Specifications
CIRCUIT IDEAS
when surrounding temperature is 0dig
C. (To cater to resistance tolerances and
calibration, resistor R6 (22-ohm) and 1-
kilo-ohm preset VR1 were added at EFY
lab. During testing.) Ideally, at 0oC, the
bridge has to be in balanced condition
and, for other temperatures, the bridge
will be unbalanced. The unbalanced volt-
age of the bridge is converted into suit-
able value in the range 0V to 5V (corre-
sponding to temperatures 0oC to 100oC,
respectively) by the instrumentation
amplifier formed by op-amps IC1
through IC3 (uA741). Output of instru-
mentation amplifier is given to voltage
compactors for driving the display
LEDs.
Before using this circuit, the follow-
ing adjustments have to be made. First,
immerse the RTD in ice water (0oC)
and adjust preset VR1 such that the
bridge becomes balanced and the out-
put of IC3 becomes )V. Next, immerse
the RTD in boiling water and slightly
adjust preset VR2 such that the output
of IC3 becomes 6V. Respeat the above
two steps four to five times.
To control the temperature of wa-
ter, ‘on’/’off’ type controller is used.
Lower threshold point is set at 97oC.
An electric heater coil is used for heat-
ing the water. When power supply is
switched ‘on’, the heater starts heating
the water. When temperature reaches
80oC, output of IC5(b) goes ‘high’. This
turns ‘on’ relay driver transistor T1 to
energise relay RL1. In this state, relay
RL2. Relay RL2 in energised state cuts
off power supply to the heater coil. Re-
lay RL2, once energised, remains so due
to the latching arrangement provided
by its second pair of contacts. Simulta-
neously, the buzzer also sounds, due to
forward biasing of transistor T3.
Since the supply to the heater is cut-
off, the temperature of water starts de-
creasing. Gradually, the buzzer goes
‘off’, as output of IC5(d) goes ‘low’. When
temperature goes below 80oC, output
of IC5(b) goes ‘low’ to turn ‘off’ transis-
tor T1 and relay RL1. As a result, the
power supply provided to relay RL2 (via
RL1 N/O contacts) is cut off and relay
RL2 de-energises. This will again turn
‘on’ the mains electric power supply to
the heater coil. Once again, the tem-
perature of water starts increasing and
the cycle repeats to maintain water tem-
perature within the limits 80oC to 97oC.
This controller can be used to con-
trol the temperature of water in water
heaters, boilers, etc. The lower and up-
per threshold points can be changed by
connecting the base terminals of tran-
sistors T1 and T2 to different output
terminals of voltage comparators (IC4
and IC5). Base terminals of transistors
T1 and t2 are meant for lower and up-
per threshold points, respectively.
EMERGENCY LIGHT
S.C. DWIVEDI
RAJESH KAMBOJ
T
he circuit of emergency light pre-
sented here is unique in the
sense that it is automatic, com-
pact, reliable, low-cost, and easy to as-
semble for anyone. The circuit consists
of four sections, namely, battery charg-
ing section, inverter section, changeover
section, and low battery voltage indica-
tion section.
In the battery charging section, 230V
AC mains is converted to 9V AC using
step-down transformer X1. The diodes
D1 and D2 from a full-wave rectifier,
and capacitor C1 filters the rectified
voltage. The output of filter is about
12V DC, which is connected to the col-
lector of transistor T1 provides a fixed
bias of 8.2V. Thus, transistor T1 works
as a regulator and provides a constant
voltage for charging the lead-acid bat-
tery. LED1 indicates the charging of bat-
tery.
The inverter section comprises
transformer X2, transistor T2, capaci-
tor C2 and resistor R3. Transformer X2
is ferrite core type. Its winding details
are shown in Fig. 2. While core details
are shown in Fig. 3. Resistor R3 pro-
vides DC bias to the base of transistor
T2, while capacitor C2 couples the posi-
tive AC feed-back from winding L1 to
the base of transistor T2 to sustain the
oscillations. The AC power developed
across primary winding L2 is trans-
ferred to secondary winding L3, which
ultimately lights up the fluorescent
tubes.
The
changeover
section uses
diodes D3
and D4 as an
automatic
switch. In the
presence of
AC mains
supply, diode
D3 keeps
transistor T2
in its cut-off
state, while
diode D4 pro-
99