Industrial Automation MCT 373
By:
Dr. Hassan Metered
Prof. of Vehicle Dynamics and Control
Helwan University
Lecture No. 4
Relay Logic Control and Diagram
Symbols, Working and Examples
It consists of relays wired up in a particular fashion to perform
the desired switching operations. The circuit incorporates
relays along with other components such as switches, motors,
timers, actuators, contactors, etc. The relay logic control
works efficiently to perform basic ON/OFF operations by
opening or closing the relay contacts, but it involves a huge
wiring. Here we will learn about Relay Logic Control Circuit,
its symbols, working and how they can be used as Digital
Logic Gates. Also, called Ladder Diagram.
Definitions:
Rails: Two vertical lines labeled L1 and L2 that connect the
rungs of a PLC diagram.
Rung: Horizontal line in a relay logic diagram that has input
devices and an output device.
In rung 1, the push button is Off and hence
Working of Relay Logic Circuit
it does not allow the current to pass through
it. Therefore, there is no output through rung
1.
In rung 2, the push button is On and
therefore, current passes from the high
voltage rail to the low voltage rail and the
Pilot Lamp 1 glows.
In rung 3, the contact is Normally Open,
therefore Pilot lamp 2 remains Off and there
is no flow of current or output through the
rung.
In rung 4, the contact is normally Close,
thereby allowing the current to pass through
it and giving an output to the low voltage
rung.
In rung 5, no current flows through the
main rung as the contact is normally Open
but due to the presence of the sub-rung,
which contains a normally close contact,
there is a flow of current and hence the pilot
lamp 4 glows.
Basic Logic Gates Using Relay Logic
1. OR Gate
In this, the Pilot lamp will turn
On whenever any one of the
inputs becomes one which makes
the contact associated with that
input as normally close.
Otherwise, the contact remains
Normally open.
2. AND Gate
The contacts are connected in series for
AND gate. This means that the pilot
lamp will turn ON if and only if both
the contacts are Normally close i.e.
when both the inputs are 1.
3. NAND Gate
As two Normally close contacts are
connected in parallel, the pilot lamp lights
up when one or both the inputs are 0.
However, if both the inputs become 1, both
the contacts become Normally Open and
hence the output becomes 0 i.e. the pilot
lamp doesn’t light up.
4. NOR Gate
Here, two normally close contacts are
connected in series which means the
pilot lamp will light up only if both the
inputs are 0. If any one of the input
becomes 1, that contact changes to
normally open and hence the flow of
current is interrupted, thereby causing
the pilot lamp not to light up,
indicating 0 output.
Rules:
1. Input devices are placed near the left corner of the rung.
2. One (and only one) output is placed near the right corner of
the rung.
3. Input devices can be connected in series, parallel, or a
combination of series and parallel.
4. Output devices cannot be connected in series. Only parallel
output devices can be placed in a rung.
5. While input devices can be represented multiple times in
the relay logic diagram, each output device can only be
represented once in the diagram.
6. All the input and output devices must be placed
horizontally. This means that no vertically positioned input
or output devices are allowed in a relay logic diagram.
7. Current in the relay logic diagram must flow from left to
right.
8. Diagrams must be numbered
and commented correctly. Rung
numbers are placed on the left
side of each rung. The instruction
symbols and comments are placed
above the instruction. Rung
comments are placed on the right
side of each rung.
9. The location of each contact
associated with a coil can be
recorded by the right-hand rail
near the coil.
Example: 1
Two normally open switches (SW1
and LS1) and one normally open
pushbutton (PB1). Three discrete
input devices are connected in series
with a motor starter.
Example: 2
A relay logic circuit can have several
relay coils and contacts. Note that
contacts associated with a relay coil
can be normally open, normally
closed, or a combination of both.
Example: 3
Power Circuit Control Circuit
Example: 4
Press the start pushbutton for running
the motor continuously. Press the jog
pushbutton for running the motor
temporarily.
Example: 5
Press the forward pushbutton for
running the motor in forward
direction. Then press stop pushbutton
to stop the motor. After that, press the
reverse pushbutton for running the
motor in reverse direction.
Thanks for
your attention
Questions?
