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Operation
As a sensor
for checking the battery out-put,
resistors
R10,
Rll
R12 and a zener diode (ZD) form a bridge.
Two points A and B on the
bridge are
connected
to the base and emitter
of TR6
respetively,
as
shown
in Fig. 2-29.
Potential
difference across
the two points turns
''on"' or ''off!' the
TR6,
depending
on the polarity;
If the point A is negative
with respect
to the point B,
TR6
is turned "'on''.
If positive
TR6
is turned
"'off''.
As
considered,
if the battery has enough out-put voltage,
the point B
becomes
positive with respect to the point A,
since the ZD is biased
with the voltage higher than its zener level.
Then TR6 turns "on", produc-
ing the collector
current
i2.
This i2 in R12 causes the potential difference across R12,
by which
TR7
is turned "on".
Collector current of TR7 (i4) lights the indicator
lamp.
The lamp which draws
the current
i4 from the source
is also used
as a load to the source (battery).
If the battery is low,
the ZD turns
out tobe a high resistor.
Thereby
TR6
is turned
"off'' and operation to be followed
is just contrary to when
TR6 is turned "on",
Adjustment
For adjustment,
VR3
must
be adjusted
so that the potential difference
across the ZD with the battery out-put 5.0 V, is equal to the zener voltage
level of the ZD.
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