A power supply circuit board should contain the majority of the components needed to transform and supply power to the equipment it is used in. These boards can be found in equipment that is plugged into an outlet supplying alternating current (AC) or battery-operated devices running on direct current (DC). Supplies often have a label specifying the amount of operating voltage and current. The board should have a fuse to prevent damage to the supply and the rest of the circuitry in the event of an overload. Variable power supplies are also available for testing and experimental use.
Very basic power supply circuit boards may consist of a transformer, four diodes or a block rectifier, and one or more capacitors for filtering. The board will have a connection for the input voltage and a switch to turn the supply on or off. A fuse should be incorporated in the circuit. Some electronic components can become hot while operating, and these may be attached to heat sinks to prevent overheating.
An AC power supply circuit board contains transformers that step down or step up voltages. Step down transformers simply reduce the incoming voltage to the circuit. While most common household devices in the U.S. run on 110 to 120 volts, some heavy-duty devices require a transformer to increase the available voltage. Other countries may have supply voltages of 220 to 230 volts. Converters can be purchased that allow 120-volt devices to operate on 240-volt sources.
In an AC to DC power supply circuit board, a transformer is used to reduce the incoming supply voltage to the amount at which the board is designed to operate. After the transformer portion of the circuit, the AC voltage is rectified to DC voltage. Capacitors or inductors are used as filters to reduce the amount of noise and ripple in the voltage.
DC power supply circuit boards operate on an incoming voltage supplied by a battery. The voltage does not need to be rectified, or changed, from AC but still needs to be regulated. The regulation can be performed by an integrated circuit (IC) or set of Zener diodes. Filtering of the voltage may be completed with electrolytic capacitors.
Multiple voltages can also be produced for devices that require this. Some equipment requires both +5 and -5 volts, as well as +/- 12 or 15 volts. One power supply circuit board can be used to provide all the necessary voltages for the device. A variable power supply, or Variac, is a device that allows the user to change the output voltage as needed.
Circuit protection should be provided in the event of an overload or surge. Fuses for some power supply circuit boards are replaceable by the user, while others will not be accessible. If a fuse is damaged and can be replaced, the replacement should have the same current rating. Bypassing this fuse can cause extreme damage to the equipment and possibly even start a fire.