LEDs operate at a voltage of several volts DC so that powering LEDs from 120V requires a transformer to step down the voltage and a rectifier to change the AC to DC. The easiest way to obtain a low voltage, DC supply is to use a wall transformer or a "brick" of the kind used to power laptops or other DC devices. These transformers are not regulated so it is important to match their voltage and current to that of the LED system. The voltage can be matched by connecting a resistor in series with each LED, and the current can be matched by connecting several LED and resistor branches in parallel.
Calculate the current and voltage needed for the wall transformer from the specifications for the LEDs. For a single system, all the LEDs should be the same. Find the forward voltage and continuous current of the LEDs. Typical LED voltages are around 3 volts and standard LEDs carry 20 or 30 milliamps, while power LEDs may carry 200 milliamps.
The voltage of the wall transformer should be several times that of the LEDs to allow for the resistor connected in series. The higher voltage will mean that the LED voltage and current will fluctuate less when there are changes in the circuit due to temperature or loading. Add the currents of all the LEDs. The transformer rated output current should be slightly more than the total LED current. Typical wall transformer ratings are 12V DC and 800 milliamps.
Calculate the value of the resistors. Subtract the LED voltage from the wall transformer output voltage. For the example given above, it would be 9 volts. Resistance "R" equals voltage "V" divided by current "I." If the current for each LED is 200 milliamps or 0.2A, the resistance required will be R=9/0.2 or 45 ohm. The watt rating of the resistance will be V x I or 9 x 0.2 = 1.8 Watts. The next larger size, perhaps 2 Watts would be suitable.
Wire up the circuit. Build one branch of the LED circuit by wiring one side of the resistor to the positive side of one of the LEDs. The positive side has the longer lead, and the negative side is beside a flat spot on the LED casing. Then wire the negative side of the LED to the negative output of the wall transformer. The wall transformer nameplate shows which wire is positive and which is negative. Wire the other side of the resistor to the positive output of the wall transformer. Wire up additional LED branches and connect them in the same way to fully load the wall transformer. Switch on the circuit and use the multimeter to measure the branch currents to make sure they are not above the specified maximum LED currents.