How to find LED voltage and current

I got this question from Awaji about the light-emitting diode (LED):

Simple circuit to find LED voltage

[Awaji]: How can I know the LED voltage and the current ratings of an LED? Also, how is the resistor of an LED chosen? Thanks


Here are three different ways to find the LED voltage and current:

  1. The easiest way is to look it up in the datasheet, or the store where you bought it. Look for LED forward voltage and the Test Current.
  2. You could find the LED voltage by using a multimeter with a diode function.
  3. You could connect a battery to the LED and a potentiometer. Start with high resistance on the potentiometer and gradually decrease it until you have a nice brightness. Then measure the voltage over the LED, and measure the current going through the circuit.

How to choose a resistor for your LED

First, figure out the voltage that your LED needs by using one of the three methods described above.

Then figure out the current you want to flow through the LED. If you’re not sure, aim for somewhere between 1 mA and 10 mA.

A rule-of-thumb you can use with LEDs is that they will “grab” the voltage they need (if you have it set up correctly). This means that if you connect a 9V battery to a 2V LED with a resistor in series, the LED will have 2V across and the resistor the rest (7V).

This means you can calculate the resistor you need using Ohm’s law:

You know the voltage across the resistor (ex 7V), and you know the current you want (ex 10mA/0.01A). To calculate the resistance you need, just divide the voltage by the current:

R = V / I

R = 7V / 0.01 A = 700 Ω

So you need a resistor of 700 Ω to get 10 mA of LED current.

The resistor that you place in series with an LED is called a current limiting resistor because it limits how much current that can flow.

More Light-Emitting Diodes (LED) Tutorials

7 thoughts on “How to find LED voltage and current”

  1. I don’t have the data sheet but I have about 50+ LEDS all sizes and colors. Since I don’t know the ratings is there a general rule of thumb on what size resistor to use?


  2. BTW I am really interested in building a 1w – 1.5w AMP for my phone/mp3 player do you have any plans on that or anywhere that can explain to me how to build one? I see a lot of sites that give away plans but they don’t explain where to add the potentiometer or power, etc and I am brand new to the world of electronics and i have bought just about every necessary tools to do stuff with since i have this new passion.

  3. I have a fully unknown LED that I would really like to know the voltage of. All I know is that it’s a 10mm red LED and that it doesn’t seem to mind 2.5v…

    • You could try hooking it up to a power source of 5-10V with a 1k ohm resistor in series, then measure the voltage drop across it.


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