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Power LED polarity

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Hero999

Banned
Ask the seller?
 

_helio_

New Member
Yes, the easiest and fastest way, so the best one. But, how about a rule of the thumb to identify the anode (or cathode) in all LEDS?
Thanks
 

KMoffett

Well-Known Member
Take a look at the eBay ad pictures. The CAD drawing shows a + on one of the tabs.


.....Never mind!.... When I enlarged the image it's a hole. The pixels on my monitor made it look exactly like a "+". :eek:

Ken
 

_helio_

New Member
Thanks to your keen view, I can see the positive now :)

My expensive LED will be handled with care.

Thanks again
 

Hero999

Banned
I can't see a positive sign anywhere or is it just me?

I hope you know that you can't connect this LED directly to 12V, you need a constant current driver.
 

KMoffett

Well-Known Member
hero999,

Re my post:
".....Never mind!.... When I enlarged the image it's a hole. The pixels on my monitor made it look exactly like a "+". " :D

Ken
 

audioguru

Well-Known Member
Most Helpful Member
Don't buy a cheap Chinese no-name-brand LED at a high price. It doesn't even have a datasheet. It might not work.
 

Hero999

Banned
Not necessarily so, it could be a cheap LED from China, resold for a high price. :D
 

_helio_

New Member
thanks for the clue, I've just noticed that my super chinese power supply, digital readout, current controlled (price $70), doesn't control the current

with LEDs the history is different: I often see that the most innovative LEDs appear first on ebay, and after some months they are sold by retailers of the big brands
 

_helio_

New Member
KMoffett,
the LED works very well thanks to your suggestion

there are not '+' on the drawing, but there is a further hole on the photo, and this terminal presents a dent too

I've considered this the anode (and I was right), then applied 12 V, AND NO LIGHT

I've reversed the supply and applied power just for a second, and still no light

then I've again reversed the supply and increased smoothly the voltage: the first dim was at 14V, the full light at 18V; the current was 0.7 A
 

_helio_

New Member
Hero,
when I have to do with a LED of unknown polarity I gently increase the voltage by starting at 1 V and keeping track of the current

I confess, if no light appears I reverse the polarity just for a fraction of time, but I'd like a rule to individuate the polarity

I've noticed that if I read the voltage between the anode and the cathode it is ≈ 70 mV, and that if I reverse the polarity the voltage is ≈ 15 mV: can this constitute a rule?
 

Hero999

Banned
Hero,
when I have to do with a LED of unknown polarity I gently increase the voltage by starting at 1 V and keeping track of the current

I confess, if no light appears I reverse the polarity just for a fraction of time, but I'd like a rule to individuate the polarity

I've noticed that if I read the voltage between the anode and the cathode it is ≈ 70 mV, and that if I reverse the polarity the voltage is ≈ 15 mV: can this constitute a rule?
The LED is really three banks of three series LEDs connected in parallel. This means that the maximum reverse voltage will probably be 15V.

Just connect the LED to a 15V supply in series with a 47R resistor.
 

mneary

New Member
Shine a bright light on the diode. It will generate a small voltage in the same polarity as it needs to be connected. (The anode will show the positive voltage.)

A white or blue LED needs a very bright light containing blue or ultraviolet wavelengths (e.g. sunshine or fluorescent) and it will generate a few millivolts and a few microamps.
 
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audioguru

Well-Known Member
Most Helpful Member
If you can't read the Chinese datasheet then you don't have a clue what they sell.
Maybe they don't know what they sell.
 

_helio_

New Member
The LED is really three banks of three series LEDs connected in parallel. This means that the maximum reverse voltage will probably be 15V.

Just connect the LED to a 15V supply in series with a 47R resistor.
Hero, the maximum voltage I've applied is 22 V, without damages (current 0.7-0.8 A); at 15 V this LED only starts to glow

sure about the need of resistors? strangely, I've tried the LED from 1 to 22 V by increasing gradually the voltage without current limiters, and it never showed signs of extra current nor of overheating
 
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