digits must also be controlled.
The same codes from the Adder/Subtractor
that address U6 are also sent to U1. They are then decoded to turn on each
digit, in sequence, from LSB to MSB.
The Display Shutoff, Brightness and Blanking Control (U7) is used to
remove cathode voltages from the LEDs, thereby eliminating the display. The
Annunicators are driven and controlled by the Extender Logic and the BCD to
Decimal Decoder circuits.
i. Power Supply Circuits.
The Model 5345A power supplies are short
circuit proof and will automatically shut down if operated at too high a
temperature. A sophisticated ground system requires that each supply line
be measured to its own return line, e.g., +5V and +5V RET (return). Figure
8 is a schematic of the instrument's input power transformer (T1) wired for
120V operation.
Four secondary windings provide rms outputs of 22.3V,
17.0V, 36.0V and 21.9V. A shield line connects the primary to the center
taps of three of the windings, and to the thermal cutouts in the regulator
assemblies.
(1) Switching Regulator. Figure 9 is a simplified representation of
the switching Regulator which provides +5V Display, +5V, and -5.2V for
distribution throughout the instrument.
(a) +5V Display Supply. Rectifier diodes CR1 and CR2 receive 17V
rms from the secondary of T1. The resultant 8 Vdc is filtered and switched
through Q2 before being applied to the input of linear regulator U3.
U3
regulates the output voltage at +5V and provides fold back current
protection.
(b) Thermal and Electronic Shutdown. when the power switch is set
to Standby or when the ambient temperature exceeds 90C, a thermal cutout
causes Q2 in the electronic switch to cease conduction, thereby disabling
the +5V Display supply.
Likewise, Q1 in the electronic shutdown control
starts to conduct, which turns off the +5V and -5.2V supplies.
The
unregulated +8V supply is not affected.
(c) +5V and -5.2V Regulated Supplies. Both supplies receive power
from the 22.3 rms secondary of T1. Both supplies are full wave rectified by
CR5 into +14 and -14 Vdc. U1 and U2 are connected as switching type voltage
regulators to maintain the two supplies at their rated values. Q9 and Q10
supply output current boost. Should the output voltage become excessive due
to a failure in the supply, the crowbars (CR14-19) prevent damage by
shorting the output lines to common.
(2) Linear Regulator.
Figure 10 is a simplified schematic of the
Linear Regulator circuits which provide regulated +12V, +15V, +20V, +11V, -
15V, and unregulated +22V for distribution throughout the instrument. Power
supply protection is provided by the Thermal and
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