Relays are electromechanical switches, used to control the several circuits by using low-power signal or one signal. These are found in all sorts of devices. Relays allow one circuit to switch a second circuit which can be completely separate from the first. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical only.
Basically a Relay consists of an electromagnet, an armature, a spring and a series of electrical contacts. The electromagnet coil gets power through a switch or a relay driver and causes the armature to get connected such that the load gets the power supply. The armature movement is done using a spring. Thus the relay consists of two separate electrical circuits which are connected to each other only through magnetic connection and the relay is controlled by controlling the switching of the electromagnet.
Current moving through the coil of the relay makes a magnetic field which attracts a lever and changes the switch contacts. The loop or coil current can be on or off so relays have two switch positions and generally have double throw (changeover) switch contacts. Relays are usually SPDT or DPDT however they can have numerous sets of switch contacts.
The contacts are usually common (COM), normally open (NO) and normally closed (NC). The normally closed contact will be connected to the common contact when no power is applied to the coil. The normally open contact will be open when the no power is applied to the coil. When the coil is energized the common is connected to the normally open contact and the normally closed contact is left floating. The double pole versions are the same as the single pole version except there are two switches that open and close together.
Application of Relay:
- Control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers
- Control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile
- Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers
- Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short delay would use a copper disk between the armature and moving blade assembly.