31-10-2012, 11:06 AM
Relays and Reed Switches
Relays and Reed Switches.doc (Size: 86.5 KB / Downloads: 27)
Relays:
A relay is a switch worked by an electromagnet. It is useful if we want a small current in one circuit to control another circuit containing a device such as a lamp or electric motor which requires a large current, or if we wish several different switch contacts to be operated simultaneously.When the controlling current flows through the coil, the soft iron core is magnetized and attracts the L-shaped soft iron armature. This rocks on its pivot and opens, closes or changes over, the electrical contacts in the circuit being controlled it closes the contacts.
RELAY CONTROL
By using a level sensor we will get the level of feed water. This signal controls the feed water valve for maintaining a constant level. In our project we are using an ON-OFF control valve i.e. solenoid valve. The advantage of using ON-OFF control valve is
1. There is no dead time
2. There is no transfer lag.
CIRCUIT DIAGRAM DESCRIPTION:
In this circuit transistor BC547 is used as a switch. The control signal is given to the base terminal of the transistor. The collector is attached to the relay coil. Relays are electromechanical devices. There are two types of relays.
1. Normally closed
2. Normally opened
We are using normally opened type relay. When the controller output from the PC is high the transistor will be in the ON state, so relay is energized. When the controller output from the PC is low the transistor will be in the OFF state, so relay is de-energized the valve will open. When the relay is de-energized the valve will close. So according to the controller output the valve will open or close and thuslevelismaintained.