This is just a gripe. Throughout the electronics world polarized components are marked with the stripe or other distinctive marking on the positive terminal. That's fine. But why is that on SMD LEDs the dot and/or green stripe is on the negative side (the cathode)? For through-hole versions, LEDs follow the usual convention, with the positive terminal having the long lead. But only in the SMD versions is it reversed. I know this, of course, so I adjust accordingly, but every time I need to remind myself of this I wonder how such standard-setting error could have happened. Does anyone know?

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Comment by Jack Crossfire on September 13, 2008 at 3:02pm
Seems the through hole diodes all have stripes on the lower voltage side and the LEDs all have longer leads on the higher voltage side. I never remember the lead length because they're always recycled LED's.
Comment by bGatti on September 15, 2008 at 11:18am

Both Diodes and Capacitors have long market the negative terminal.
One could say LED's are market by "cutting" the negative lead "Shorter" - so that's really not a convention.

Ground is often the presumptive basis for electrical systems, with the "positive voltage" being the variable, interesting, measurable and inconsistent value.
Comment by MattTay on September 16, 2008 at 12:35am
There's no standard for part marking. You will only get a pin 1 designator. From there, you have to refer to the spec sheet. Your LED did have a pin 1 designator. You just don't know if pin is is anode or cathode :)

If you use a sot23 LED, you'll never have to check polarity again. Ever.


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