Agilent Technologies 34970A, Service Manual, Page: 127 / 273

Power Supplies
Unless otherwise noted, components in this discussion are located on
the A1 circuit assembly (34970-66501).
The schematics are included
in Chapter 8 starting on page 224.
The instrument uses two types of power supplies: floating supplies
and earth referenced supplies . The floating supply outputs are
±
18 Vdc,
+5 Vdc, and a 6 Vrms center tapped filament supply for the vacuum
fluorescent display. The earth referenced and backplane circuits are
powered from a 5 Vdc supply.
The ac mains are connected by module P1. This module includes the
functions of mains connection and line voltage selection (100/120/
220/240). The internal DMM automatically configures for the applied
line frequency by counting the frequency of the output of clamp
circuit CR106, R102, C103 (LSENSE).
The +5 volt floating supply is produced by bridge rectifier CR105,
filter capacitor C104, and regulator U103. The output of CR105 is
sensed by U104 and compared to the the voltage from battery BT101.
U104 turns on the +5 V floating supply through Q120. If the output
of CR105 falls below 6.8 V, U104 provides a PWRFAIL signal to the
main processor. At initial power on, U104 resets the main processor
with the FLT_RST line. This supply powers all floating logic.
The internal DMM relay drive circuits are also powered from this supply.
The floating
±
18 volt supplies are produced by bridge rectifier CR109,
filter capacitors C107 and C109, and regulators U105 and U106.
These supplies are used to power all measuring circuits. In addition,
the vacuum fluorescent display is driven from the
±
18 volt supplies.
A separate winding of T1 provides a center tapped 6 Vrms filament
supply for the display. Q110A and Q110B turn on and off the
filament supply in response to the FILPWR signal from the main
controller through U107A.
5
Chapter 5 Theory of Operation
Power Supplies
125