30-07-2012, 12:38 PM
AUTOMATIC EMERGENCY TORCH
Automatic emergency tourch.pdf (Size: 109.1 KB / Downloads: 41)
Just don’t think that this is yet
another addition to other emergency
light circuits published in
EFY earlier. This circuit is a hit different.
Its main features are:
1. Very reliable operation.
2. As transformer is not used, it is
compact and cost-effective.
3. The torch bulb glows automatically
at power off and goes out on restoration
of power.
4. Since Ni-Cd battery is used, no
maintenance is required. Also, battery life
is very long, nearly 4-5 years (though this
depends on frequency of usage and also on
ampere-hour rating of the battery used).
Sounds interesting, doesn’t it? Read on
then. The circuit is very simple, comprising
just a handful of components. This
implies that the circuit operation also is
very simple. The circuit consists of two
parts:
1. Power supply for charging the Ni-
Cd battery.
2. Switchover circuit which detects
mains failure and switches the bulb ‘on’.
In the power supply section, capacitors
C1 and C2 function as non-dissipating, reactive
impedances which limit the current
to a safe value. With the values of capacitors
as shown, the maximum current that
can be drawn is limited to about 70 mA
at 230V AC. Resistor R2 limits the initial
surge current and resistor R1 assists in
discharging the capacitors after switch off.
Diodes D1 through D4 form a conventional
bridge rectifier while capacitor C3 is the
filter capacitor. Fuse F1 is for protection
and is very helpful in the event of any
component giving up the ghost. This supply
charges the battery as long as mains
is present.
In the ‘switchover’ section, transistor
T1 is used as switch. Normally, when AC
mains supply is present, the rectifier output
charges the battery through resistor R4 and
LED D5 combination at about 50mA rate.
The glowing LED (D5) also gives an indication
of mains presence. Further, due to the
LED (D5), base of transistor T1 is about
1.6V (drop across D5) more positive than its
emitter. This voltage is more than sufficient
to keep the transistor at cut-off.
As soon as the mains voltage fails, the
base of transistor T1 is pulled low through
resistor R3 which drives transistor T1 to
saturation thereby turning the bulb ‘on’.
Since the transistor is in its saturated
state, the voltage drop across it is very
low. Hence the bulb glows with full brilliance.
The bulb can be switched off by
the ON/OFF switch, when not required.
With this bulb (2.2V, 250mA) the torch can
work continuously for about two hours.
The batteries should be charged for about
14 hours after they are discharged.
You can verify following voltages in
the circuit:
1. Base voltage of the transistor must
be 1.8V to 2.0V, i.e. about 0.6V less than
the battery voltage.
2. Emitter voltage must be equal to the
battery voltage.
3. Collector voltage must be 2.0V
to 2.2V, i.e. nearly equal to the battery
voltage.
All above voltages should be checked
with AC mains off. If any of the abovementioned
voltages is absent it indicates
that the transistor is bad and it should be
replaced by a good one.
Here is a word of caution now. Since
the circuit is not isolated from AC mains. it
may be hazardous to touch any component
when the mains supply is on, especially
if the supply wires (live and neutral) get
interchanged. It is strongly recommended
to use an all-plastic enclosure (including
the reflector for the bulb) for the circuit.
Also the ON/OFF switch used should have
a plastic lever. Take proper care and precautions
while building, testing and using
the circuit, and never ever allow the supply
wires to interchange. It is advisable to provide
a plug for the mains input on the box
itself so that it can be plugged directly into
a mains outlet. This reduces the chances of
mains supply wires getting interchanged.
With proper precautions and a little
care, it is hoped that this small circuit will
help make life a bit more comfortable.