Chapter 4 Theory of Operation
AC Input and Bias Supplies
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AC Input and Bias Supplies
Referring to the schematic shown on page 127, 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 T1 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 T1 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.