Calculating the value of resistor and testing a simple LED circuit

You often got encountered with circuits having many resistors of different colors. We also often think that how a circuit designer comes to know what value of resistor is to be connected with the particular component like LED, IC or in any other biasing circuit. We are going to resolve this today. 

In earlier times, there was a scientist named Georg Ohm who blessed electronics by giving a law known as (well you know that!!) Ohm’s Law. According to which “A resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.” Yes this looks to be a bit confusing but the moral of the story what Georg wants to let us know can be simplified by its mathematical formula:

R=V/I

Where R= Resistance

       V= Voltage

       I= current 

Resistance is the property of any conductor which opposes the flow of current through it. The element that posses this property is called as Resistor but every element like a good conductor such as copper also possess very small amount of resistance. The opposition or “Friction” provided by these elements to the flow of current is dissipated in the form of heat. This is one of the many reasons behind heating up of electronic devices.

Why we need resistors when we got a power supply of 5 volts: 

In previous tutorial, we made power supply that delivers us a voltage of 5V (volts). This voltage level is comfortable when working with IC’s but for LED’s, this value is very high. To lower the input voltage supply to the LED, we will give some resistance to the flow of current using resistor.

Calculating the resistor value for LED:

To calculate resistor value, we need to know about two things about LED, its input current and the voltage on which it is working on. The general purpose LED’s comes with a rating of 2.5 volts and 20mA (Milli-amperes). So to calculate the resistance, follow the steps below:

1.      Determine what value of voltage you are providing to the circuit. In our case, it is 5 volts.

2.      Now find out what is the value of voltage your LED is seeking for. In our case, it is 2.5 volts.

3.      Now subtract the voltage required by the LED from the Input voltage .i.e. 5v-2.5v= 2.5 volts.

4.      Now divide this voltage by the current value which in our case is 0.020 Amperes (Ohm's law).

R=V/I

R= 2.5/0.020

Therefore, R= 125 ohms

So, we have to use a resistor of rating 125 ohm to light up the LED. Now when you will go to market to buy a resistor of this rating, you will never find out one having exact rating, well nothing to worry about, you can always use a resistor which have ratings close to that we calculated.

Also, if you want to directly light up the LED with your battery i.e. without making a power supply unit, Then in that case, if you battery is of 9 volts then use resistor of value ranging between 325 ohms to 470 ohms.

Positive and negative lead in LED (Polarity):

 

Sometimes or I must prefer saying mostly beginners use to detect the poles of LED by checking the length of its leads or legs. In general, the longer leg is positive and the shorter one is negative leg. But what if both the legs have same length?

 

Well to find the correct polarity, take a closer look at the LED. You will see something identical to pic below :

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The LED has a semiconductor chip, which in this case is wider part. This is the positive lead of LED and the other lead connected to less wider part is called as lead frame. It acts as negative polarity of the LED. Hence, now you can find out the polarity of LED with an ease.  

To have a quick check of polarities in LED, set your multimeter on diode mode. Now touch the positive strobe of multimeter to positive leg (Anode) of LED and negative strobe of multimeter with the negative leg (cathode) of LED. You will see that LED started glowing.

 

We now have a resistor of appropriate value, a power supply unit and that’s all we need in our circuit to implement. Let’s start making one.

Components you will need:

 

1.      LED.

2.      Resistor. (We have calculated the value of resistance for 5 volts and 9 volts. If you are using Battery/voltage source of any other voltage rating, calculate the same by following above steps.)

3.      Potentiometer (It is explained in implementation part.)

4.      Voltage source/battery (more than 5 volts).

5.      Our power supply unit from previous post.

6.      And one more thing from last post Enthusiasm…!

So, here we go:

Step 1).

 Take the power supply unit that we made in our previous tutorial. (If this is the first time you are reading my blog click here take a tutorial over making a power supply).

 

Step 2). 

Now insert the LED in the component bus as shown in the pic below.

As our component bus is connected vertically from inside, in case if you don’t know about breadboard inside, click here. 

From above our circuit will appear somewhat like the one below:

Step 3).

 Now it’s time for resistor. Insert the resistor of value you have calculated from above steps and insert one of the legs of resistor in the same vertical component bus in which you inserted your LED pins. You can insert the resistor in either of the pins that doesn’t matter. Now insert another leg of resistor in the source bus. Take care of polarity of LED. But remember, either way you do, current should first cross the resistor and then enter the LED. Connect the left LED pin to source bus by a single core wire as shown below:

Step 4). 

Okay… the circuit is completed. Insert positive of battery in the pin number one of IC 7805 and negative in the pin number 2 of IC 7805 and BAM! Our LED will start glowing.

Watch the video showing the step by step implementation of above circuit below:

 

So, this was the easiest circuit built on the breadboard. From now, I hope you have become pro in breadboard connections. If not, read my previous blog and do little work out on it. Therefore, considering this I will not repeat the breadboard connections in the upcoming tutorials because sometimes we get irritated by reading something which we already know.

I know you didn’t get the feel of electronics till yet. But in the next tutorial, I will introduce you to the sensor called LDR and will make a fun circuit which lights up LED in dark automatically and turn it off in presence of light. We will also take a closer look on integrated circuit called as Op-am. So keep reading and keep learning.

Thank you for the read. Have a nice day….