Chapter 4 Theory of Operation
AC Input and Bias Supplies
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AC Input and Bias Supplies
The ac mains are connected by a fused power module. This module incorporates the
functions of mains connection, fusing, and line voltage selection (100/115/230 Vac).
The line voltage selection function of the module selects which primary winding of
power transformer is energized. The transformer secondary windings are connected
to the main pc board through connectors.
The bias supplies are consists of four sections; ±15 Vdc for the +6V power circuits;
±15 Vdc, +5 Vdc and -5.1 Vdc for the ±25V power circuits and floating logic; ±16.4
Vdc for the display; and +5 Vdc for the earth referenced logic. Power-on reset signals
are provided by the +5 Vdc supply of the floating logic.
The ±15 Vdc for the +6V power circuits are produced by rectifiers CR15 and CR16,
filter capacitors C43 and C44, and voltage regulators U24 and U25 on the bottom
board. These supplies provide bias voltages for the +6V power circuits.
The ±16.4 Vdc for the display circuits are produced by rectifier CR20, filter capacitors
C44 and C45, and voltage regulators U24 and U25 on the top board. A separate
winding of power transformer provides a center tapped 6 Vrms filament supply for
the display.
The ±15 Vdc for the ±25V power circuits are derived from ±16.4 Vdc by diodes CR15
through CR18 on the top board. These supplies provide bias voltages for the ±25V
power circuits.
The floating +5 Vdc and -5.1 Vdc supplies are produced from ±16.4 Vdc supplies.
VR2 on the top board forms a clamp circuit to provide over voltage protection in the
event of a regulator failure. The ADC REF signal is derived from +15 Vdc supply
and the TURN_ON RESET signal is derived from +5 Vdc supply. The FAN FAIL
signal is asserted when the fan current through R73 on the top board is not detected.
The TURN_ON RESET signal holds the main controller and other logic in a reset
state until after the +5 Vdc logic power is fully operational. This signal is generally
active only following application of line power to the instrument.
The +5V dc earth referenced supply is produced by rectifier CR3, filter capacitor C15,
and regulator U11 on the bottom board. The GPIB (IEEE-488) and RS-232 computer
interfaces are powered from this supply.
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