Theory of Operation 4
Keysight 34420A Service Guide 127
Power Supplies
Referring to the Chapter 8, "Power Supply Schematic", the meter uses two types
of power supplies: floating supplies and earth referenced supplies. The floating
supply outputs are± 18Vdc, +5 Vdc, and a 6 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 Pl. This module includes the functions of
mains connection, power line filters, on/off switching, and line voltage selection
(100/120/220/240). The meter 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 ofU553 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. 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. In addition, the
vacuum fluorescent display is driven from the ± 18 volt supplies. A separate
winding of Tl 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. A -5V supply
is obtained from U609 and U611 to power the DAC, U610, to provide the chart
recorder (analog) output.
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