UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201
CIRCUIT THEORY I
SINGLE-POLE DOUBLE-THROW RELAY
BACKGROUND
A single-pole double-throw relay, (seen below in Figure 1), can be thought of as a “controlled”
switch. The switch, (or contacts), may be either opened or closed by the application of an
appropriate voltage or current to the input (coil) terminals.
Relay
Coil
K
Contacts/Term inals
1m H 1 Ohm
Figure 1. A single-pole double-throw relay consisting of a coil and contacts.
The relay coil has an inductance of 1 mH and a resistance of 1 Ω.
The “switch” is not connected to the input, and the circuit which is either completed or disabled by
the switch is independent from the input (unless specifically connected otherwise).
The CONTACT assembly (there may be more than one) consists of poles and contacts which
may be opened or closed (as an ordinary switch) according to conditions at the COIL assembly.
The COIL assembly (wire wound around a metal center, or core) activates (opens or closes
contacts) when an appropriate voltage is applied across its terminals, causing a specific amount
of current to flow. The amount of current required to activate the coil is defined as ION, the “turnon” current.
Once the coil is activated, it will remain activated as long as the current in the coil does not fall
below a certain minimum value known as IHD, the “holding current”. If the current should drop
below the holding current, the coil de-activates. In order to re-activate the coil, a current equal to
the turn-on current must be provided.
Relays are described by two sets of specifications, one set for the coil, the other set for the
contacts. The coil specification usually denotes either the voltage or the current necessary to
activate the coil (Example: VCOIL = 5 Volts, or ION = 75 mA).
The contact specification will include the number of poles and throws of the switch contacts as
well as the maximum voltage and current which the contacts can withstand (Example: single-poledouble-throw (SPDT) 125 Volt, 2Ampere contacts).
Some other terminology about relay contacts might include “NORMALLY OPEN” (NO),
“NORMALLY CLOSED” (NC), and “COMMON” (COM) as shown in Figure 2.
K1
NO
COM
K
NC
1m H 1 Ohm
Figure 2. The contacts are designated as NORMALLY CLOSED (NC),
NORMALLY OPEN (NO), and COMMON.
If the switch contact is in the lower position when the relay is not energized, we say that the
contact is “closed” and a connection is made between that contact and the COMMON terminal.
(This position is defined as NORMALLY CLOSED.) Meanwhile, the connection between the
COMMON terminal and the upper contact is “open”, and that contact is said to be NORMALLY
OPEN.
When the relay is energized, the switch will change position, connecting the upper contact with
the COMMON and opening the connection between the lower contact and COMMON.
K1
NO
COM
K
NC
1m H 1 Ohm
Figure 3. The relay under “energized” conditions.
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SIMPLE RELAY APPLICATIONS
Emergency Lighting System
The circuit shown below in Figure 4 can be used to simulate an Emergency Lighting System, that
is, a scheme to illuminate some lights from a Backup Power source when either the Primary
Power source or a strategic Lamp fails.
K1
COM
K
J1
NO
NC
1mH 1 Ω
Key = O
PrimaryPower
12 V
Main
9V
Emergency
9V
BackupPower
9V
Figure 4. An Emergency Lighting System.
The Main lamp is connected to the Primary Power through the relay coil. The
Emergency lamp will be connected to the Backup Power if either the Primary
Power or the Main lamp fails. The circuit is shown in the normal mode of
operation, with the Main lamp connected to the Primary Power.
In this scheme, the relay coil and Main lamp are connected in series to the Primary Power, a 12
Volt battery. As long as the Primary Power remains on and the lamp is good, the relay will be
energized, closing the normally open (NO) contact, and the Emergency lamp will not be
connected to the Backup Power. If the relay coil becomes de-energized due to either the loss of
the PRIMARY POWER or the failure of the Main lamp (an open circuit), the relay contact will
return to the normally closed (NC) position, connecting the Emergency lamp to the Backup Power,
restoring lighting. In a “real-life” situation, the relay coil and Main lamp would be connected to 120
Volts AC while the Emergency lamp would be powered by a 12 Volt battery. In addition, there
would be provisions to keep the 12 Volt battery fully charged while the Primary Power is on.
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Backup Power System
The circuit shown in Figure 5 shows a method of providing Backup Power to a load (the lamp in
this case) when the Primary Power source fails.
K1
J1
K
Key = O
NO
COM
NC
1mH 1 Ω
PrimaryPower
12 V
X1
9V
BackupPower
9V
Figure 5. A Backup Power system.
The lamp X1 is normally energized by the 12 Volt Primary Power source.
The circuit is shown in the Backup mode due to the loss of Primary Power.
The Primary Power, a 12 Volt battery, is connected across the relay coil. As long as the relay is
energized, the lamp will be connected to the Primary Power via the NO relay contact. When the
Primary Power is removed,or fails, as shown, the lamp will be connected to the 9 Volt Backup
Battery via the NC relay contact.
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PRELIMINARY WORK / DESIGN PROBLEM
Design, build, and test a simple alarm system that could be used to monitor the water
temperature, oil pressure, and available fuel level for an engine. When a fault occurs, the system
should TRIP by energizing a relay, causing an audible 1.9 kHz buzzer to sound and a Red LED to
illuminate.
The basic “sensors” are SPST switches. These individual switches will be “normally open” (NO),
and will “close” when any of the following fault conditions occur:
the temperature is too high
the oil pressure is too low
the fuel level falls below a certain minimum level.
In addition, the system should provide
a Green LED to show NORMAL status indication (switches are open),
a Yellow LED to indicate which parameter has developed a fault condition,
a method to SILENCE and RESET the system following a TRIP condition, and
a scheme to TEST the system (by means of a separate push-button switch).
The system will be powered by the engine’s 12 Volt starting battery (in our case, a 12 Volt DC
Power Supply).
PROCEDURE / RESULTS
1. Construct the circuit of Figure 4, but substitute a Red LED for each lamp. You might need to
include a 100 Ω resistor in series with the Emergency LED in order to limit the current.
The relay coil should energize and the “Main” LED should glow. Test your configuration by
shutting down the 12 Volt Primary Power. Does the “Emergency” LED glow? Restore the 12 Volt
Primary Power. The circuit should now be operating normally. Remove the Main LED to simulate
an open-circuit failure (it’s OK to remove the LED with the power applied). What happens?
2. Construct the circuit of Figure 5 using a Red LED for the lamp. Again, you might need a series
resistor to limit the LED current.
The relay coil should energize and the LED should glow. Test the circuit by shutting down the
Primary Power. What happens? Is the LED illuminated at the same intensity? Why or why not?
3. Construct the circuit which you designed in the Preliminary Work. Test the circuit’s
performance, and modify your design if necessary. In your lab notebook, provide a complete
circuit diagram, parts list, and technical description of your final circuit.
4. Provide a MultiSim simulation of the final configuration of the circuit which you designed in the
Preliminary Work and tested in the laboratory.
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