In other words relay is a device which is necessary for the implementation of step changes in the condition of an electric circuit in the result of the given input stimuli. Initially, the term "relay" was applied to electromagnetic relays, which were used to enhance the electrical Telegraph signals, attenuated in long transmission lines to the values required for operation of Telegraph apparatus.Electromagnetic relay consists of an electromagnet and one or more contact groups that are controlled by the drive mechanism associated with the armature of the electromagnet. The principle of operation of the relay based on the action of electromagnetic forces generated in the metallic core during the passage of current through the turns of its coil. Over the core of the electromagnet is a movable anchor (plate) with the contacts opposite which are located the stationary contacts.Initially, the anchor is held by the spring. When you tension the electromagnet attracts the armature and closes or opens the contact. After the external signal is terminated, the contacts return to their original position, i.e. the contacts of relay two working positions – closed and open.Electromagnetic relay is a universal switch analog and pulse signals. It performs several important functions. The relay is galvanic isolation between the control circuit and load circuit. Thanks to the relay control signal is propagated into multiple output signals, this unit allows you to enhance the power of the control signal. Relay gives you the ability to independently manage multiple output circuits with different levels of current and voltage divided circuit with different levels of working currents and voltages as well as DC circuit and AC. Due to the electromagnetic relay possible conversion and regulation of electrical signals.