Backwards marked LEDs...?

[mjg]

I've just spent about half an hour trying to work out why the rate LED on my "Shark Fin" build wasn't flashing. 

I tried some other coloured LEDs, and they worked fine.  But as soon as I put in a blue LED, it would stay on, rather than flash.  Tried resistors in series, which made it dimmer, but still didn't flash. 

After stuffing around for a while, I accidentally tried the blue LED 'backwards'  - it started flashing.    $&@&!!

Looking closer at the blue LEDs I've got, they are marked with the flat edge on the side with the longer leg.  Which is totally wrong isn't it?     I was using the flat edge to judge polarity, but the legs were telling a different story. 

I then went through all the LEDs I got in that batch (yes, a cheap eBay buy from China).  The blue and red LEDs have the flat edge on the longer leg... the yellow and white LEDs have the flat edge on the shorter leg. 

So I guess I've learned two things - look at the leg, not the flat bit on the body... and don't buy cheap eBay LEDs from China? 

[jimilee]

I've run across it from time to time, I started checking them with my meter before using them, it's always a hassle to remove leds


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[flanagan0718]

I keep a key FOB battery on my work bench to test all the LEDs I use. I've run into this before (or maybe my brain wasn't working) and they smell so bad when you blow them up. So I just test them on the battery just before soldering in place.

[davent]

I check them all with power just before i install or i'd be sure to mess it up.

dave

[mjg]

I guess I'm lucky that I socketed that LED - I was being indecisive about where to mount it. 

If I had have soldered it in backwards that would have sucked.  I'll be double checking them from now on before soldering. 

[jimilee]

If you use your mm on the diode setting, you don't need a battery, the LED will light up.


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[Timko]

I check them all with power just before i install or i'd be sure to mess it up.

dave

I do this too; I'll power the pedal up then insert the LED to make sure it's working correctly.

[somnif]

If you use your mm on the diode setting, you don't need a battery, the LED will light up.

That depends a bit on the color and the DMM. The diode checker on mine caps out around 2v, so no luck on most waterclears (even reds just barely glow).

[Willybomb]

If you use your mm on the diode setting, you don't need a battery, the LED will light up.

I read that as "If you use your mum on the diode setting".  Confused the hell out of me.

[jimilee]

If you use your mm on the diode setting, you don't need a battery, the LED will light up.

That depends a bit on the color and the DMM. The diode checker on mine caps out around 2v, so no luck on most waterclears (even reds just barely glow).

Right, that's all you need to check polarity though, right?


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[reddesert]

Of the two internal contacts in an LED, the larger one is the negative side.

I don't know if this is always-always true for all types, but it has been for every one I've looked at.

[somnif]

That depends a bit on the color and the DMM. The diode checker on mine caps out around 2v, so no luck on most waterclears (even reds just barely glow).

Right, that's all you need to check polarity though, right?

[/quote]

For reds certainly, but waterclear green, blues, or whites I need to rig up a battery and CLR to test. (Well, I rigged up one permanently with a 50k pot so I could test the best R to use with any given LED, but still)