Manual stabilisers are
still popular because of their simple construction, low cost, and high
reliability due to the absence of any relays while covering a wide range
of mains AC voltages compared to that handled by automatic voltage
stabilisers. These are used mostly in homes and in business centres for
loads such as lighting, TV, and fridge, and in certain areas where the
mains AC voltage fluctuates between very low (during peak hours) and
abnormally high (during non-peak hours). Some manual stabilisers
available in the market incorporate the high-voltage auto-cut-off
facility to turn off the load when the output voltage of manual
stabiliser exceeds a certain preset high voltage limit.
The output voltage may become high due to the rise in AC mains
voltage or due to improper selection by the rotary switch on manual
stabiliser. One of the major disadvantage of using a manual stabiliser
in areas with a wide range of voltage fluctuations is that one has to
keep a watch on the manual stabiliser’s output voltage that is displayed
on a voltmeter and keep changing the same using its rotary switch. Or
else, the output voltage may reach the preset auto cut-off limit to
switch off the load without the user’s knowledge. To turn on the load
again, one has to readjust the stabiliser voltage using its rotary
switch. Such operation is very irritating and inconvenient for the user.
This under-/over-voltage audio alarm circuit designed as an add-on
circuit for the existing manual stabilisers overcomes the above problem.
Whenever the stabiliser’s output voltage falls below a preset low-level
voltage or rises above a preset high-level voltage, it produces
different beep sounds for ‘high’ and ‘low’ voltage levels—short-duration
beeps with short intervals between successive beeps for ‘high’ voltage
level and slightly longer-duration beeps with longer interval between
successive beeps for ‘low’ voltage level. By using these two different
types of beep sounds one can readily readjust the stabiliser’s AC
voltage output with the help of the rotary switch.
There is no need of frequently checking voltmeter reading. It is
advisable to preset the high-level voltage 10V to 20V less than the
required high-voltage limit for auto-cut-off operation. Similarly, for
low level one may preset low-level AC voltage 20V to 30V above minimum
operating voltage for a given load. The primary winding terminals of
step-down transformer X1 are connected to the output terminals of the
manual stabiliser. Thus, 9V DC available across capacitor C1 will vary
in accordance with the voltage available at the output terminals of the
manual stabiliser, which is used to sense high or low voltage in this
Transistor T1 in conjunction with zener diode ZD1 and preset VR1 is
used to sense and adjust the high-voltage level for beep indication.
Similarly, transistor T2 along with zener ZD2 and preset VR2 is used to
sense and adjust low voltage level for beep indication. When the DC
voltage across capacitor C1 rises above the preset high-level voltage or
falls below the preset low-level voltage, the collector of transistor
T2 becomes high due to non-conduction of transistor T2, in either case.
However, if the DC voltage sampled across C1 is within the preset high-
and low-level voltage, transistor T2 conducts and its collector voltage
gets pulled to the ground level.
These changes in the collector voltage of transistor T2 are used to
start or stop oscillations in the astable multivibrator circuit that is
built around transistors T3 and T4. The collector of transistor T4 is
connected to the base of buzzer driver transistor T5 through resistor
R8. Thus when the collector voltage of transistor T4 goes high, the
buzzer sounds. Preset VR3 is used to control the volume of buzzer sound.
In normal condition, the DC voltage sampled across capacitor C1 is
within the permissible window voltage zone. The base of transistor T3 is
pulled low due to conduction of diode D2 and transistor T2.
As a result, capacitor C2 is discharged. The astable multivibrator
stops oscillating and transistor T4 starts conducting because transistor
T3 is in cut-off state. No beep sound is heard in the buzzer due to
conduction of transistor T4 and non-conduction of transistor T5. When
the DC voltage across capacitor C1 goes above or below the window
voltage level, transistor T2 is cut off. Its collector voltage goes high
and diode D2 stops conducting. Thus there is no discharge path for
capacitor C2 through diode D2. The astable multivibrator starts
oscillating. The time period for which the beep is heard and the time
interval between two successive beeps are achieved with the help of the
DC supply voltage, which is low during low-level voltage sampling and
high during high-level voltage sampling.
The time taken for charging capacitors C2 and C3 is less when the DC
voltage is high and slightly greater when the DC voltage is low for
astable multivibrator operation. Thus during low-level voltage sensing
the buzzer beeps for longer duration with longer interval between
successive beeps compared to that during high-voltage level sensing.
This circuit can be added to any existing stabiliser (automatic or
manual) or UPS to monitor its performance.