I am repairing a disco light unit, this consists of 7 multicolour circular led arrays,
blue was not lighting up or flickering in the centre array only.
i replaced the obviously dead leds, but still flickering,
the control board tested fine.
having eliminated the control board , i decided to replace all the blue leds,
this is where things went downhill- i ordered new leds from a reputable supplier,
and replaced them, the led outline was clearly printed on the board,what could go wrong?
well something did, the leds didnt work and something seemed to be getting warm!
after some hours diagnosis i eliminated everything apart from the leds.
it turned out that the flat on the leds was on the "wrong"side.
(normally i would use the lead length but as the polartiy was clearly marked on the board)

i queried this with the supplier, he said this was normal and the manufacturers put the flat on the anode side to indicate the type of led. and i should have known this.
i would have thought that industry standard meant just that.
i would be interested to hear your views and perhaps an advisory.

tibbles

 

My view? Never trust the advertising. This is why factories have incoming parts inspection departments. Even if you buy a good brand from a known vendor, mistakes happen and counterfeits exist.

 

There are a handful of identifiers for finding the positive and negative pins on an LED. You can try to find the longer leg, which should indicate the positive, anode pin.
Or, if someone’s trimmed the legs, try finding the flat edge on the LED’s outer casing. The pin nearest the flat edge will be the negative, cathode pin.

They lie! Lies I tells ya!
OK, have a LED sitting here, just a little generic red in color LED. The longer leg is on the flat so how can the longer leg be the anode and cathode both? Actually in this case the longer leg is the cathode and is on the flat. I have seen them in all flavors and configurations and if there is a "standard" it has managed to elude me.
With a large lot it is wise to just, at random, check a few.

Ron

 

hi #12
thanks for your reply
"Even if you buy a good brand from a known vendor, mistakes happen and counterfeits exist."
that was my first thoughts, but the guy seemed to know what he was talking about-"position of flat to indicate InGaN (3.2v or GaP (2v)"

 

Hello,

Have a look at the following page:
http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html

Bertus

 

Hello,

Have a look at the following page:
http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html

Bertus

I don't see anything on that site that talks about packaging and polarity marks at all.

 

i queried this with the supplier, he said this was normal and the manufacturers put the flat on the anode side to indicate the type of led. and i should have known this.
i would have thought that industry standard meant just that.
i would be interested to hear your views and perhaps an advisory.

In general it is your responsibility to be familiar with the specs of the components you use. That doesn't make the assumption you made too unreasonable, but at the end of the day it really WAS your responsibility. Sooner or later we all get burned by something like this -- component packaging that superficially appears to follow the usual convention but, in actuality, is not. I have aluminum electrolytic capacitors that use a black stripe on the side to indicated the negative terminal and others that use the same thing to indicate the positive terminal. It can be very frustrating, especially if you aren't aware of it and you go and mix the two types in your parts bin!

As for "industry standard" -- I'm not aware of any industry standard that addresses this (though there might well be). If you can come up with the standard number, it would be interesting to take a look at.

 

When you make the spec wider you can get a less expensive component. An end user such as a lighting unit has different concerns than a LED that is observed directly: it can well tolerate color shifts, brightness shifts, and so on.

Now imagine you are a lighting maker and some sales guy cals you to say " Our new assembly guy made a batch of LEDs with the tab on the wrong side. If I sell them for 50% off would you be interested in 10,000?'

I'd sure jump at that offer.

 

Hello,

Did you see this image:

It shows the inner works of the led with the polarity.
The cup is the cathode and the arm is the anode.

Bertus

 

While the cup is usually the cathode, there are exceptions. Depends on the type of substrate the LED is made from. Also, many lens materials make it effectively impossible to discern the inner details. But, still, it's a useful check, if not completely definitive.

As an aside -- I still find it utterly amazing that such a complex device can be manufactured and sold for pennies.