How to Wire LEDs in a Series

The shorter lead is the cathode, or negative terminal, of the LED.

LED--light-emitting diode--lights can be wired in series or parallel. Wired in series, the supply voltage is divided between the LEDs. Connected in parallel, each LED receives the full supply voltage, which must be attenuated using a separate resistor for each one. For this reason, it is much simpler to wire small numbers of LEDs in series. Power and voltage requirements, however, rise quickly with the number of LEDs.

Things You'll Need

• LEDs
• Resistor
• DC power supply

Instructions

1. Look at your LEDs datasheet for the maximum rated forward voltage across the LED. This is the maximum voltage that the LED can handle without burning out. Datasheets are available online from electronics parts manufacturers and vendors. If you can't find a datasheet, 3 volts is a safe assumption for a white LED.

2. Divide the supply voltage by the number of LEDs you want to use. The result is the voltage drop across each LED. For example, a 9 V battery powering three LEDs in series drops 3 V across each LED. If it's powering two LEDs there is a 4.5 V drop across each LED, which is probably too much. If the voltage drop will be too high, you need to use a resistor in series with the LEDs.

3. Calculate the resistor value using Ohm's law, which states that voltage equals current times resistance, or V = IR. Rearranged to solve for resistance, the equation is R = V/I. Replace V with the amount of voltage you need to drop across the resistor. For example, if you are powering 2 LEDs needing 3 V each with a 9 V source, you need to drop the remaining 3 V across the resistor. Replace I with the LED current. You can find the rated current for the LED in its datasheet, but 20 mA is a standard value; 20 mA is .02 amps, so your equation is R = 3/.02, which equals 150. You need to wire a minimum 150 ohm resistor in series with the LEDs. Resistors come in standard values, so it's okay if you can only get one that's a little bit stronger. You should not, however, use a smaller resistor. Using a substantially stronger resistor will drop less voltage across the LEDs, dimming them.

4. Set up your circuit on a bread board. Connect one leg of the resistor to the positive terminal of the power supply and the other leg to the cathode (negative terminal) of the first LED. Connect the anode (positive terminal) of the first LED to the cathode of the next LED, and so on until all the LEDs are connected. Connect the anode of the last LED to the negative terminal of the power supply, and they will all light up. You can tell the cathode of an LED from the anode by looking at the leads. The lead for the cathode is almost always shorter. If you look inside the bulb, you can see that each lead connects to a small metal part, separated from the other by a gap. The cathode is the larger of the two components in the bulb.

References

• The LED Light: How to Hook Up LEDs
• LLamma: How to Properly Wire LEDs
• Metku: Current Limiting Resistor Calculator for LEDs