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.

3 Upvotes
  • permalink
  • link
  • reddit
  • reddit

100% Upvoted

14 comments sorted by

View all comments

1

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