Linear Regulator - Variable Regulators

Variable Regulators

Variable regulators come in the form of ICs, with a capability to adjust the output voltage by using external resistors of specific values. This family includes low power devices like LM723 and medium power devices like LM317 and L200. Some of the variable regulators are available in packages with more than three pins, including dual in-line packages.

For output voltages notded by standard fixed regulators and load currents of less than 7 amperes, commonly available adjustable three-terminal linear regulators may be used. An adjustable regulator generates a fixed low nominal voltage between its output and its 'adjust' terminal (equivalent to the ground terminal in a fixed regulator). The LM317 series (+1.25V) regulates positive voltages while the LM337 series (-1.25V) regulates negative voltages. The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

Single IC dual tracking adjustable regulators are available for applications such as op-amp circuits needing matched positive and negative DC supplies. Some have selectable current limiting as well. Some regulators require a minimum load.