Industrial Electronics – II
Chapter # 7 Relays
Dated 11 – 02- 2013
7-4 LATCHING RELAYS
Define latching relay and where it is used ?
Electro-mechanical latching relays are designed to hold the relay closed after power has been removed from
the coil.
Latching relays are used where it is necessary to stay open or closed even though the coil is energized
momentarily.
7.16) Describe the operation of latching relay that use two coils
There are two conditions “Latch and Un-latch” in latching relay that
uses two coils.
a) The latch coil is momentarily energized to set the latch and
hold the relay in latched position
b) The un-latch or release coil is momentarily is energized to disengage the mechanical latch and return relay to the un-latched
position.
Note :
the relay coil is energized momentarily, it does not have to
be continuously energized to hold the latch and un-latch position
What are the advantages of Latching relays ?
a) the relay will remain in its original latch or un-latch state when power is cut-off or restored, this
arrangment is often known as memory relay.
b) The sequence of circuit operation continue from the point of interruption after power is resumed;
rather then returning to the beginning of the sequence.
c) In case of running out of a part on assembly line may signal the shutdown of the process by
momentarily energizing the unlatch coil.
Self – Test :
28. True or False? Latching relays are designed to hold the relay closed after power has been removed from
the coil.
(True)
29. In a latching relay that uses two coils, the coils are identified as the ____________coil and
___________coil.
(Latch, Un-latch)
30. True or False? Latching relays can provide circuit continuity during power failure.
(True)
7-5 LOGIC RELAYS
7.17) Why can a relay be considered digital in nature ?
A relay can be considered digital in nature because it is basically an ON / OFF, two sates device. The coil is the
input and the contacts are the output.
7.18) What type of relay logic circuit would be used to :
a. Produce an output, when an input is not present
The NOT or inverting Gate logic produce an output
when no input is present.
Fig(a) shows a circuit that indicates the open (OFF
light) and closed light state of the pressure switch.
b. produce an output when each of several input condition is met
An AND Logic produce an output when each of several
input condition is met, it can be defined as
“Control circuitry that requires two or more functions to
be completed as a prediction for another event to take
place describe the AND circuit.
From fig(B) Both push-buttons PB1 and PB2 must be
pushed at the same time to energize the solenoid to
operate.
d) produce an output, when anyone of several input condition is met
An OR Gate logic produce an output when anyone of
several input condition is met, which can be define
as;
“Control circuitry in which one condition or another
separate condition can cause an event to take place
to describe the OR circuitry”
e) Upon failure of power, have output remain in its original state when power is restored.
Memory is used in logic circuits to recall or restore the past events,
which remembers the last state of output until the power is turned
OFF.
From fig (c), The operation of the circuit is similar to that of
magnetic starter with a 3-wire control system. Momentarily
actuating PB2 turns the motor on and it will remain ON until PB1 is
momentarily actuated.
7.19) List three different technologies that can be used to implement logic control.
Logic control can be implemented by 3 technologies,
1) Relay Control was first, and it is still being used.
2) Solid State Logic modules, a minor player because it led to the development of inexpensive electronic
computers
3) Micro-Computers technology is latest, which has many important advantages..
Self-Test
31. A relay can be considered digital in nature because it is basically an ___________ state. (ON / OFF)
32. Control circuitry that requires two or more functions to be implemented for another event to take place
describes the logic________________ circuit.
(AND)
33. Control system in which one condition or another separate condition can cause an event to take place
describe the logic_________________ circuit.
(OR)
34. The requirement of logic NOT circuit is that it will produce an output when an ___________ is not present.
.
(Input)
35. Normally relay memory is implemented using an _______________ relay.
(Latching)
36. True or False? Relay logic control is the most popular choice for complex control systems. (False)
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7th chapter End
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