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expertengr
Guest
Hi, Normally we use a resistance in series to limit current for LED between 15-20mA otherwise it burns. I am wondering how to calculate the value of resistance when LED is connected to Microcontroller. Let's suppose we have a LED which needs 20 mA current. At some places they consider voltage drop of 2 V across the LED and then they calculate the resistance value as given below.
R = V/I = (5V-2V)/20 mA = 150 Ohm
But logically, a LED in forward bais drop 0.7 V like an ordinary PN diode, is that right ? If so then the calculation should be like this
R = V/I = (5V-0.7V)/20 mA = 215 Ohm
The other way of solving this is by including the internal resistance of LED, at some places it says 25 Ohm. The total resistance will be then the series resistance + the LED internal resistance. The calculation of series resistance in this case will be then
R + Internal Resistance = V/I
R + 25 Ohm = 5/20 mA
R + 25 Ohm = 250 Ohm
R = 225 Ohm
Q1: What is the actual voltage drop across the LED in circuit ?
Q2: What is an internal resistance of LED ?
R = V/I = (5V-2V)/20 mA = 150 Ohm
But logically, a LED in forward bais drop 0.7 V like an ordinary PN diode, is that right ? If so then the calculation should be like this
R = V/I = (5V-0.7V)/20 mA = 215 Ohm
The other way of solving this is by including the internal resistance of LED, at some places it says 25 Ohm. The total resistance will be then the series resistance + the LED internal resistance. The calculation of series resistance in this case will be then
R + Internal Resistance = V/I
R + 25 Ohm = 5/20 mA
R + 25 Ohm = 250 Ohm
R = 225 Ohm
Q1: What is the actual voltage drop across the LED in circuit ?
Q2: What is an internal resistance of LED ?