What Size Resistor should I use for an LED

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We are going to do some sums here, but don't panic! it really is straight forward.

LED's like electricity, and they are Greedy! They will take as much current as you give them. Sometimes they take so much they go POP!

You must limit the amount of current going through a LED with a resistor, Here is how you decide on the size.

To make an LED light requires a minimum amount of voltage. This amount depends on the LED, but usually it will be between 1.5V and 3.2V

If you have the following information, it will help

What is the Voltage rating (Vf) of the LED           I don't know - go to the bottom of the guide

What is the current Rating (If) of the LED            I don't know - go to the bottom of the guide

What is the voltage of your supply (Vs)               

With these three pieces of information, we can work out our resistor size

It is simply, the SupplyVoltage - LED Voltage all divided by the Current    (Vs-Vf)/If

An example 9v battery, Vf=1.8v If=20mA (remember mA=Amps/1000)

9-1.8 =7.2v   divided by 0.02 (20mA) = 360 ohms  choose the nearest value larger than this  (in this case 390 ohms)    Done!

If 2 leds in series, 9-1.8-1.8 =5.4V divided by 0.02 = 270ohms ( a standard value)

 

I don't have all the information!

Very slightly trickier, assume the worst case and see what happens

For most LED's it is ok to assume a Vf of 1.8Volts and a If of 20mA (if your LED is a low power version, these calculations will make it pop.)

Remember Car electrics will need to be based on 13.8Volts, not 12Volts (battery charging voltage i.e. when the engine is running)

If the LED is dim, try the next size resistor down, and then down again until it is lit brightly. Always work from highest resistor down during this trial and error stage

 

WATTAGE

Resistors come in various power ratings. To choose the one you need, reuse some of the figures from above

Watts= Voltage * Voltage /Resistance  

so, using the 9V example above

Vf was 1.8V  so Vs-Vf was 7.2V

Resistor was 360 ohms

Therefore Watts = 7.2 x 7.2  all divided by 360    =0.144    as 1/4 watt is 0.25 this is perfect!

 

If you found this guide helpful, please tick yes below.  Thanks, Andrew

 

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