### Introduction

Designing LED displays may be fun, but very often we are just left thinking how to wire led lights? Learn through a formula how simple it is to design your own LED displays.

We already know that a LED requires a particular forward voltage (FV) to get lit. For example a red LED will need a FV of 1.2 V, a green Led will require 1.6 V and for a yellow LED it is around 2 V.

The modern LEDs are all specified with 3V forward voltage irrespective of their colors.

But mind you, at these voltages the relevant LEDs will just begin getting illuminated and be hardly visible. To get an optimum light output from them, you have to at least apply a voltage which is 25% higher than the FWD voltage of the particular LED.

But since LEDs are very “current sensitive” may just blow off and get permanently damaged on doing the above explained method of illuminating them.

### Using Current Limiter Resistor

This problem is very easily solved by just adding a series resistor to the LED. It may be connected either to the positive or the negative line of the circuit.

The value of this resistor may be simply calculated through the below given formula:

**R = (supply voltage VS – LED forward voltage VF) / LED current**

For a series connection the total forward voltage will be required in the formula, by multiplying FV of each LED by the total number of LEDs in the series.

A 20 mA of current is enough for most of the purpose.

**How to Connect the LED Lights?**

To understand this let's read the following discussion:

· If for example you want to design a LED display using 90 red LEDs with a supply power of 12 volts, begin the procedure of construction first by dividing the supply voltage by 3 volts (Forward voltage of red LED for optimum brightness).

· The answer will be obviously = 4. This is the number of LEDs required for each series of LEDs.

However we won't connect 4 LEDs in series, because that would make the light pretty dim, Let's make it 3 for getting optimum brightness.

· Now dividing the total number of LEDs (90) by 3, we get an answer that's equal to 30. So you will have to make 30 numbers of series, each consisting of 3 LEDs.

· All the 30 series will naturally have a positive and a negative free ends.

· Now just connect the common positives ends of the LEDs and the resistor negatives ends of each series and apply 12 volts supply to these terminals.· You will instantly find the whole design glowing up brightly with an uniform intensity.

· You may align and organize these LED series as per the design of the display.

· A situation may arise when your designed display may contain LEDs in odd numbers.· For example if in the above case instead of 90 if the display would consisted of 101 LEDs, then we could have made 33 series of 3 LEDs each, but still there would be 2 LEDs remaining to wire.

· The problem is easily solved by just changing the resistor value for the remaining smaller series. This can be again done using the formula already explained in the article.

Anirudh Vyas says

i guess many such series can be paralleled.

How many such ckts can i parallel.It is used to illuminate my aquarium.I want a light equivalent to that of a 11 to 14 watt CFL . The main purpose of using LED's is to save energy.

Swagatam says

when you add leds in series, the whole string will consume the same current, but in parallel it's just the opposite.

therefore it's always a good idea to put leds in series and accommodate as many of them as feasible, depending upon the supply voltage.

for example in 12v you can engage 3 leds in series,that's the maximum number of leds withe optimum brightness, beyond this number you will have to incorporate parallel connections of more such series.

Anirudh Vyas says

Thank you sir,

I really liked this blog,Immediate response.

(a) How many LED's can i use,in this ckt

(b)And i planned to apply the following calculation.I just used elementary electronics not sure,

2A=2000mA so,2000mA/350mA (as 350 mA for each led)=5.71

and to prevent the damage i ll run them at 80% brightness so

5.71*0.8=4.57=(4 leds at 70% brightness or 80% brightness 5 leds)individual LED's parallel each other

Is it feasible i dont know the consequence of it.

Swagatam says

Hi Anirudh,

That's not the right way of calculating LEDs in a circuit.

use this formula:

R = (U – total Fwd V)/I, here R is the resistance, U is the supply voltage, Total Fwd is the forward voltage of the LEDs, and I is the safe maximum LED current.

Total fwd voltage refers to the added value of all forward voltages of the LEDs in series, for example if you are using three white leds in series, then the total fwd voltage of the series will be 3.3 x 3 = 9.9V

I hope you got it.

Anirudh Vyas says

sir i have a DC adaptor rated 12V 2A can i use it to power multiple 1W leds

Swagatam says

Yes, you can power 1watt leds with it, put them in series of three with 6 Ohm 1 watt series resistor.

Anonymous says

sir how is it 180E resistor? what is the wattage of the resistor?

Swagatam says

please read the above comment, wattage will be 1/4 watt

Bibin Edmond says

Hi swagatam,

As said above

Connect the calculated value (180Ohms nominal) of resistor to one of the free ends of each series.

how do u get that 180ohms…

The formula is like this right.. ?

R = (supply voltage VS – LED forward voltage VF) / LED current

Supply voltage- 12v

led forward voltage -3 (total of 4 leds so VF= 12)

Led Current – 0.020

so R=( 12-12)/0.020

ie is =0 right.. ?

Then how do u get that 180 ohms yaar.. please help.. !

Swagatam says

Hi Bibin,

Your calculation is correct, if the forward voltage is equal to the supply voltage it simply means that the system does not require any resistance (zero resistance) and the supply can be directly connected to the series.

The above article needs one correction, the fwd voltage of red LED should have been put as 1.2v and not 3v, so if we divide 12 by 1.2, it gives 10, that means without resistance 10 red led should be introduced for 12V, but that would make the light quite dim.

Therefore we need to reduce the number of Leds and add a calculated resistor in series for safety.

So now you can apply the formula as given above, it will give correct results if the fwd voltage drop is added correctly as per the specs of the particular LED.

Regards.

Bibin Edmond says

ok… Thanks swagatam… !

Anonymous says

Thanks sir….you are a genius among mortal men

Swagatam says

Thanks!

In parallel the current simply adds, if one string of LED consumes 20mA of current, two in parallel would consume 40mA and so on, however in parallel the strings must be uniform with the number of LEDs and the resistor values, these are the only criteria to be followed for LEDs in parallel.

In series if one LED goes bad, it would mean, the forward voltage of that LED being added into the string, overloading it slightly more, however as long as a resistor is there in series this won't cause much of a problem to the rest of the LEDs.

Anonymous says

Sir,thanks for the innovative formula,for led in series,now i would like to know if 1 Led goes bad would in affecd the whole led. And i would also like you to give me the formula for led in parallel.