Signal Processing Elements

Relay

We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic field is generated. This magnetic field sucks the contacts of the relay in, causing them to make a connection. These contacts can be considered to be a switch. They allow current to flow between 2 points thereby closing the circuit. The most important feature of relay is self holding. There are different types of relays according to the application. They are differ by coil supply as 24 DCV, 220ACV, 110ACV or number of contact points as 2 or 4 n/c - n/o points or shape of pins as LY, MY.

“Self holding” and “Interlocking”

In automatons m/c the signals to control cct is given by sensors or switches and as they operate in real time they give pulse to cct as signals. So it is needed to memories them in the cct. This feature can develop using self-holding method.
In some cases until one action stopped other action should not activate. That means if two actions want to be prevented operate at same time we can use interlocking method.

Contractors

Contractor is a scale up device of relay. Using contactor we can drive any load according to their rating. But relay can only drive few amperes. So relay is used to drive contractor and contractor can drive higher load (motor). For small power applications (solenoid valve) relays can directly use to drive them. Using contractors only it can be developed a control cct so there are auxiliary devices to increase the number of contact points. Using this method we can drive motor forward and reverse with star to delta manual changing. I practiced induction motor control with star delta changing, forward reverse driving using contactors.

Main points to select proper relay are as follows
(1) Points for contact

1) Forms and number of contact combinations
2) States of contact load AC or DC?; resistive or inductive or capacitive
or lamp?; occurrence of back electromotive force or inrush current?
3) Level of contact load Large or small current?; Switch on or
not for current load?; Switch off or not for current load?
4) Frequency in switching operation
5) Demand for life in switching operation

(2) Points for coil

1) Coil power source DC or AC?
2) Fluctuation in supply voltage
3) Energizing method and circuit Necessity of special energizing circuit
4) Coil resistance
5) Operate and release voltages
6) Operate time and release time

(3)Insulation

1) Dielectric strength
2) Insulation resistance
3) Surge strength

(4) Environment

1) Range of ambient temperature or humidity
2) Environmental atmosphere
3) Vibration and shock

(5) Mounting

1) Outside dimensions
2) Fixing and soldering of terminals
3) Mounting method

(6) Others

1) Safety standards and other standards
2) Special specifications or conditions

Timers and Counters

We could study about the timers and counters used in automation systems. Timers used to get time interval and switch signals to control cct. User can adjust this time and when supply given the timer count time and switch the points after set time. There are two types of timers called delay ON and delay OFF timers. In delay ON timers after set time normally open points will be closed and normally closed points will be opened. In delay off timers, just after supplies given normally open points will be closed and normally open points will be closed. Then after the set time, normally open points will be opened and normally closed points will be closed. Timers used in many applications. When motor wants to auto-change to star – delta timer used to get time interval. Auto-change over in generator uses timers.