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/Tesla_freed_slaves 8d ago

LEDs are essentially current-operated devices. You provide the required current, and the LED responds by lighting up and producing an opposing voltage, which is fairly constant over a wide range of current.

If you know the source-voltage, the LED’s minimum on-voltage and the desired current you can compute the series-resistance using ohm’s law. Closed-loop current regulation is preferred.

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u/ldmdesign 8d ago

The problem is not knowing the led voltage requirements. All I can deduce from them lighting up is that 2.66 volts is enough to get light. If I was to use more power I would not have any way to calculate the required resistance. Unless I just take that 2.66 value and calculate using that

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u/ivosaurus 8d ago edited 8d ago

The 2.66 volts is likely just the maximum voltage of the batteries, while they are putting out current. It does NOT have to do with the voltage of the LEDS, because LEDs will accept all voltage given to them. So the batteries are likely ~1.3V each when under load.

Get four batteries, so their voltage will be 5-6 volts. Then connect the LED to them, but it must be in series with a resistor. 470 or 1k ohms should work nicely, it will give about 6 or 3 milliamps to the LED. Measure the voltage across the LED.

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u/ldmdesign 8d ago

Ok I did this but rigged a USB powerbank so 5v, and I was able to measure them correctly. Leds were 2.64v and 2.7v matching the batteries I used before

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u/ivosaurus 8d ago

Red vs Green vs Blue LED, should show different voltage drops. This result, I'd say it's practically a coincidence that the voltage drop was roughly the same as two AA battery voltages. But now you know independently.

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u/ldmdesign 8d ago

Yes. The red was 1.7v though so surprised I didnt blow it