[Music] Hello viewers, welcome to Tech StudyCell. In this video, we will see how to calculate the resistance value while connecting an LED in a circuit. It is very important to connect a resistance in series with the LED if the forward voltage of the LED is less than the input voltage of the circuit, otherwise the LED may burn. So in this video we’ll discuss the calculation of the resistance value for different input voltage with some practical experiment. Here I have used a 5 mm green LED and from the datasheet we can see the forward voltage of the LED is 3.0 – 3.6 volt and the current rating is 30 milli ampere. From this data we can conclude the current through the LED should not be greater than the 30 milli ampere. In this schematic you can see this is the input voltage (Vi) and I have use a resistance (R) in series with the LED to limit the current within 30 milliampere as the current rating of this LED is 30 milliampere. Now here I will use a 9-volt battery as an input so the input voltage will be 9 volt and we have already got the forward voltage of the LED which is 3 volt and if we deduct 3 volt from the 9 volt then we will get the voltage drop across the resistance. So the voltage drop across the resistance will be 6 volt. From this equation we can easily measure the value of the resistance. we have already got the voltage across the resistance and we know the current rating of the LED, the (I) should be within limit of 30 milli ampere which is 0.03 ampere. So if we calculate the value of the (R), we will find 200 ohm. So here in the circuit I will use 220 ohm to limit the current within 30 milli ampere. Now to find a wattage of this register we can use this simple formula which is (W)=(V)*(I). Here the (V) is 6 volt which is the voltage drop across the resistance and (I) is the current through the resistance which is 30 milliampere or 0.03 Ampere. So if you multiply these two value, we will get the wattage of the resistance should be. So here we are getting 0.18 watt so the wattage value of the resistor, we should use should be greater than this 0.18 watt. So here I will use 0.25 watt register to make the circuit. now we will build the circuit and measure the voltage and current value accordingly here I have connect a 220 ohm register with a positive terminal of the battery as you can see we have already calculated the resistance value which is 220 ohm to limit the current within 30 milli ampere and I have connected the cathode of the LED with the negative terminal of the battery as you can see the cathode is connected with a negative terminal of the battery. Now if you connect these two point you can see the LED is glowing. now we measure the voltage and current with the voltmeter and ammeter. Let me connect the voltmeter and ammeter with the circuit so that I can show you the value of the current. Now here you can see I have connected an ammeter in series, so we can measure the current and you can see the value is 26.5 milli ampere. now we will measure the voltage. First we’ll measure the input voltage. you can see the voltage is 8.8V which is aprox 9 volt. next we’ll measure the voltage across this led. now if we measure the voltage across the LED which is 3.1 Volt that is (VL)=3V. Now we are going to measure the voltage across the resistance. now you can see the voltage across resistance is 5.6 volt and from the calculation we have calculated the voltage across the resistance would be 6 volt. so all the data you can see practically which we have calculated earlier. Now we’ll change the input voltage to 12 volt and calculate the resistance value. Here if the input voltage is 12 volt and the forward voltage is 3 volt as we are using same LED. The voltage across the resistance will be (12 – 3)Volt which is 9 volt. so 9 volt should be drop across this resistance and the current rating should be 0.03 ampere which is 30 milliampere. so with the help of same formula, we can easily calculate the value of the resistance (R) for the 12 volt input. Voltage across the resistance is 9 volt as we have calculated and the current will be 0.03 ampere. so the value of the R will be 300 ohm. now here I don’t have 300 ohm resistor so I will use two 160-ohm resistors. So the total value of the resistance will be 320 ohm. Here I will use this variable power source as 12 volt source. first we will connect the 320 ohm resistor with the positive terminal of the DC source. here I have used two 160 ohm resistors in series to get total 320 ohm resistance. now we’ll connect the negative terminal of the DC power source with the cathode of LED. Now our circuit is ready if I connect these two terminal you can see the LED starts glowing. now we’ll measure the voltage and current with the help of these two multimeters. here in this multimeter you can see the value of the current through the circuit which is 28 milli ampere now we’ll measure the voltage first we’ll measure the input voltage you can see the input voltage is 12 volt. now we’ll measure the voltage across the LED, which would be 3 volt as we know the forward voltage of this LED 3 volt. so in a reading we are getting 3 volt or 3.1 volt in this multimeter. now we’ll measure the voltage across this resistance (R) which should be 9 volt. we have chosen the value of the resistance such that the voltage drop across the resistance should be 9 volt. let me show you you can see it’s showing 9-volt across the resistance. so we can easily calculate the resistance value for different input voltage while connecting an LED with the circuit to limit the current within the current rating of this LED. 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