# Help with reading an LED Datasheet

#### Jamie1994

Jun 15, 2022
2
Hello

I am trying to calculate the resistance needed to safely power an LED.

I am using the formula:
Resistor = (Battery Voltage – LED voltage) / desired LED current

But I as I am new to electronics, I am not sure how to determine the voltage and current values to use from my LED datasheet:

Here it does not specify the voltage drop across the LED.

There is also two values for current (peak and constant), and I am not sure which value to use in my formula.

Any help would be greatly appreciated.

Jamie

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
This table shows max. ratings only. Usually you'd find diagrams showing the I vs. V characteristics in the datasheet, to. From these you take the voltage drop at a given current.
There is also two values for current (peak and constant), and I am not sure which value to use in my formula.
As it says: "continuous forward current" - that is the current you can drive savely through the LEd for prolonged periods. If you were to pulse (blink) the LED, you should also find a graph showing pulse current vs. duty cycle in the datasheet.

If you don't have the datasheet with curves, you can measure the voltage drop at a given current using a power supply with variable current limit: Set the current limit to e.g. 30 mA and measure the voltage drop across the LED.
You don't have such a power supply? experiment with e.g. a 9 V battery and different resistors to approximate a current f 30 mA, then measure the voltage drop across the LED. As a starting point you may use the table shown here to approximate the expected voltage drop for use in your equation for the resistor.

#### Jamie1994

Jun 15, 2022
2
Hello Harald

Thank you this is extremely helpful. I had a look and you are right, there is more information on the datasheet and I found the value of the forward voltage:

So I can now substitute the values for continuous current and forward voltage into my formula:

Resistor = (Battery Voltage – 2 V) / 30 mA

Jamie

#### Martaine2005

May 12, 2015
4,960
Hi Jamie, if the LED is too bright for your liking, you can of course lower the current. Modern LEDs are still very bright at 5mA.

Martin

#### CircutScoper

Mar 29, 2022
300
Hello Harald

Thank you this is extremely helpful. I had a look and you are right, there is more information on the datasheet and I found the value of the forward voltage:

View attachment 55437

So I can now substitute the values for continuous current and forward voltage into my formula:

Resistor = (Battery Voltage – 2 V) / 30 mA

Jamie

Note that 30mA is the absolute maximum rating. Continuous operation at absmax may not be compatible with a long service life.

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