Connecting LEDs Together in Series

We can connect LED’s together in series to increase the number required or to increase the light level when used in displays. As with series resistors, LED’s connected in series all have the same forward current, I_F flowing through them as just one. As all the LEDs connected in series pass the same current it is generally best if they are all of the same colour or type.

Connecting LED’s in Series

Although the LED series chain has the same current flowing through it, the series voltage drop across them needs to be considered when calculating the required resistance of the current limiting resistor, R_S. If we assume that each LED has a voltage drop across it when illuminated of 1.2 volts, then the voltage drop across all three will be 3 x 1.2v = 3.6 volts.

If we also assume that the three LEDs are to be illuminated from the same 5 volt logic device or supply with a forward current of about 10mA, the same as above. Then the voltage drop across the resistor, R_S and its resistance value will be calculated as:

Again, in the E12 (10% tolerance) series of resistors there is no 140Ω resistor so we would need to choose the next highest value, which is 150Ω.