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
R1
100 Ohm
NO
COM
R2
K
NC
100 Ohm
1m H 1 Ohm
Prim aryPow er
12 V
BackupPow er
9V
Main
9V
Em ergency
5V
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 contact will
be in the normally open (NO) position 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 change 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
NO
COM
K
NC
J1
Key = Space
1m H 750 Ohm
Lam p
Prim aryPow er
12 V
12 V
BackupPow er
9V
Figure 5. A relay circuit which provides Backup Power to a load (the lamp). The
load is normally energized by the 12 Volt Primary Power source.
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, the lamp will be connected to the 9 Volt Backup Battery via the
NC relay contact.
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PRE-LAB 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).
Simulate your circuit using Multisim, and include a copy of the working simulation in your Pre-Lab.
PROCEDURE / RESULTS
1. Construct the circuit of Figure 4, but substitute a Red LED for each lamp. 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 the 12 Volt lamp. The relay coil should energize and the
lamp should glow. Test the circuit by shutting down the Primary Power. What happens? Is the
lamp at illuminated at the same intensity? Why or why not?
3. Construct the circuit which you designed in the Pre-Lab. 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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