Help with reading an LED Datasheet

[Jamie1994]

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]

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]

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]

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]

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.