A power supply is a device that is used to convert the electrical current that enters from a power source to the voltage and current values necessary for powering a load. In electrical systems, the load is a device which consumes electrical energy in the form of current and transforms it into other forms like heat, light, work, etc. A power supply must transfer current in a controlled manner to a wide range of loads, sometimes simultaneously, all without letting changes in the input voltage or other connected devices affect the output. A power supply can be external, such as charging devices for a laptop computer, or internal, as in larger devices like desktop computers. Furthermore, a power supply can either be regulated or unregulated. In a regulated power supply, the changes pertaining to the input voltage do not affect the output, whereas in an unregulated power supply, the output depends on the input.