It will be easier to understand the operation of the circuit if we
separate the under and over-voltage protection circuitry comprising
transistors T1, T2 and associated components from the rest of the
regulator circuit. Diode Dl arid. capacitor C2 provide 24V DC supply
required to operate the relays.
Diode D2 and capacitor Cl provide the sample DC
voltage for generating reference voltages for the cutout. For initial
setting of cutout, collector of Tl is not connected to the base of T2.
Also the time delay capacitor C4 is only connected after both under- and
over- voltage points have been preset independently. Initially, due to
the potential divider action of resistors R4, R6 and preset VR2, a
voltage would appear at the base of T2. · lf this voltage is 0.6 ·V or
more above the zener diode voltage of D4, transistor T2 would conduct,
energising the relay and connecting the hot tap of the auto-transformer
to the universal socket’s hot terminal through N/O contacts of the
relay RLl. _ Resistor R7 is for limiting base current. Therefore poten-
tiometer VR2 is preset in a position where the relay would just switch
off below the required lower trip point. On the sample prototype this
was adjusted at l55V input, a value at which the output voltage of the
regulator was 200V—the lower limit for our specified regulated output.
Whenever output voltage goes below this safe operating voltage, the
relay releases and disconnects the power to the appliance and
simultaneously gives a visible indication of this by applying power to
the neon light through its N/ C contacts. At this point the collector of
Tl is connected to the base of T2. Again a sample voltage would be
available at the cathode of D3 due to the potential divider action—of
Rl, VR1 and R3. If this voltage exceeds the zener voltage of D3,
transistor T1 would be able to conduct. This would clamp the base of T2
to ground. Since the base ofT2 has been pulled to ground, T2 would be no
longer conductive and hence once again the cutout relay would be
released, disconnecting the supply to the load. Preset VRl is so
adjusted that as soon as the output supply voltage exceeds the upper
limit (240·volts in our case), transistor T1 conducts and releases the
relay. Once the over- and under-voltage points have been set, capacitor
C4 (whose value may be found out experimentally) may be added between
the base of T2 and ground. This capacitor is essential to introduce a
time lag in the operation of the cutout when rapid variations occur in
mains supply. With capacitor C4 connected, the cutout waits till a
steady level of voltage is reached. This also helps in avoiding erratic
behaviour of the cutout. Referring to only one section, say around T3
and T4, of our regulator circuitry, we find that transistor T3 would
conduct when its base is at a negative potential as compared to its
emitter. The emitter of T3 is fed with a fixed reference voltage from
the anode of D5 which remains more or less constant over the input
voltage range. Preset VR3 is adjusted to a point where, any further
increase in the input voltage switches T3 on, and consequently
T4,·pulling_ RL2 on. Further stages are similarly set at different
voltages. Setting Having checked up the wiring, procure a variable auto-
transformer (known as variac) to set the correct input-output voltage
range. A very simple test for making sure that all stages of your
circuit are operating is to feed input mains voltage and see that all
the relays can be switched on or off by merely changing the settings of
presets. At this stage no output load should be connected to the
regulator. lf any part of the circuit remains permanently on or off,
then the components as well as wiring for that particular stage may be
checked thoroughly for faults. lf everything seems correct then connect a
good quality multimeter, switched to read 250V AC, at the output
socket. The cheap type of AC voltmeter mounted on the regulator panel is
generally not a reliable instrument for accurate readings and should
periodically be checked for calibration. Using a variac apply an input
between 155 to 160 volts till you get an output just around 200 volts.
Reduce this voltage slightly and make sure that as soon as it reaches
below 200 volts (threshold between 195 and 200V) the low voltage cutout
transistor T2 switches off, releasing RL1. For this test, base of T2
should not be connected to collector of Tl, so also positive of C4,_ as
described under the cutout section. At this point no voltage would be
available at socket SOI and neon Ll would be lit. Increase and decrease
this voltage at the threshold range a number of times to make sure the
steady repeat accuracy of the cutout. Having set this, preset VR2 may be
sealed with a blob of paint. Next keep on increasing the input volts
till the output just reaches 240V our upper limit for regulated voltage.
Till this _ stage all the presets of the regulator must be so
positioned that none of the relays is on. So far as the auto-transformer
is concerned this would mean that the input hot line is connected to
point 1 of the transformer with output at point 4. incidentally this is
the maximum step-up given by the transformer for the lowest input range.
As soon as output voltage tries to shoot beyond 240V, preset VR3 should
be adjusted to switch on RL2. ` This would change the input hot end
from point l to point 2 of X2, thus reducing the step·up ratio of X2.
Keep on increasing the input voltage with variac till again it reaches
240V output. Now the relay RL3 should be adjusted with preset VR4 to
switch on, further reducing the step-up ratio.
