Theory of Operation 5
Keysight 34970A/34972A Service Guide 133
Power Supplies
Unless otherwise noted, components in this discussion are located on the A1
circuit assembly (34970-66501).
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 single +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 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.
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