How to Power Three LEDs

Note down the voltage drop across the LEDs and the rated current. Select a wall transformer which can supply more than the rated current at 18 V DC. Multiply the LED voltage drop, which is usually around 3.6 volts, by 3 to get the total voltage drop across the 3 LEDs.

Perform the calculation to get a total voltage drop of about 10.8 volts for small, colored or white LEDs having a rated current of about 20 milliamps or large, power LEDs with a rated current of up to 800 milliamps. Check to make sure the wall transformer can supply 50 milliamps for the smaller LEDs and 1 amp for the larger ones.

Calculate the size of the resistor. The resistor current is the rated LED current and the resistor voltage is 18 V DC minus the total voltage drop across the LEDs. Calculate the resistance by dividing the voltage in volts by the current in amps. Calculate the resistor power by multiplying the voltage by the current to get power in watts.

For the power LED example of Step 1, the resistance required is 18 V minus 10.8 V equals 7.2 V divided by 0.8 amps equals 9 ohms. The power is 7.2 V times 0.8 amps equal to 6 watts.

Connect the circuit together. Connect the plus side of the DC power supply to the open switch, and the other side of the switch to the resistor. Connect the other side of the resistor to the positive lead of the first LED. Connect the negative lead of the first LED to the positive lead of the second LED, and the negative lead of the second LED to the positive lead of the third LED. Connect the negative lead of the third LED to the negative side of the DC power supply. Close the switch and measure the current in the circuit with the multimeter to make sure that the current is equal to or just less than the rated LED current. If the current is too high, add small resistors in series to bring it down.