732B/734A
Instruction Manual
4-14
Low-Battery Shutdown Circuitry 4-25.
Sealed lead-acid batteries such as are used in this instrument are very susceptible to
damage and/or decreased capacity if subjected to deep discharge. To prevent this from
occurring, battery power is turned off when the voltage decreases to the level that
coincides with loss of IN CAL.
Comparator U501A (inputs in parallel with comparator U507A). detects loss-of-cal
status when the battery voltage is at a nominal 11.2V. U501 pin 2 switches to the
conducting state (low) and sinks current through the photodiode of optoisolator U514.,
turning on the output transistor of U514. U514 pin 5 drives U516 pin 3, the clock input
of a CMOS D-type flip-flop. U516 pin 1 (Output) goes low and turns off N-channel
MOSFET Q508. With Q508 off, there is no return path to battery (-), and the instrument
is turned off. The only remaining load on the battery is the supply current to U516,
which is less than the self-discharge current of the battery itself.
The battery voltage quickly recovers to approximately 11.5V, but the latching action of
the flip-flop prevents the instrument turning on again. At this point it is necessary to
reconnect the instrument to the ac mains voltage in order to restore operation. This
enables the 15V supply and turns on the photodiode of U515. The resulting high level at
U515 pin 5 sets U516 pin 1 high which then turns Q508 on and normal operation
resumes.
The IN CAL annunciator will be off and can be reset by SW501 only if the oven
temperature is within limits (the time required to reach this condition is dependent on
how long the instrument remained upowered).
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