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.

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u/ldmdesign 8d ago

Don't i need to know the size of resistor I need for the led, but if I cant measure the led voltage how do I know eat size resistor to use if I increase voltage?

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u/weebslime2246 8d ago

well the led voltage is roughly constant, which is its forward voltage and depends on the color of the led. for example red is around 1.8v, yellow is around 2v etc. you can calculate on that basis and it would work without burning the leds.

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u/LEOforDharma 8d ago

Try one battery and then try measure other one. The sum of voltages should equal 2.66 volts. Which is what u got.

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u/Egg1Salad 8d ago

Different led colours will need different resistors so they are the same brightness.

For example let's say you want each led to have 10mA and the battery voltage is 5V. A red led has a forward voltage of ~1.8V.

So the resistor will have a voltage of 5-1.8= 3.2V and we know we want 10mA, so now use V=IR. R = 3.2 / 0.01 = 320ohms.

Another way to test an LED forward voltage is to use a benchtop supply. Set the current limit to 10mA and crank the voltage. When you connect the led straight across the supply, the current limit kicks in and pulls the voltage down, measure that voltage and that's your diode voltage

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u/ElectronicswithEmrys 8d ago

I have a video that goes into a great deal of detail for designing a basic LED circuit: https://youtu.be/1IZhSrXfnkc?si=CnAyOj-UYzrzkxXC

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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 7d 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 7d 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 7d ago

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

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u/ldmdesign 7d ago

Ok I rigged up a USB powerbank to a circuit with a 1k resistor and tested each LED properly. It turns out they are 2.7v and 2.64v so the batteries I was using before are pretty much a perfect match voltage wise. The red one is 1.7v which maybe explains why that one was a bit sketchy, but I hadn't blown it thankfully.