EET273 Worksheet #3 Relays and Solenoids
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This weeks topic is relays and solenoids. At the end of this worksheet you will be able to:
Recognize Relays
Understand how Relays work
Understand how basic Solenoids work
Understand the difference between open for a Solenoid and open for an electrical circuit.
In preparation for class you should read and outline:
• CH11 in Lessons In Industrial Instrumentation
• CH10 Section: Intro, 10.1, 10.3.1, 10.3.2 in Lessons In Industrial Instrumentation
If you are still confused or want more information, there are other links on the page and much more
information on the web.
When reading, be sure to take notes. But remember notes, is not copying down everything
you read. A better approach is to create an outline of important points. This outline should
include page references so you can find more information later if you need it.
In class we will discuss your reading and work on your outline. You are expected to complete the WS
problems for discussion.
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Questions
Question 1
In one to two words describe the function of a relay.
Suggestions for Socratic discussion
• What are some other elements that you have studied that perform this same function?
• What are some advantages and disadvantages of using a relay vs using a transistor?
file q0006
Question 2
In trying to figure out how a system works you come across a relay you have never seen. It is part #
R50-E2-Y2-24 from Potter & Brumfield.
Find the data sheet online and answer these questions.
• Draw the schematic symbol for this relay.
• Is this a Form A, Form B, or Form C relay?
• How many NO contacts are in this relay?
• Is the coil controlled by AC or DC voltage?
• The coil is designed to close with what voltage?
• The contact connects a 100 Ohm light with a 120Vac source. Is this ok? Explain why or why not.
file q0009
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Question 3
This is a pressure alarm circuit, designed to energize a warning light if the process pressure sensed by
the pressure switch ever crosses a certain threshold value:
To 480 volt AC source
H1
H3
H2
H4
480/120 volt
step-down transformer
X1
2
1
8
7
3
4
5
6
Relay
X2
1
2
3
4
5
6
7
Light bulb
8
Com
NC
NO
Pressure switch
impulse tube
To process
connection
First, determine if this is a low-pressure alarm or a high-pressure alarm (i.e. under what type of process
pressure condition will the light bulb energize, an abnormally low pressure or an abnormally high pressure?).
Next, determine the effect of a bad wire connection (“open” fault) at terminal 2 of the control relay on
the status of the warning light.
Suggestions for Socratic discussion
• Identify how the circuit could be altered to alarm in the opposite condition it does now (i.e. high pressure
instead of low pressure, or low pressure instead of high pressure, whichever you have determined the
circuit to be in its present configuration).
Kuphaldt file i04508
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Question 4
Sketch connecting wires such that the relay will select one of two different photocells to send signals to
a light indicator. Use the following schematic diagram as a guide:
+
-
Battery
Relay
(plugged into socket)
Light indicator
B
A
Schematic diagram
A
Light
indicator
B
Kuphaldt file qi01420
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Question 5
There is a problem somewhere in this relay logic circuit. Lamp 2 operates exactly as it should, but lamp
1 never turns on. Identify all possible failures in the circuit that could cause this problem, and then explain
how you would troubleshoot the problem as efficiently as possible (taking the least amount of electrical
measurements to identify the specific problem).
L1
L2
A
CR1
B
CR2
CR1-1
CR3
CR2-1
CR3-1
CR1-2
Lamp 1
CR2-2
Lamp 2
Next, suppose an electrician tried to force lamp 1 to energize by connecting a temporary jumper wire
in parallel with that lamp. Explain why this strategy will not work, and in fact will likely cause damage to
the circuit.
Suggestions for Socratic discussion
• Identify how ladder-logic type diagrams differ from standard electronic schematics, particularly with
regard to how electromechanical relay coils and contacts are shown in each.
• Suppose you were asked to diagnose the problem in this circuit without using any test equipment, but
simply by observing and listening to the circuit function. Knowing that an electromechanical relay
typically makes a soft “click” sound when its coil changes state, explain how you could isolate certain
faults in this circuit without a multimeter.
• Explain why a relay coil failing shorted will not necessarily yield the same results as a relay contact
failing shorted.
Kuphaldt file i02320
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Question 6
Time-delay relays are important circuit elements in many applications. Determine what each of the
lamps will do in the following circuit when pushbutton “A” is pressed for 10 seconds and then released:
L1
L2
A
TD1
4 seconds
TD1
TD1
TD1
TD1
Lamp #1
Lamp #2
Lamp #3
Lamp #4
Show your answer by completing this timing diagram:
A
on
Lamp #1
off
on
Lamp #2
off
on
Lamp #3
off
on
Lamp #4
off
Time scale (1 second per mark)
For each of the relay contacts shown in this circuit, identify whether it would be properly called an
on-delay or an off-delay contact.
Kuphaldt file i02500
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Question 7
An electric motor is used to power a large conveyor belt. Before the motor actually starts, a warning
siren activates to alert workers of the conveyor’s forthcoming action. The following relay circuit accomplishes
both tasks (motor control plus siren alert):
L1
L2
Stop
Start
CR1
CR1
OL
TD1
Siren
M1
M1
TD1
5 sec
Study this ladder logic diagram, then explain how the system works.
Kuphaldt file i02501
Question 8
Draw the loop diagram and flow diagram symbol for NC solenoid. The solenoid is unidirectional. Be
sure to draw arrows showing the direction of flow.
Explain how the solenoid is operated.
Explain how to open and close the valve to create fluid flow and to prevent fluid flow.
Suggestions for Socratic discussion
• How is a solenoid similar to a relay?
• Explain the difference between NC for a solenoid valve and a Relay contact.
• Explain the difference between a 2-way and 3-way solenoid.
file q0007
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Question 9
Explain what this electric solenoid valve will cause the pneumatically-actuated control valve to do when
de-energized:
Control
signal
I
FV
S
E
FY
D
Vent
Kuphaldt file qi00918
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/P
Question 10
The wire coil of an electric solenoid valve has the ability to store energy in its magnetic field, just like
any inductance (U = 12 LI 2 ). The potential for energy release from a solenoid coil igniting a hazardous
atmosphere is particularly high when the coil is energized by DC: when the circuit is suddenly broken by
the opening of a switch contact or by a loose wire connection, the rapid collapse of the solenoid’s magnetic
di
field induces a large voltage according to the formula v = L dt
.
One way to prevent this from occurring is to connect a commutating diode in parallel with the solenoid
coil, as such:
Classified area
Non-classified area
Commutating diode
Switch
Two-wire cable
S
Solenoid
valve
In order to explain why this diode eliminates the problem of “kickback voltage” generated by the solenoid
coil, you must be able to explain the behavior of an inductor as a load and as a source. Determine both
the direction of current through the inductor and the polarity of voltage across the inductor in the following
two example circuits, and identify whether the inductor is acting as an energy source or as an energy load
in both circuits. Finally, explain how this relates to the solenoid circuit shown above:
Switch closes
Switch opens
Current increases in the inductor
Current decreases in the inductor
Suggestions for Socratic discussion
• How would the circuit function if the commutating diode’s polarity were reversed?
• Explain why the addition of a commutating diode slows down the decay of the solenoid’s magnetic field
when the switch is opened. In other words, explain why adding the diode to the circuit makes it so the
solenoid valve takes longer to return to its “normal” (de-energized) state.
Kuphaldt file i02472
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Answers
Answer 1
Electromechanical Switch
Answer 2
You will have to google the data sheet. All of the answers can be found directly on the data sheet. The
coil is controlled by DC. The contact can carry AC or DC so the concern with the contact is how much
current can safely flow through the contact.
Answer 3
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Answer 4
+
-
Battery
Relay
(plugged into socket)
Light indicator
B
A
Schematic diagram
A
Light
indicator
B
Answer 5
This is a problem worthy of a good in-class discussion with your peers! Of course, several things could
be wrong in this circuit to cause lamp 1 to never energize. When you explain what measurements you would
take in isolating the problem, be sure to describe whether or not you are actuating either of the pushbutton
switches when you take those measurements.
Jumpering across lamp 1 creates a short-circuit condition by removing the only load in that “rung” of
the circuit!
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Answer 6
A
on
Lamp #1
off
on
Lamp #2
off
on
Lamp #3
off
on
Lamp #4
off
Time scale (1 second per mark)
Each contact with an arrowhead pointed toward the energized position is an on-delay contact, whereas
each contact with an arrowhead pointed away from the energized position (i.e. toward the “normal” state)
is an off-delay contact.
Time-delay relays are not the easiest for some students to understand. The purpose of this question
is to introduce students to the four basic types of time-delay relay contacts and their respective behaviors.
Discuss with your students how the contact symbols make sense (arrows on the switch actuators describing
direction of delay).
Note to your students how it is possible to have different types of time-delay contacts actuated by the
same relay coil.
Answer 7
The siren immediately activates when the “Start” pushbutton is pressed, and then cuts out 5 seconds
later when the motor actually starts.
Answer 8
A solenoid is an electrically controlled valve. A relay is an electrically controlled switch. Both rely on
electromagnetism to open or close the valve/switch.
NC – valve no flow, NC contact – short circuit= max flow.
2-way : SPST 3-way : SPDT (note this is not a complete answer)
Answer 9
When de-energized, the solenoid will cause the control valve to vent. This will put the valve in its closed
position.
Answer 10
The inductor acts as a load when the switch initially closes, and as a source when it opens. The polarity
reversal as the inductor transitions from load to source (with current maintaining the same direction) is
what turns the diode on to provide a discharge current path for the inductor.
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Kuphaldt, Tony. ”Socratic Electronics.” Socratic Electronics. Ibiblio.org, n.d. Web. 28 Dec. 2014.
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