Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Linear Power Supplies:

Converting power from AC to DC can be done a couple different ways. The first method we will discuss is by using a "linear power supply", which is a very simple (but effective) process for power conversion.

A linear power supply takes the incoming AC voltage and steps it down via a transformer (ex: 120Vac is stepped down to 48Vac). The 48Vac is then rectified through a full-wave bridge rectifier. A bridge rectifier is really just four diodes in a bridge arrangement. Also a filter capacitor is thrown in to help maintain a constant DC level.

The output voltage is controlled by a power transistor operating in its linear region, it acts as a variable resistor in series with the load. The power transistor receives its control from a circuit that senses output voltage. The control circuit modifies the transistor bias to maintain a constant voltage output, regardless of changes in the load current.

Below is a simple schematic illustrating how a linear power supply works.

Linear Power Supply Example¹

The main drawback to this design is efficiency, not just in terms of power conversion, but also the physical size. First, a large transformer is needed to step down the incoming AC voltage, adding significant weight. Second, the transistor (which is acting like a variable resistor) must dissipate quite a bit of power (which is lost as heat). The power dissipated (in Watts) can be calculated by this simple equation: (V_i - V_o) x I_o or in layman's terms it would be the "Voltage In" minus the "Voltage Out" times the "Amperage Out".