A universal power supply is a device that steps 110 or 220-volt AC household current down and converts it to DC so it can power an electronic device, appliance, or some other electric equipment. It consists of a transformer encased in a box fitted with a wall plug on one end and a cord on the other. There is a switch on the box that allows the user to select the output voltage, usually in a range between 1 and 20 volts, and connected to the output are a series of diodes to convert the AC current to DC. A universal power supply comes with a variety of output jacks, each designed to fit the input of a device that will run on one of the selected voltage settings.
Laptops and household appliances, as well as audio and video and other other electronic equipment, come with their own mobile power supply so you can run them from a household circuit. The transformer and diodes in the power supply are necessary to protect the delicate electronic circuitry in these devices, and losing the power supply, an all-too-frequent occurrence, renders them inoperable. There is no standard input voltage that is appropriate for all devices, so having a universal power supply on hand is an easy way to regain the use of one if its power supply has been lost.
A transformer has an input and output coil, both wrapped around a central core, and the ratio of the input voltage to the output voltage depends on the relative number of turns of these two coils. The switch on a universal power supply alters the output voltage by changing the position of the point of connection to increase or decrease the number of output coils. The diodes convert the current from AC to DC by passing it in one direction while canceling the current in the return direction.
While a universal power supply can be used to power a variety of different equipment, there isn't one that is appropriate for all devices. Before purchasing one as a substitute for a laptop power supply or a supply for a sensitive piece of electronic equipment, buyers should check the range of voltages for which it is rated, as well the output jacks that come with it, to make sure it will work. The power supply output current rating should also be the same as that of the device for which it will be used.