Linear Power Supply

• A linear power supply first reduces voltage from 110 to some lower value, such as 5, 9 or 12 volts. It does this with a transformer, which has a pair of tightly-wound copper coils surrounded by an iron frame. After the transformer, a rectifier turns AC to DC, which it then filters and regulates. Filtering removes electrical noise and regulation holds the voltage to an exact value. Without regulation, a circuit's varying power demands cause voltage to wander.
  
  

Switching Power Supply

• A switching power supply rectifies the 110-volt AC directly, dispensing with the large transformer used by a linear design. A transistor driven by a square wave switches the 110-volt DC on and off. The square wave runs at between 50 kHz and 1 MHz and feeds a small transformer, which reduces the voltage. A regulator circuit monitor's the supply's voltage and changes the switching action, leaving the transistor "on" slightly longer for each cycle if voltage is too low, and "off" longer if it is too high. The switching regulator's high frequency operation allows for a smaller transformer than a linear supply has. Though more complex, this design is more efficient, losing less energy as heat.
  
  

Size, Weight And Cost

• Because the transformer is the largest single component in a power supply, the linear design is larger and heavier than the switching supply. Depending on the details of the design, the switching supply can also have a lower cost. The first switching supplies, because of their greater complexity, were more expensive, but with mass production, the price of the solid-state switching components has come down, making them less expensive than linear supplies.
  
  

Noise

• Even with careful design, a switching power supply's square wave can generate high-frequency radio and electrical noise. For computers and related equipment, the noise does not cause problems, but is not acceptable in audio and scientific equipment. Linear supplies also produce certain kinds of noise, but it tends to be lower in frequency and confined to the supply. Sensitive equipment can pick up the radio frequency noise produced by a switching power supply, even if it is not directly connected.