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8.13 APPLICATIONS
The Applications section of this chapter will discuss the constant current
characteristic in the design of security systems, the bridge circuit in
a common residential smoke detector, and the nodal voltages of a digital
logic probe.
Constant Current Alarm Systems
The basic components of an alarm system employing a constant current
supply are provided in Fig. 8.86. This design is improved over that provided
in Chapter 5 in the sense that it is less sensitive to changes in
resistance in the circuit due to heating, humidity, changes in the length
of the line to the sensors, and so on. The 1.5-k� rheostat (total resistance
between points a and b) is adjusted to ensure a current of 5 mA
through the single-series security circuit. The adjustable rheostat is necessary
to compensate for variations in the total resistance of the circuit
introduced by the resistance of the wire, sensors, sensing relay, and milliammeter.
The milliammeter is included to set the rheostat and ensure
a current of 5 mA.
Door
switch
Window
foil
Magnetic
switch
≅ 5 mA
1 kΩ
FIG. 8.86
Constant current alarm system.
Sensing relay
a
To bell circuit
N/C
N/O 5 V @ 5 mA
1 kΩ
10-mA
movement
Rheostat
0 1.5 kΩ
E
b +
10 V
–
If any of the sensors should open, the current through the entire circuit
will drop to zero, the coil of the relay will release the plunger, and
contact will be made with the N/C position of the relay. This action will
complete the circuit for the bell circuit, and the alarm will sound. For
the future, keep in mind that switch positions for a relay are always
shown with no power to the network, resulting in the N/C position of
Fig. 8.86. When power is applied, the switch will have the position indicated
by the dashed line. That is, various factors, such as a change in
resistance of any of the elements due to heating, humidity, and so on,
would cause the applied voltage to redistribute itself and create a sensitive
situation. With an adjusted 5 mA, the loading can change, but the
current will always be 5 mA and the chance of tripping reduced. Take
note of the fact that the relay is rated as 5 V at 5 mA, indicating that in
the on state the voltage across the relay is 5 V and the current through
the relay 5 mA. Its internal resistance is therefore 5 V/5 mA � 1 k� in
this state.
A more advanced alarm system using a constant current is provided
in Fig. 8.87. In this case an electronic system employing a single tran-
APPLICATIONS ⏐⏐⏐ 301