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Old 21st June 2010, 03:20 PM   #1
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Default LED polarity

I was wondering if anybody knew of a simple way of telling the polarity of an LED by simply looking at it?
Jeff
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Old 21st June 2010, 03:27 PM   #2
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Try this

led polarity - Google Search
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Old 21st June 2010, 04:08 PM   #3
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Old 1st July 2010, 10:00 AM   #4
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Can u see the pins inside. Is it a non-white LED.
The smaller looking one is positive i.e. anode.
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Old 2nd July 2010, 07:42 PM   #5
BrianVG is offline BrianVG  Belgium
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The long pin is the positive and the shorter is the negative. If you have a multimeter you can also test it with the mode to test resistance, if a material has a lower resistance than xxxOhm(200 with my decive) it wil 'beep'. So when the LED lights up you can see from you probes wich is positive and negative(COM=negative). The multimeter will send a little current trough the led to test resistance but this will also let you LED work.
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Old 3rd July 2010, 06:03 AM   #6
wwenze is offline wwenze  Singapore
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Originally Posted by BrianVG View Post
The long pin is the positive and the shorter is the negative. If you have a multimeter you can also test it with the mode to test resistance, if a material has a lower resistance than xxxOhm(200 with my decive) it wil 'beep'. So when the LED lights up you can see from you probes wich is positive and negative(COM=negative). The multimeter will send a little current trough the led to test resistance but this will also let you LED work.
May or may not work depending on the band gap voltage of the LED, typically a few volts, vs the voltage from the multimeter, typically in millivolts.
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Old 3rd July 2010, 07:33 AM   #7
Mooly is offline Mooly  United Kingdom
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Looking at the LED is an old trick... many new ones don't conform nowadays.
It's never a problem, a DVM on diode check lights most to confirm, if it doesn't a 9 volt battery and 10 k resistor does.

And just to go slightly off topic, where it does get really spooky is identifying the drain and source on a common JFET.

I have the "official" answer from the man himself (Bob Pease of National Semiconductor) if anyones interested
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Old 3rd July 2010, 07:41 AM   #8
BrianVG is offline BrianVG  Belgium
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Originally Posted by wwenze View Post
May or may not work depending on the band gap voltage of the LED, typically a few volts, vs the voltage from the multimeter, typically in millivolts.
Worked perfectly for my 7-segment displays. It won't work with high brightness LED's I think. He can only try to know it
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Old 3rd July 2010, 09:39 AM   #9
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Hi all

Unfortunately the old trick of checking the lower wire with the chip attached might not be a reliable guide - some LEDS have anode substrate others cathode substrate.

Many LEDS are not able to handle high reverse voltages. Even a 9V battery used in many digital meters might be too high (according to the data sheet).

The forward voltage of a LED might be too high for measuring the resistances on an old meter with 1.5V battery (and Avo's 15 V for high resistance may cause more damage).

Simplest way is to hook up a 4.5V battery through a 220 ohm resistor and see which way it lights.

Cheers
John
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Old 3rd July 2010, 10:14 AM   #10
SY is offline SY  United States
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With packages like T1 and its relatives, the cathode is marked by a small flat spot on the rim.

Pease uses a battery and resistor, FWIW.
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