How to Wire LEDs for Use on a 12V Source
by Phil LamassYou can wire the maximum number of LEDs to a 12-volt source by connecting series LED strings to the source in parallel. It is best to use LEDs with the same forward voltage and forward current characteristics. Voltage and current calculations will be easier, and circuit behavior will be consistent. When you connect LEDs in series their forward voltages accumulate. The total current through the series string equals the forward current of a single LED. The total forward current that the source must handle will be the sum of the forward currents of all of the series strings connected to in parallel.
Calculate the Total Number of LEDs That Can Be Wired to the Source
Calculate the number of LEDs in a series string. Divide the source voltage by the forward voltage of a single LED. For an LED with a forward voltage of 3.3 volts, the calculation is 12 volts/3.3 volts = 3.6. The total forward voltage of the series string should be less than the source voltage, so use 3 LEDs. The total forward voltage of a series string will be 9.9 volts, a little more than 80 percent of the supply voltage. The total current will be 25milliamps.
Calculate the series limiting resistor value and the power that the resistor will dissipate. To calculate the resistance value, subtract the total voltage drop across the LED string from the supply voltage and divide the result by the forward current. Assume the forward current is 25 milliamps. The series resistor value is (12 volts minus 9.9 volts)/.025 equals 84 ohms. The power dissipated by the resistor is (12 volts minus 9.9 volts) times .025 equals 52 milliwatts. You can use an 82 ohm (the closest standard value) 1/4 watt (250 milliwatts) resistor.
Calculate the number of series strings that you can connect in parallel. This will be limited by the maximum current that the source can supply. Assume that the source can supply up to 500 milliamps. Applying the 80 percent rule, the total current demand should be no more than 400 milliamps. The number of series strings that can be connected in parallel is 400 milliamps/25 milliamps equals 16 series strings.
Calculate the total number of LEDs by multiplying the number of parallel strings by the number of LEDs in each string: 16 strings x 3 LEDs per string equals 48 LEDs.
Wire and Connect the Series Strings to the Source.
Connect the positive terminal of the power supply to one lead of the series limiting resistor.
Connect the other lead of the limiting resistor to the positive lead (usually the longest of the two leads) of the first LED.
Connect the negative lead of the first LED to the positive lead of the next LED in the string. Repeat this step for the remaining LEDs in the string
Connect the negative lead of the last LED in the string to the negative terminal of the power supply.
Repeat steps 1-4 for the remaining series strings.
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