Power Supplies - Keithley 6512 Instruction Manual

clock. When a latch output is enabled, segments in the select-
ed digit are turned on.
Digit selection is performed by data from the PA3, PA4, and
PA5 outputs of the MPU to control U2 0 3. A single U203 out-
put, corresponding to the selected digit, will go low when the
correct data appears on its inputs. For example, if 010
pears on the inputs of U2 0 3, DS201 will be turned on.
Selection of the display annunciators is performed in a simi-
lar manner. The data stored in the corresponding segment
latches will then turn on the appropriate annunciator LEDs.
Front panel switches are read in a somewhat similar manner
by using a row-column matrix along with U206. To select a
particular row, data is transmitted out the MPU PA3, PA4, and
PA5 lines to U206. This action selects one of the rows by plac-
ing the corresponding output of U206 high. Matrix columns
are then read by reading data in through PA0, PA1, and PA2,
of the MPU. If a particular switch contact is closed, the data
bit will be high; if the switch is open, the data bit will be low.
6.7

Power supplies

The Model 6512 has numerous power supplies, which are
used to power the analog and digital circuits. Diagrams of
these supplies may be found on drawing numbers 6512-106,
page 1 and 6512-166 located at the end of Section 8.
The power supplies are essentially divided into two groups:
one group powers the digital circuits, while a second group
of supplies is used to power the analog circuits. Each group
of supplies has its own transformer secondary. This configu-
ration is used to maintain proper isolation between the digital
circuits and sensitive analog circuitry.
AC power is applied to J1011, which is the power connector
located on the rear panel of the instrument. S101 is the POW-
ER switch located on the front panel, while S102 is the line
voltage selection switch. When S102 is in the 115V position,
the primary windings are connected in parallel; when S102 is
in the 230V position, the T101 windings are placed in series.
T101 has a secondary winding, which is used to supply the
+5V supply used to power the digital circuitry. Rectification
is done by elements of CR110, while filtering is performed
by C101. The supply voltage is regulated by U101, which is
a standard 7805 IC regulator.
ap-
Other secondaries of T101 supply power to all the DC sup-
2
plies that power the analog circuits.
The ±210V supplies, which are used to power the preampli-
fier output circuit, are generated by one secondary winding
of T101 and two voltage-doubling circuits. CR301, CR303,
C301, and C304 perform the rectifier and filtering functions
for the +210V supply, while CR302, CR304, C302, and
C303 are similar components in the -210V supply. The
±210V supplies are not regulated and are referenced to ana-
log and signal common.
Another secondary winding on T101 feeds power to compo-
nents that generate the +5V analog and -9V analog supplies.
The +5V supply is used to power the ranging amplifier, A/D
converter, and other components (such as U301) located in
the analog section, while the -9V supply powers the -2V ref-
erence source. +5V supply components include CR311,
which rectifies AC input, C305 for filtering, and Q309,
VR302, and U306 for regulation. Elements of CR311 also
provide rectification for the -9V supply, while C306 filters,
and VR303 regulates the output voltage. Both of these sup-
plies are referenced to analog and signal common.
The ±5V sources supply power to much of the input pream-
plifier section. These two supplies, which are referenced to
bootstrap common, utilize half-wave rectification and IC
regulators. CR320, C316, and U308 are associated with the
+5V supply, and CR319, C315, and U307 perform similar
functions for the -5V supply. R344 and R345 are intended to
trim the -5V supply voltage to a precise value.
The ±24V sources supply power to the preamp output stage
when the circuit is configured for amps or coulombs. Each
supply is a simple half-wave rectifier, filter capacitor pair,
with CR323 and C317 supplying +24V, and CR322 and
C311 performing the same functions in the -24V supply.
Both these supplies are referenced to the preamplifier output.
Theory of Operation
6-11