Control relays are used in electric and electronic equipment. Transistors, motors, power plants, uninterrupted power supply systems and power meters are some of the equipment where control relays are used.
A control relay is a device which opens or closes a switch in accordance with the current flow through a conducting coil. It is an electromagnetic device and the switches do not come in direct contact with the conducting coil. In most cases, it controls the power flow in a circuit.
The control relay operates in two modes. When current flows through the electromagnetic coil it attracts a magnetic substance which closes the switch and vice versa when there is no current flow through the coil. A spring will assist in closing and opening of the switches. The relay will not go into an intermediate stage between on and off. Electronic circuits can be coupled to the relays to make them operate automatically. The switching process will be pre-programmed in a circuit according to the need of a particular application. Logical controllers or programmed microcontrollers are used in electronic and complex controls. Ladder logic is the logic mainly used in relay control.
The relay operates basically by the principle of magnetization and de-magnetization. As current flows through the coil it creates magnetic poles in the coil, called the North Pole and South Pole. These poles attract an armature which will be coupled to a spring. As long as the current is sufficient in the coil, the armature stays attracted to the coil and the switch will stay in the on condition. As the current value drops, the attractive force of the coil is enhanced by the potential energy of the spring and thus the spring pulls back the armature and the switch goes on to the off position. The efficiency of the relay is measured by taking into account how fast it can switch between the stages. Relays of very fast switching times are available now.
Many factors are considered while designing relays. The conducting and magnetic properties of the coil and the nature of material of the coil are important factors. Also, the magnitude current flow should also be taken into consideration. If the current exceeded the capacity of the coil then the relay and the equipment connected to it can get damaged. Relays should be operated in optimal temperature conditions as it affects the life cycle of the device. Temperature should be kept below a level which the relay can tolerate.
Source by Jeremy P Stanfords