Transformers are probably the most familiar part of any electric circuit, and although available in a huge range of different sizes, their basic function remains the same. They transfer energy by means of electromagnetic induction involving the use of two sets of windings, one for the input and the other for the output circuit. A core is also almost invariably used. The most common use of a transformer is to provide a different AC voltage at the output than that at the input, with the step-down type being most frequently encountered. Small transformers are suitable for PCB use and are generally very reliable. Short-circuit proof types are available. Many of the smaller transformers are of the toroidal type, in which both sets of windings are in close proximity on a ring-shaped (toroidal) core. The variable transformer employs this principle but also features a movable contact in potentiometer style, effectively altering the length of one set of windings. Voltage stabilisers and noise protection transformers have equally rated inputs and outputs but work to remove any transients or wave-form imperfections from the mains supply. Some auto-transformers allow devices such as power tools to be used with what would otherwise be an incompatible mains supply, such as 110V tools with a 230V mains.