SECTION 10: CIRCUIT DESCRIPTION
Part No. 715-0080, Rev. B
- 81 -
POWER FAIL ALARM
The
PWR FAIL
alarm activates when the battery voltage drops below 10 volts, causing
Z1
to turn off
Q1
, which supplies the battery voltage. This removes all power from the control circuitry. The drain of
Q1
will go low which will turn
Q2
on. Transistor
Q2
then supplies power from the battery to sound the
alarm and turn on the
PWR FAIL
indicator. Although the battery has insufficient power to operate the
unit, it does have plenty of power to operate the alarm indicators. The unloaded battery voltage will
rebound and exceed the 10 volt level, but because
Q1
is off,
Z1
is turned off and cannot turn
Q1
back
on. This is a latched condition and can be changed only by cycling the power switch, or by application
of an external power source.
Without the application of external power, turning the power switch off
will charge a capacitor (
C3
) on
the display board through
R72
. Then when the power switch is turned on, this capacitor is switched to
pin 8 of
Z1
. This provides sufficient power to
Z1
to allow it to turn on
Q1
, if the battery voltage is
sufficient. When
Q1
turns on, power is then supplied to
Z1
through
CR9
.
VOLTAGE REFERENCE
Operational amplifier
Z9A
amplifies the 0.2 volts reference generated by
Z8
. Potentiometer
R4
adjusts
the gain to provide 5 volts at pin 1 of
Z9
,
TP4
. This establishes the voltage reference for the control
loop amplifier circuits and all alarm circuits except the fail-safe circuit. The voltage reference for the
fail-safe circuit is generated by
Z2
and adjusted by means of
R10
to be 5 volts at
TP10
.
LAMP DRIVE CIRCUIT
Lamp power is regulated by maintaining a constant current through the lamp. A voltage reference
within
Z8
provides 0.2 volts at pin 1 which is connected to pins 8 and 3. The amplifier drives the base
of
Q7
to a voltage that causes the voltage at
Z8
pin 2 to be 0.2 volts. The constant voltage across the
resistance of
R38
generates a constant current in the emitter of
Q7
and the emitter and collector
currents are approximately equal. Therefore, the collector or lamp current is constant and regulated at
0.42 amps. The constant current drive provides the soft start effect when the lamp is turned on.
Содержание AirBORNE A750i
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