Relay Circuit Assignment - Job #4
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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series wiring
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parallel wiring
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deenergize
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coil voltage
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spst
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spdt
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dpst
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dpdt
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com (common)
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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.
Background Information
Spend about ten minutes looking over the online material presented at
this website produced by the National Association of Relay Manufacturers.
SOURCE - Read
through this background information.
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Circuit symbol for a relay
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Relays
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Relay showing coil and switch contacts
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A relay is an electrically operated switch. Current flowing through
the coil of the relay creates a magnetic field which attracts a lever and
changes the switch contacts. The coil current can be on or off so relays have
two switch positions and they are double throw (changeover)
switches.
Relays allow one circuit to switch a second circuit which can be completely
separate from the first. For example a low voltage battery circuit can use a
relay to switch a 230V AC mains circuit. There is no electrical connection
inside the relay between the two circuits, the link is magnetic and mechanical.
The coil of a relay passes a relatively large current, typically 30mA for a
12V relay, but it can be as much as 100mA for relays designed to operate from
lower voltages. Most ICs (chips) cannot provide this current and a transistor
is usually used to amplify the small IC current to the larger value required for
the relay coil. The maximum output current for the popular 555 timer IC is 200mA
so these devices can supply relay coils directly without amplification.
Relays are usually SPDT or DPDT but they can have many more sets of switch
contacts, for example relays with 4 sets of changeover contacts are readily
available. For further information about switch contacts and the terms used to
describe them please see the page on switches.
Most relays are designed for PCB mounting but you can solder wires directly
to the pins providing you take care to avoid melting the plastic case of the
relay.
The supplier's catalogue should show you the relay's connections. The coil
will be obvious and it may be connected either way round. Relay coils produce
brief high voltage 'spikes' when they are switched off and this can destroy
transistors and ICs in the circuit. To prevent damage you must connect a protection
diode across the relay coil.
The animated picture shows a working relay with its coil and switch
contacts. You can see a lever on the left being attracted by magnetism when the
coil is switched on. This lever moves the switch contacts. There is one set of
contacts (SPDT) in the foreground and another behind them, making the relay
DPDT.
The relay's switch connections are usually labeled COM, NC and NO:
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COM = Common, always connect to this, it is the moving part of
the switch.
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NC = Normally Closed, COM is connected to this when the relay
coil is off.
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NO = Normally Open, COM is connected to this when the relay coil
is on.
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Connect to COM and NO if you want the switched circuit to be on
when the relay coil is on.
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Connect to COM and NC if you want the switched circuit to be on
when the relay coil is off.
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Choosing a relay
You need to consider several features when choosing a relay:
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Physical size and pin arrangement
If you are choosing a relay for an existing PCB you will need to ensure that
its dimensions and pin arrangement are suitable. You should find this
information in the supplier's catalogue.
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Coil voltage
The relay's coil voltage rating and resistance must suit the circuit
powering the relay coil. Many relays have a coil rated for a 12V supply but
5V and 24V relays are also readily available. Some relays operate perfectly
well with a supply voltage which is a little lower than their rated value.
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Coil resistance
The circuit must be able to supply the current required by the relay coil.
You can use Ohm's law
to calculate the current:
Relay coil current =
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supply voltage
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coil resistance
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For example: A 12V supply relay with a coil resistance of 400
passes a current of 30mA. This is OK for a 555 timer IC (maximum output
current 200mA), but it is too much for most ICs and they will require a transistor
to amplify the current.
Switch ratings (voltage and current)
The relay's switch contacts must be suitable for the circuit they are to
control. You will need to check the voltage and current ratings. Note that
the voltage rating is usually higher for AC, for example: "5A at 24V DC
or 125V AC".
Switch contact arrangement (SPDT, DPDT etc)
Most relays are SPDT or DPDT which are often described as "single pole
changeover" (SPCO) or "double pole changeover" (DPCO). For
further information please see the page on switches.
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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Answers
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