A three-pin flasher relay works on principles of electromechanics to appropriately power turn and hazard signals on most automobiles manufactured since the late 1930s. Historically, turn signal systems evolved from relays and mechanisms to solid state apparatuses. All flasher relays have an audible and visible output when connected as designed.
A flasher relay is powered off of the automobile's 12-volt DC main. It is grounded (positive or negative ground) to match the rest of the car's electrical system. Its circuit is designed for a maximum current load to power the turn and emergency signals only. Any further demand or overload will trip a circuit breaker or blow a fuse or fusible link.
The heart of the flasher is the relay. The relay is the electromechanical device that makes the operator switch-selections happen. Typically, a switch is engaged, which energises the electromagnet in the relay. This electromagnet closes contacts that power the flasher and lighting circuit. Left, right or both sides can be selected.
The flasher is a thermostatic switch designed to power on and off at a set rate. Typically, flashers are set for 6 or 12 volts, depending on the application. The flasher is a complete unit and not typically designed for service.