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LED Series resistor value calculation
LED always has a series resistor along with it. This is” Ballast resistor”, the life saving device of
LED. It control the forward current to the LED to a safer limit and protect it from burning. Value
of the resistor if the factor that determines the forward current and hence the brightness. The
simple equation Vs – Vf / If solves the problem of resistor value. Vs represents input voltage, Vf
the forward voltage and If the allowable current through the LED. The resulting value will be in
Ohms. It is better to restrict the current to a safer limit of 20 mA.
LED along with the limiting resistor R4 is the power on status indicator. A significant voltage
drop ( about 2 volts ) occurs across the LED when it passes forward current. The forward voltage
drops of various LEDs are shown in Table 2.
Table 2
Red Orange Yellow Green Blue White
1.8 V 2 V 2.1 V 2.2 V 3.6 V 3.6 V
A typical LED can pass 30 –40 mA current without destroying the device. Normal current that
gives sufficient brightness to a standard Red LED is 20 mA . But this may be 40 mA for Blue and
White LEDs. Current limiting resistor R4 protects LED from excess current that is flowing
through it. The value of R4 should be carefully selected to prevent damage to LED and also to get
sufficient brightness at 20 mA current. The current limiting resistor can be selected using the
formula
R = V / I
Where R is the value of resistor in ohms, V is the supply voltage and I is the allowable current in
Amps. For a typical Red LED, the voltage drop is 1.8 volts. So if the supply voltage is 12 V ( Vs
) , voltage drop across the LED is 1.8 V ( Vf ) and the allowable current is 20 mA ( If ) then the
value of R4 will be
Vs – Vf / If = 12 – 1.8 / 20 mA = 10.2 / 0.02 A = 510 Ohms.
A suitable available value of resistor is 470 Ohms. But is advisable to use 1 K resistor to increase
the life of the LED even though there will be a slight reduction in the brightness. Since the LED
takes 1.8 volts , the output voltage will be around 10 volts. So if the circuit requires 12 volts, it is
necessary to increase the value of Zener slightly. Table 3 is a ready reckoner for selecting limiting
resistor for various versions of LEDs at different voltages.
Table 3
Supply
voltage
Red Orange Yellow Green Blue White
12 V 470 E 470 E 470 E 470 E 390 E 390 E
9 V 330 E 330 E 330 E 330 E 270 E 270 E
6 V 180 E 180 E 180 E 180 E 120 E 120 E
5 V 180 E 150 E 150 E 150 E 68 E 68 E
3 V 56 E 47 E 47 E 33 E - -
* Available resistor values in ohms