Published on 31 July 2013
To use my USB charger boards in the car, I need to convert the nominal 12 volt supply to 5 volts at up to 2 amps (a 10 watt output). The first choice in voltage regulation is often the ubiquitous 78xx (where xx is the desired output voltage). For low power loads, this is an effective, inexpensive choice, but for higher power applications, these regulators don't work so well.
The input current to a linear regulator will be equal to the output current at the lower voltage plus a negiliable amount to operate the regulator itself. In this case, the output is 2 amps at 5 volts, equaling a 10 watt output. The input is therefore 2 amps at 14 volts (when the car is running), which equals 28 watts of input power. 28 watts input for 10 watts output??? Where does the extra 18 watts go? HEAT. A nightlight bulb is 4 watts for comparison and that's more than enough to burn your fingers, so 18 watts is a huge waste! It would take a large heat sink to dissipate this much heat to keep a linear regulator at a safe temperature.