Tab Electronics Guide to Understanding Electricity ... - Sciences Club

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Special-Purpose Diodes and Optoelectronic Devices
227
Figure 7-13
An electronic fuse
monitor circuit.
R1
D1
D3
F1
D2
D4
ZD1
12 Volt
C1
100 F
Load
16 WVdc
Show the Blow
As my last entry into this section of circuit potpourri, I submit the
blown fuse alarm circuit illustrated in Fig. 7-13. In most homes, there
are situations where it is critical to maintain electrical power to certain
devices. For example, a chest freezer or a sump pump located in the
basement have critical needs for constant power. A blown fuse (or
tripped circuit breaker) to either of these appliances could result in a
flooded basement, or the loss of hundreds of dollars’ worth of food.
Unfortunately, it is likely that the blown fuse will not be discovered
until the damage has already occurred. The circuit shown in Fig. 7-13
solves that problem by providing an alarm when a blown-fuse condition
occurs.
When a fuse blows, it represents an infinite resistance (like an open
switch) within an electric circuit. Therefore, the entire source voltage for
the circuit will be dropped across it. This is how the fuse-monitoring
circuit obtains its operational power.
Assume that the fuse (F1) is protecting a 120-volt AC circuit (I do not
recommend this monitoring circuit for AC voltages higher than 120
volts AC). On blowing, it will apply 120 volts AC to the monitor circuit.
R1 limits the current, and drops most of the applied voltage. Diodes D1
through D4 rectify the voltage and apply pulsating DC to the zener
diode (ZD1). C1 filters the pulsating DC to apply smooth DC to the load.
The load can be a piezo buzzer (such as used in smoke detectors) or any
other type of low-power, low-voltage visible or audible indicator.
The resistance value and power rating of R1 will depend on the load
requirement. Build the circuit as illustrated using a 1-amp, 200-volt PIV
bridge rectifier (or comparable diodes), a 1-watt zener diode, and a 100-
Kohm, 1 2
-watt resistor for R1. If the load will not operate when 120 volts
AC is applied to the circuit, start decreasing the value of R1 a little at a