7815 is a positive 15V1A fixed output voltage regulator integrated circuit, because it has only input, output, common ground 3 feet, so called three-terminal regulator, input 18V to 40v voltage, the output remains unchanged 15V. Note: If the voltage is stable, the load is not less than 10mA, and the input and output terminals are connected to the ground with a capacitor greater than 0.1 microfa. Power consumption (through the current multiplied by the input and output voltage drop) should not be greater than 2W without a radiator. Internal overcurrent protection works at about 1.6A.