3. Theory of Operation
Functional Block Descriptions
147
This current opens the ac-line fuse before the energy flowing through the
above mentioned components can damage them. Resistor A1R6 pulls the
gate of the SCR (A1CR1) to ground so it cannot float - causing a false
crowbar trip. A1R5 and A1C1 provide some filtering in case there are any
spikes on this line (again, to prevent false tripping).
A1R4 bleeds the voltage down on A1C4, as a safety feature, when power is
removed from the instrument. Together A1R13 and A1C22 are a filtering
circuit that reduce conducted emissions.
A1C3 provides filtering for switching signals emanating from the A12 Dc-Dc
Power Converter Module. A1R7 and A1R8 tie the otherwise floating lines
going to the A12 module to analog ground. This is a preventive measure to
keep these lines from floating up to above the 56V (72V for Opt. 048) limit on
the input to the dc-dc converter ICs inside A12.
There are also diodes (A1CR4-A1CR6) placed on all A12 outputs to ensure
the outputs never go out of range and destroy the assemblies/modules
connected to them. This is in addition to the over-voltage protection already
supplied by A12 itself.
External DC Input Circuit
A1J1 receives the dc input from an external dc source (22 to 42V) (40-58 Vdc
on Opt. 048). The dc voltage is then routed to a filter consisting of A1L1 and
A1C2 before being sent to A11 Power Steering Logic Assembly via connector
A1J4. Capacitor C2, is rated at 80V (100V for Opt. 048), as the maximum
allowable input voltage is 42V (58V for Opt. 048). Diode A1CR2 prevents any
damage to the instrument if the external dc-input source is reversed. The
current handling capability of A1CR2 is as high as that of the external input
fuse (5A) (2.5AT for Opt. 048) so the fuse blows before the diode fails.
A11 Power Steering Logic Assembly
A11 directs the outputs of three power sources ( two in case of Opt. 048) to
the main instrument input bus in a hierarchical order. The three power
sources are: 1) the dc from the ac line (through a transformer, rectifier, and
filter) 2) the external dc input, and 3) the internal standby 24V battery, (this
does not apply for Opt. 048). The main instrument power input goes directly
to A12 Dc-Dc Power Converter Module which supplies power to all
instrument components (except A11 Power Steering Logic Assembly itself,
which has its own supply). A11 circuits are divided up into six functional
blocks: external dc, housekeeping, dc from ac line, battery logic, battery
charger, and power fail detection. (For Option 048 instruments the A11
circuits are divided up into three functional blocks only: external dc,
housekeeping, and dc from ac line).
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