LED Protection Resistors

Most LED devices will be damaged unless a protection resistor (R) is connected in series with them.

The resistor acts as a current limiter as the LED is a non-Ohmic device and does not regulate the current flowing through itself. The resistor value is determined by the type of LED being used and the supply/battery voltage used to power the circuit. Diagram 1 below shows the simple protection circuit needed.

diagram 1
diagram 2

 

How to work out the resistor value (R) required

1, First gather the information you need:

We have already seen that the value of the protection resistor is determined by the type of LED and the power supply voltage in the circuit.

From the internet or a component supplier's magazine, look up the colour and size or LED you are using. They will supply two important pieces of information:-

The first is the current it requires - this is known as its If value.
The second is the voltage it needs across it - this is known as its Vf value.

(See values in diagram 2 above).

The final piece of information you need is the battery or supply voltage Vs. This is easy to find out as batteries always state this on their case. You could use a multimeter to make sure of this too.

2, Now use this formula, with the information you have found, to work out the resistor value. Alternatively, you may find it easier to put the information into the protection resistor calculator above.

 

Try this Example with the Calculator at the Top

A "Super Bright" LED was found to have the values of Vf and If as shown in the table below.

If the LED is to be placed in the circuit above, with a 12 volt power supply, use the calculator at the top of the page, to find the preferred protection resistor you would need.

Supply Voltage
Vs

(volts)
LED
Vf

(volts)
LED
If

(current in milliamps)
Example 3mm
Super Bright Red LED
12
2.1
30

How The Above Formula Was Produced

 

You may be interested to see how Kirchoff's 2nd (voltage) Law and Ohms Law allow us to calculate what is happening in circuits.

This is how they have been used to produce the above formula...

 


Step 1,

Kirchoff's 2nd Law tells us that the power supply voltage (Vs) is equal to the sum of the voltages across the other components in the circuit: (Vr + Vf).

We can use this to write our 1st equation: Vs = Vr + Vf

 


Step 2,

Ohms Law tells us that the voltage across the resistor (Vr) is equal to the current (If) in the circuit multiplied by the value of the resistor (R).

We can use this to write our 2nd equation: Vr = If x R

 


Step 3,

Using some simple algebra we can now combine the 2 above equations to provide the single equation we need to calculate the protection resistor value.

From equation 2 we know Vr = If x R ,

(So, in
equation 1, we can replace Vr with If x R)

Doing this in equation 1 gives us: Vs = (If x R) + Vf

(We use brackets to show that If and R need to be multiplied together first.
The result of this multiplication can then be added to Vf).


Step 4,

This is the formula we need, but as we are interested in finding out the value of R, we now need to rearrange Vs = (If x R) + Vf , so R is on its own on the left.

Starting with Vs = (If x R) + Vf

make
change 1

subtract Vf from both sides Vs - Vf = If x R
make
change 2
move left side to right side If x R = Vs - Vf
make
change 3
divide both sides by If.
(Note:
If / If = 1)
R = Vs - Vf / If

 

Which gives us our circuit formula
to calculate
the LED protection resistor R:



(Note: This is the algorithm used in the LED protection resistor calculator.)

PREFERED RESISTOR VALUES
After using the formula or calculator, select the nearest higher value of resistor from the list below (eg. If calculated resistor = 135 then go up to 150):
22 - 27 - 33 - 47 - 56 - 68 - 100 - 120 - 150 - 180 - 220 - 270 - 330 - 390 - 470 - 560 - 680 - 820 - 1000