In an industry there can be many
switches and sensors together forming a control system for a single output as
well as there are major possibilities that most of our outputs might be
connected to each other i.e. the status of one output might be turning off
another.
The sensors and switches have only two
states so they provide discrete signals i.e. high (1) or low (0). Their status
can be referred as inputs if used to control a certain output. Switches
connected in different sequence might result in a distinct process
control. Now we do know some logic gates that turn the output ON only if
certain conditions of the inputs are found true such as AND gate, OR gate, NOT
gate. Let us go through them and figure out the Relay logic that can be
designed for such logic.
To simplify and make it easy to understand we would be only considering two inputs for now(i.e. only two switches).
AND
Gate:-
The
output turns high only if all the inputs are high.
A . B = Output
Checking out the truth tale it clearly
notifies that the output turns high(1) only when both the inputs turn high.
If we are using two switches for inputs
then we can say that the output should turn ON only when both the switches are
pressed together.
 |
| RLD for AND gate |
For example let us consider a ceiling
fan and the connections are made such that we need to press two switches to
turn it ON that simply tells that the current might pass through one switch
when it is pressed but it needs the other switch pressed to reach the fan, i.e.
is they might be connected in series
Let us check the following RLD for AND
gate and compare it with the Truth-Table.
Checking out the RLD it
explains that only when both of the switches are pressed the output coil will
turn ON. Even when we compare to the
truth table to RLD
Concluding that if the
value of a input is high that signifies that the switch is pressed in RLD
similarly 0 is for released (no action)
i.e. 1(high) in truth-table
= switch pressed
0(low) in truth-table
= switch released (no action)
OR Gate:-
The output turns high
if any of the input is high.
A + B = Output
If we are using two switches for inputs then we can
say that the output should turn ON when any of the switch is pressed that is
one is bypassing the other
For the same celling fan and two switches the
connections are different as any one of the switch can turn on the fan i.e. there
are two different paths and current flow in any of them can turn on the output.
Simply the switches are connected in parallel so that one switch can bypass the
other.
The Relay Logic Design clarifies more
of it
A new conclusion can be drawn to
clarify further Relay Logic designing.
AND = A . B = Output = Series
OR = A + B = Output = Parallel
OR Gate:-
Also called as inverter
for its specific function it uses a single input the status of the O/P is
always inverted form of the input i.e. if input is high output is low and vice
versa.
Inverted A = B
Speaking about a switch
whenever the switch is pressed (activated/energized) the output is low and
whenever the input is released (de-energized/no action taken) the output in ON.
In electrical the NC
switch works in the same way as the word itself tells that it is connected in
normal condition i.e. when the switch is neither pressed nor activated it works
as an closed contact and when pressed it breaks the contact resulting open
circuit and hence turning the output OFF.
Here we can conclude with
a very important logic that is necessary to draw out the Relay logic for each
and every case even if there are multiple inputs or multiple output.
The logic delivers an
interconnection between the Boolean equation, the status of the switch (1 or 0
i.e. pressed or released) and type of switch (NO and NC).
We will see some more example of that while designing relay logic for universal gates in the upcoming topic(universal gates)
In case you have any queries
