The electrical hookup to power a frog is as simple as three wires and a Single-Pole Double-Throw (SPDT) switch or relay. An SPDT switch or relay will have three contacts and is very similar in function to the track turnout. One end of the relay has one contact, the other two. This can be wired to divert an incoming current to one of two paths, or to select a single exit path for one of two inputs. The latter is the way it will be used for the frog.
A wire from each rail is connected to the B end of the relay. The A end is connected to the frog. This means the frog will be powered by whichever input is selected and this will be determined by the setting of the track turnout itself.
This same relay can be wired the other way to route power to the diverging tracks of a turnout in such a way that only the route aligned is powered. Power routing is a convenient way to prevent run-away trains in yards, sidings or staging areas while also reducing the current draw on the layout as a whole.
