In the circuit diagram shown, we have used a common cathode seven segment in which all the LEDs are connected to a resistor and then to a switch. So the resistance should be approximately around 200 ohms. Each led segment draws a current of approximately 15 mA to glow properly. If we have 5 volts DC battery, then the drop across led is 2 volts. So if we want to connect resistor for LEDs segments of red light then the resistance can be calculated as below. For example forward voltage drop of blue led is 3.6 volts whereas it is 2 to 2.2 volts for red LEDs.Įxternal resistors in series are connected with these LEDs to control the forward voltage drop. Different colored LEDs have different forward voltage drop. So we need to control this forward current by using resistors to stop LEDs from getting damaged. If this forward current exceeds the limit then it can damage the led.
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