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A 3-terminal regulator is a device which takes an unregulated input voltage and produces a constant well-defined output voltage. It has just 3 connections (or terminals: the input, the output and a common ground. The 78/79 seriesintegrated circuits are a very common example. 78 devices take positive input and output whereas 79 series take negative. Two further digits in the type code indicate the voltage, for example 7805 produces +5V output.