User`s guide

Chapter 4 Theory of Operation

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 three sections; +5 Vdc and -5.1 Vdc for the power

circuits and floating logic; ±17.4 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 ±17.4 Vdc for the display circuits are produced by rectifier CR8, filter capacitors

C18 and C19, and voltage regulators U12 and U16. A separate winding of transformer

provides a center tapped 6 Vrms filament supply for the display.

The floating +5 Vdc is produced from the separate winding of transformer. The

+5_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 R20 is not detected. The TURN_ON RESET signal holds the main controller

and other logic in a reset state until 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 CR4, filter capacitor C23,

and regulator U9. The GPIB (IEEE-488) and RS-232 computer interfaces are powered

from this supply.