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.)
|
|