How do you tell “ordinary” LEDs (that need a resistor) from 12v LEDS (that don’t)?

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#1 21683325 13 Oct 2019 15:33

Michelle OBrien Michelle OBrien

Anonymous

Post #1
21683325 13 Oct 2019 15:33

Is there a way to tell an "ordinary" LED (that needs a resistor inserting in the circuit to protect the LED from excess voltage that would risk burning it out) from a 12v LED (that doesn't because it has a built-in resistor)? The two types of LED look exactly alike.Michelle O'Brien
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#2 21683326 14 Oct 2019 07:57

PeterTraneus Anderson PeterTraneus Anderson

Anonymous

Post #2
21683326 14 Oct 2019 07:57

Connect a 1000 ohm resistor in series with the LED, and measure the voltage across the LED as you apply 3V to 12V across the resistor-LED series combination. An "ordinary" LED will display the relatively-constant voltage of a diode. A 12V resistor-LED will show varying voltage depending on the voltage drop of the resistor inside the LED, since you now have two resistors in series with the diode.
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#3 21683327 14 Oct 2019 14:12

David Ashton David Ashton

Anonymous

Post #3
21683327 14 Oct 2019 14:12

Peter's method is good Michelle, but he did not include the usual LED voltage. If (with a 1K series resistor) it lights and has around 1.6 to 1.7V across it (for a RED LED, green/yellow LEDs are slightly more, blue leds a bit more still and white LEDs up to 3v) it is a plain LED. If it has any more than the above voltages it's probably got a built in limiter.
#4 21683328 16 Oct 2019 16:53

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21683328 16 Oct 2019 16:53

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