The voltage regulator (commonly referred to as the "three terminal regulator") was one of the most welcome arrivals in the early age of integrated circuits in the late 1960's. Before that, voltage regulation circuits were complicated and unreliable.

Most modern regulators incorporate current overload and reverse voltage as well as thermal protection. There are versions for negative voltage as well as positive voltage systems but they are not interchangeable. The most common positive regulators are the 7800 and the LM340T series and the most common negative regulators are the 7900 or the LM320T series. The regulators come with a variety of voltage outputs as well as some with adjustable outputs. With most three terminal regulators, it is very easy to make the regulator put out a higher voltage than normal with the simple addition of two resistors added to the reference input.

Regulators generally need a power source at least 2-3 volts higher than the output voltage. If that isn't possible there are low dropout or "low overhead" regulators such as the LM2931 or LM2940 that can operate with inputs only slightly higher than the output.

The voltage regulator has a variety of uses in model railroading including things like constant voltage lighting control and overcurrent protection. One important item missed, however, in almost all model train articles involving voltage regulators is the requirement for input and output capacitors. A 7805 regulator, for instance, must have at least a 0.22 microfarad capacitor between the output and ground or it will most likely oscillate around the desired output. An LM2940 requires about 100 microfarad. While a few regulators can operate without output capacitors, if you omit them, the regulator will usually become unstable and/or unpredictable.