Relay Circuit Assignment - Job #6
Here are the tasks to complete for this assignment. You may want to print
this page. Check off each task when done. Then move on to the next task.
Definitions Before You Begin
Be sure you have an understanding of these terms before beginning this exercise.
You may need to use a dictionary, a code book, a theory book, or even ask other
electricians you may know for help. This is part of the learning activity. Begin
now.
terminology
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definition
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sequence operation
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overloads
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PLC
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Read this
As background, here are some details which will help you understand this
assignment. If you encounter words, terms, phrases or abbreviations which you do
not understand, then stop. Research the meaning before going on. This is very
important to your learning activity. The essence of any technical education
activity is comprehensive understanding of all topics which are encountered.
Relay Circuits
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A relay is a
critical component of many control systems because they offer an
indirectly operated electrical switch that can be used for remote
control and to control high current devices with a low current control
signal.
 They
operate on the same principles as a solenoid, except that the core is
fixed in place. The relay has an input coil that uses a voltage/current
to create a magnetic field. As the coil becomes magnetized it pulls a
metal switch (or reed) towards it; thus, making an electrical contact. A
contact that closes when the coil is energized is normally open. In
contrast, a contact that the reed touches when the coil is not energized
is normally closed.
In the figure to the right, the magnetic force pulls the armature
toward the coil, and through the mechanical lever, forces the electrical
contacts closed to close the controlled circuit. The springiness of the
contacts cause them to open with the coil is de-energized. Relays can
have normally open contacts (as the figure depicts), normally closed
contacts, or various combinations of both.
Relays are used to let one power source close a switch for another.
It is often the case that the other switch has a high voltage/current.
The relay can accomplish this control while keeping the switches
isolated.
Prior to the development of electronic programmable controllers,
relay functions were performed by real hardware relays that were
physically wired together. When a programmable controller is used, the
only real hardware relays required are those for the actual outputs to
the machine.
A solid state circuit used to store a sequence state or to combine
multiple relay logic paths are referred to in design and programming as
"control relays". In comparison to the hardwired relay method,
control relays are solid-state memory locations. The digital code
contents of these memory locations represent the state of a relay
contact. Instead of using a meter to determine whether a real relay
contact is set open or closed, the appropriate memory locations need
only be examined.
Relay outputs are one of the most common outputs. Relays can be used
with both AC and DC loads, as the figure to the left illustrates. A load
is a term used to described whatever is connected to our outputs. It is
called a load because the outputs are being loaded with something. If no
load is connected to the outputs, and the output is connected directly
to the power source, then damage to the outputs would most likely occur.
Some common forms of loads are solenoid, lamp, motor, and etc. These
loads come in a variety of electrical sizes.
When connecting a load to a PLC output, caution should be used so that
the maximum current consumption of the load will not exceed that of the
PLC output. An example of loads connected to a PLC is illustrated to the
right. If the load draws too much current, beyond that specified for the
PLC, damage could occur to the PLC output.
There are some loads that can be tricky, they are called
"inductive loads". These loads have a tendancy to deliver back
current when turned on. This is similar to a voltage spike. In order to
prevent any unexpected current flows diodes, varistors, and "snubber"
circuits are used.
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Go to the Diagram of this Job
Click HERE to go to the diagram, lecture, and picture for this exercise. Follow
the instructions found there. Then return to this page with your back button.
Connect the Components
Use the material list on the diagrams page to have ready all the components you
will need to complete this exercise. Connect all required components based on
the diagram.
Check your Connections
With the diagram as a guide, verify that all the connections have been made
properly.
Apply Power To Test
Wear safety glasses when testing this circuit. Locate the overcurrent protective
device for the circuit you will use. If possible, connect to a GFCI protected
circuit for the added safety. Be sure you have a full understanding of this
circuit before applying power. Then when you test the circuit, there should be
no problem with it working properly. Make corrections to the wiring as needed so
the circuit works properly. Memorize these connections.
Sketch the diagram from Memory
A sure way to determine if you have internalized this circuit is to draw a
sketch of the diagram from memory. You may choose to use a straight edge. But
that is not necessary. When you have completed the sketch, check it against the
diagram which is given for accuracy. Electricians must have the ability to
remember circuit connections without reference. Here is your chance to
demonstrate what you have learned. If you are unable to sketch the ladder
diagram from memory, refer back to the given ladder diagram. Then try again.
Repeat this activity until you feel confident that you can sketch this specific
diagram without reference.
Reconnect From Memory
Now on to the real test. Reconnect this diagramed circuit from memory. This will
ultimately tell you if you understand this circuit or not.
Review Before Answering Questions
Go back to review definitions, the diagram, the picture, your notes, your sketch
and any other details which will help you as you move on to the questions about
this diagram.
Answer the Questions
When you are satisfied that you can draw a sketch of the diagram, and wire it
all from memory, it is time to go on to answer questions about the diagram.
Click HERE to go to the questions.
Check Your Answers
When you have completed the questions, go on to check your answers. If you have
any which are different than the given answers, try to determine the reason. Go
back to the diagram. Research the question. Ask other electricians you may know.
But do all it takes to understand the correct answer to each question.
Click HERE to go to the answers.
Go On
When you can answer all questions without error, and strongly feel you
understand all answers, it is time to go on to the next assignment.
Click HERE to go on to the next assignment page.
If you would like to research any of these topics further, enter the search
term in this Google Search Box:
Use these Relay Circuit links to go to where you left off in
your online activities.
Job
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Assignment
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Diagram
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Lecture
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Picture
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Questions
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