Technical data

Power Supplies
Referring to the schematic shown on page 9-15, the multimeter uses two
types of power supplies: floating supplies and earth referenced supplies.
The floating supply outputs are
±18 Vdc, +5 Vdc, and a 5 Vrms center
tapped filament supply for the vacuum fluorescent display. The earth
referenced 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, on/off switching, and line voltage
selection (100/120/220/240). The multimeter automatically configures
for the applied line frequency by counting the frequency of the output of
clamp circuit
CR554, R555, C555 (LSENSE).
The 5 volt floating supply is produced by bridge rectifier
CR552, filter
capacitor
C556, and regulator U553. The reset output of U553 will change
to logic
LO when the unregulated dc input to the regulator falls below
5.5 volts.
XPONRST is the instrument master hardware reset signal.
This supply powers all floating logic. Relay drive circuits are also
powered from this supply.
The floating
±18 volt supplies are produced by bridge rectifier CR551,
filter capacitors
C551 and C553, and regulators U551 and U552. 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 5 Vrms filament supply
for the display. Bias circuit
CR556, R556, and C559 generates the
required cathode dc bias for the display filament supply.
The 5 volt earth referenced supply is produced by rectifier
CR751, C752,
and regulator
U751. This supply is earth referenced by the screw which
mounts the
PC board to the instrument chassis. The GPIB (IEEE-488)
and
RS-232 interfaces along with other input/output circuits are powered
from this supply.
Chapter 5 Theory of Operation
Power Supplies
106