Connecting LEDs in series

If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.

All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V (approximately for Red, Yellow and Orange LEDs) for each LED (or, 3V for Green LED, or 4V for Blue and White, approximately) plus at least another 2V for the Resistor. To work out a value for the Resistor you must add up all the LED voltages and use this for V_L.

Example calculations:

Figure out the total estimate voltage for a Red, a Yellow and a Green LED in series:

V_L (Red) = 2V

V_L (Yellow) = 2V

V_L (Green) = 3V

TOTAL V_LEDs = 7V (the three LED voltages added up)

Plus 2V for the Resistor, at least

New TOTAL = 9V (need a supply voltage)

So, a 9V battery would be ideal.

If the Supply Voltage (V_S) is 9V and the Current (I) must be 15mA = 0.015A, then, the Resistor will be:

R = (V_S - V_L) / I = (9 - 7) / 0.015 = 2 / 0.015 = 133.3 Ohms, so choose R = 139 Ohms  (the nearest standard value which is greater).

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