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Theory of Operation
Battery run time should be greater than six hours from full charge with the display backlight off.
If run time is seen to be less than this, the ac power should be momentarily interrupted, then
plugged back in for at least 2.5 hours to allow for a full battery charge.
Power-supply Assembly
The Model 1140A power-supply assembly contains the battery charging circuit, dc-to-dc
converter, and analog power supplies.
Battery Charging Circuit
The charging circuit consists of a charge controller and associated circuitry. When ac power is
applied to the unit and it is turned on, operation does not involve the batteries. In this manner, the
unit can operate on line power with fully discharged batteries.
The charge controller initiates a charge cycle whenever the unit is plugged in and the battery
voltage is low enough to warrant charging. Charging will not start if the battery temperature is
below 5°C or above 45°C. If the battery temperature is outside this range, TEMP FLT will be
shown on the display. Once the battery temperature has returned to within limits, charging will
begin if the battery requires it. If the battery pack fails, charging will not start and BATT FLT will
be shown on the display.
Once the battery is charging, it can be terminated due to over or under temperature, over or under
battery voltage, if the charging takes too long, and of course if the battery reaches full charge.
When the battery reaches full charge, the display indicates CHARGED. If the charge terminates
for any of the other reasons, BATT FLT or TEMP FLT is indicated on the display.
If either of these fault conditions is indicated and the Model 1140A is not at room temperature,
the unit should be returned to room temperature and ac power momentarily interrupted to see if
the fault recurs. If the fault recurs, it may indicate a defective battery.
When the battery charge reaches a low state, the display flashes LOW BATT to warn that there is
approximately ½ hour of running time remaining.
Dc-to-dc Converter
The dc-to-dc converter circuitry encompasses an inverter and controller, transformer, rectifiers,
and filters. The converter operates at high frequency and is regulated to produce a constant
voltage to the analog power supplies.
As the power supply shuts down due to low battery, the dc-to-dc converter alerts the
microprocessor so it can ensure that no data is lost as power shuts down. On restoration of
power, normal operation resumes. If the power supply fails to properly restore power the display
indicates POWER FAILURE. This should never be seen and is an indication of a failure in the
power-supply circuitry.
Analog Power Supplies
The analog power supplies regulate and filter several voltages for the analog circuitry. This
power is fed to the analog assembly for use in producing the required voltages and
measurements.
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