+ came OUT of the "+" lug.

However when the diode is a destination, not a pass-through, + goes IN the "+" leg.

When I took chemistry in college, they referred to electron flow as current, and since electrons flow towards the cathode, that was the positive element.  I even remember them saying, "Just remember the 't' in cathode is like a plus sign."

ruffrecords said:

I have lost count of the number of times I have connected a LED the wrong way round.

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Me too!  I check every single one now, I don’t even bother to look at em...

Just blame Benjamin. Franklin, I do.

It's much too late to switch conventions regarding the direction of current now though.  If the discovery of electrons didn't do it, probably best to just go with the... er...  flow.

Don't get too invested in simplified descriptions of atomic level interactions. The electrons (holes?) do not really flow, but bump along displacing nearby neighbors creating an apparent electrical current. 

Measure twice solder once.

JR

JohnRoberts said:

Don't get too invested in simplified descriptions of atomic level interactions...

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Exactly.  Too busy wondering whether a positron is really an electron moving widershins or backwards in time.

Otherwise, I generally flick the led against a battery when I forget which way round it needs to go.