The relays are the fundamental elements for maneuvering electric loads, as they allow the combination of logic in the control, as well as the separation of power and command circuits. The simplest relays are made of a housing with five terminals. https://www.mrosupply.com/v-belts/v-belts/a-section/178193_a69_gates-rubber/ The terminals 1,2 correspond to the excitation coil, the terminal 3 is the input terminal, and the terminals 4.5 correspond to the normally closed (NF) and normally open (NO) contacts, respectively. An important characteristic of the relays is that the voltage at the terminals (1,2) can be 5 Vdc, 12 Vdc or 24 Vdc, while at the same time the terminals (3,4,5) can work with 110 Vac or 220 Vac .
There is no physical contact between the drive terminals and the work terminals and this concept accepted the appearance of two circuits in an electrical panel. In the control circuit is the interface with the operator of the machine or device and therefore works with low currents (up to 10 A) and / or low voltages and in the power circuit, it is the circuit where the loads to be motors, heating resistors, among others. In this can circulate electric currents of the order of 10 A or more, and reach tensions of up to 760 V.