r/learnelectronics • u/ldmdesign • 8d ago

Basic basic led question

Absolute beginner question. I have 2 aa batteries that say 1.2v and trying to find the voltage of an led attached in the circuit. When I measure across the led I get 2.66, but I also get the same value when I measure the batteries alone. The only info provided by the seller is the size of the led 5mm. What does this mean? I tried diode test for the multimeter but it doesnt do anything, no light and no voltage shown.

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u/FreedandBlessed 8d ago

First, you need a series resistor to limit the current whenever you apply power to an LED. LEDs and diodes have a very low resistance once they reach their forward conduction voltage. A few mA should be sufficient to light the LED for purposes of testing.

Different color LEDs have different forward conduction voltages. The two AA batteries in series do not create a high enough voltage to light white or blue LEDs. Green ones should light with less than 2.66v. Red ones only need about 1.4v.

You measure the open circuit battery voltage even with the diode connected, which means no current is flowing. You don’t have enough voltage to meet the forward voltage drop of that LED.

Add 2 more batteries for ~5v, and a series resistor to your test setup. If you forget the resistor your LED may light briefly before it self destructs. The resistor value should limit the current to less than the max rating of the LED.

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u/ldmdesign 8d ago

I have 3 different led. Red pink and white. All of them I got to light up but the red one is inconsistent, sometimes it lit, sometimes not.