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Keithley 236 Service Manual
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SECTION
4
Principles
of
Operation
423
VII
Feedback
A
simplified
schematic
diagram
of
the
V/I
feedback
cir
cuitry
is
shown
in
Figure
4-5.
The
detailed
schematic
of
this
circuitry
is
provided
by
page
3
of
drawing
236-126.
Amps
Ranging
Current
range
selection
is
accom
plished
by
activating
the
appropriate
range
FET
and/or
relay.
The
nominal
range
resistances,
relays
and
FETs
for
each
of
the
nine
current
ranges
are
shown
in
the
illustra
tion
(Figure
4-5).
The
FET
labled
QAB
for
the
lOOmA
range
is
actually
a
composite
P
and
N-channel
FET.
A
more
detailed
look
at
the
lOOmA
range
is
shown
in
Figure
4-6.
In
reality,
QA
is
comprised
of
P-channelFETs
Q8
and
Q9,
and
QB
is
comprised
of
N-channel
FETs
Q6,
Q7,
Q37
and
Q38.
Without
special
circuitry,
current
measurement
range
changes
may
cause
voltage
spikes
to
occur
at
the
output.
Voltage
spikes
are
caused
by
contact
bounce
of
the
se
lected
range
relay.
These
spikes
occur
too
fast
for
the
bootstrap
amplifier
to
react
to
the
change
in
feedback
voltage.
As
a
result,
the
system
cannot
adjust
itseif
ac
cordingly
to
maintain
a
steady
voltage
output.
The
Model
236/237
resolves
this
problem
by
using
cir
cuitry
that,
in
effect,
shunts
the
current
ranges
with
a
100Q.
resistor
whose
voltage
drop
ramps
in
a
linear
fash
ion
towards
0V.
This
slow
rate
of
voltage
change
allows
the
system
to
adjust
itseifkeeping
the
output
voltage
con
stant
(spike
free).
The
circuitry
that
accomplishes
this
consists
of
FETs
QA
and
QB,
and
the
±15V-to-OV
ramps
that
control
them.
The
following
sequence
takes
place
when
a
current
meas
urement
range
change
occurs:
1.
Relay
K8
cioses.
2.
The
appropriate
ramp
is
activated
(RANGE
2
RAMP
for
+
polarity,
or
RANGE
1
RAMP
for
polarity).
The
ramp
on
the
gate
of
the
appropriate
FET
resuits
with
a
corresponding
voltage
ramp
across
the
1002
resistor.
3,
The
other
ramp
is
activated.
Turning
on
the
second
FET
increases
the
current
carrying
capability
of
the
circuit.
With
both
FETs
fully
on,
the
current
ranges
are
shunted
by
the
1002
resistor.
4,
The
new
range
relay
and/or
FET
is
activated.
Amps
Ranging
inA
Output
Stage
Out
+
Sense
+
To
v/I
Control
and
Readback
Ou
Figure
4-5.
Simplified
Schematic
of
VII
Feedback
Out
Com
4-6
94
96
Table of Contents
Table of Contents
21
Performance Verification
27
Introduction
27
Environmental Conditions
27
Initial Cond1Tions
27
Required Test Equtpment
28
Verifications Procedures
28
Equipment Characterization
30
Table 1-3 Model 263 I-Source Characteristics
31
Common Mode Verification
32
Figure 1-2 Setup for Common Mode Verification
33
Voltage Source Verification
34
Figure 1-3 Setup for Voltage Source Ranges Verification
35
Voltage Measure Verification
35
Figure 1-4. Setupfor Voltagemeasure Ranges Verification
36
Table 1-5 Voltage Measure Verification
37
Remote Sense Verification
37
Figure 1-5 Setup for Remote Sense Verification
38
Low Current (Ina to Loma) Measure Ranges Verification
38
Low Current (Ina to Loma) Source Ranges Verification
43
Looma Measure Range Verification
44
Figure 1-8 Setup for Looma Measure Range Verification
45
Looma Source Range Verification
46
Calibration
49
Introduction
49
Recommended Calibration Equipment
49
Environmental Conditions
50
Warmup Period
50
Cal Lock
50
Calibration Procedures
50
Equipment Characterization
51
Figure 2-2 Setup for Equipment Characterization
52
Table 2-3 Model 263 I-Source Characterization
53
Voltage Calibration
54
Figure 2-3 Setup for Volts Calibration
55
Looma Range Calibration
57
Low Current Ranges Calibration (Ina Loma)
59
Calibration Errors
61
Table 2-4 Calibration Error Messages
62
Calibration Program
64
Table 3-1 Line Fuse Selection
73
Introduction
73
Line Fuse Replacement
73
Fan Filter Cleaning
73
Handling and Cleaning Precautions
76
Disassembly Instructions
76
Figure 3-3 Resonant Converter Board (Model 237)
77
Figure 3-4 Output Board
78
Figure 3-5 Analog Board (Shields Removed)
79
Diagnostics
82
Table 3-2 Recommended Troubleshooting Equipment
82
Special Handling of Static Sensitive Devices
82
Recommended Test Equipment
82
Power Supply Checks
84
Table 3-3 Power Supply Checks
84
Analog Switching
85
Table 3-4 Analog Switching; Current Ranging
85
Table 3-5 Analog Switching; Volts Ranging
86
Table 3-6 Analog Switching; Feedback
86
Digital and Display Checks
87
Table 3-9 Digital Circuitry Checks
88
Table 3-10 Display Circuitry Checks
89
Principles of Operation
90
Introduction
90
Analog Circuitry
91
V/I Control and Readback
91
Output Stage
93
Figure 4-3 Model 236 Output Stage (Simplified Schematic Diagram)
94
Feedback
95
Figure 4-6 Damping Relay Contact Bounce
96
Resonant Converter (Model 237)
97
A/D Converter
97
Table 4-1 Volts Range Relays
97
Digital Circuttry
98
I/O Controller
98
Source/Measure Controller
99
Communications
99
A Display Circuitry
99
Powersupply
100
Introduction
102
Parts Lists
102
Ordering Information
102
Schematic Diagrams and Component Location Drawings
102
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Keithley 236 Specifications
General
Brand
Keithley
Model
236
Category
Measuring Instruments
Language
English
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