Specifications

CIRCUIT IDEAS
tion, using presets VR3 and VR4. For elec-
tronic loads such as TV and VCR, the on-
time delay may be set for 10 seconds to 20
seconds. For refrigerators, the delay should
be preset for about 2 minutes duration, to
protect the compressor motor from fre-
quently turning ‘on’ and ‘off’.
In this circuit, the on-time and off-
time delays depend on charging and dis-
charging time of capacitor C1. Here the
discharge time of capacitor C1 is quite less
to suit our requirement. We want that on
switching ‘off’ of the supply to the load, the
circuit should immediately be ready to pro-
vide the required on-time delay when AC
mains resumes after a brief interruption,
or when mains AC voltage is interrupted
for a short period due to over-/under-volt-
age cut-off operation. This circuit is also
useful against frequent power supply in-
terruptions resulting from loose electrical
connections; be it at the pole or switch or
relay contacts, or due to any other reason.
Here supply for the over- and under-
voltage sampling part of the circuit
[marked +12V(B)] and that required for
the rest of the circuit [marked +12V(A)]
are derived separately from lower half and
upper half respectively of centre-tapped
secondary of step-down transformer X1,
as shown in Fig. 1. If we use common 12V
DC supply for both parts of the circuit,
then during relay ‘on’ operation, 12V DC
to this circuit would fall below preset low
cut-off voltage and thus affect the proper
operation of the sampling circuit. The
value of filtering capacitor C4 is so chosen
that a fall in mains voltage may quickly
activate under-voltage
sensing circuit, should
the mains voltage reach
the low cut-off limit.
In the sampling part
of the circuit, wired
around transistor T1,
presets VR1 and VR2 are
used for presetting over- or under-voltage
cut-off limits, respectively. The limits are
set according to load voltage requirement,
as per manufacturer’s specifications.
Once the limits have been set, zener
D1 will conduct if upper limit has been
exceeded, resulting in cut-off of transis-
tor T2. The same condition can also re-
sult when mains voltage falls below the
under-voltage setting, as zener D2 stops
conducting. Thus, in either case, transis-
tor T2 is cut-off and transistor T3 is for-
ward biased via resistor R3. This causes
LED1 to be ‘on’. Simultaneously, capaci-
tor C2 quickly discharges via diode D5
and transistor T3. As collector of transis-
tor T3 is pulled low, transistors T4 and
T5 are both cut-off, as also transistor T5.
Thus, LED2 and LED3 are ‘off’ and the
relay is de-energised.
Now, when the mains voltage comes
within the acceptable range, transistor T2
conducts to cut-off transistor T3. LED1
goes ‘off’. Transistor T5 gets forward bi-
ased and LED2 becomes ‘on’. However,
transistors T4 and T5 are still ‘off’, since
base of T4 via ze-
ner D4 is con-
nected to capaci-
tor C1, which
was in dis-
charged condi-
tion. Thus, LED3
and relay RL1 or
load remain ‘off’.
Capacitor C1
starts charging
slowly towards
+12V(A) rail via
resistors R6 and
R7, and presets
VR3 and VR4. When the potential across
capacitor C1 reaches 6.8V (after a delay
termed as on-time delay) to breakdown
zener D4, transistor T4, as also transis-
tor T5, gets forward biased, to switch ‘on’
LED3 and relay RL1 or load, while LED2
goes ‘off’. Should the mains supply go out
of preset limits before completion of the
on-time delay, capacitor C1 will immedi-
ately discharge because of conduction of
transistor T3, and the cycle will repeat
until mains supply stablises within pre-
set limits for the on-time delay period.
The on-time delay is selected by ad-
justing presets VR3 and VR4, and resistor
R6. Zener diode D3 is used to obtain regu-
lated 9.1 volts for timing capacitor C1, so
that preset on-time delay is more or less
independent of variation in input DC volt-
age to this circuit (which would vary ac-
cording to the mains AC voltage). To switch
‘off’ the relay/load rapidly during undes-
ired mains condition, the timing capacitor
C1 is discharged rapidly to provide com-
plete control over turning ‘on’ or ‘off’ of
relay RL1 (or the load). The functioning of
the LEDs and relay, depending on the cir-
cuit condition, is summarised in Table I.
Fig. 2: Schematic diagram of over-/under-voltage cut-off with
on-time delay
TABLE I
Showing State of LEDs for Various Circuit Conditions
Circuit condition LED1 LED2 LED3 Relay/Load
Over or under voltage ON OFF OFF OFF
cut-off in operation
On-time delay in operation OFF ON OFF OFF
AC voltage normal
after on-time delay OFF OFF ON ON
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