Keithley 237 Service Manual

SECTION
4
Principles of
Operation
From V/I Contro
-3OVF
-150V
(120
mA)
To
V/
Feedback
Figure
4-3. Model 236
Output
Stage
(Simplified
Schematic
Diagram)
Figure
4—4
Mode!
237
Output Stage
(Simplified
Schematic
Diagram)
Feedback
the
error amplifier
(MAIN
drive).
The polarity
and mag-
nitude of MAIN
determines
which
transistor
is forward—
biased and
the amount of bias current
that in turn deter
mines the
magnitude of
the
output voltage.
For example,
as
a
positive
MAIN
becomes
more
positive, more
bias
current
is delivered
to
the
base
of
Q21
resulting
in a
higher
output
voltage.
Model 237
The
simplified
schematic
diagram
for
the
output
stage
for the
Model
237 is shown
in Figure 4-4.
The
detailed
schematic
of
the output stage
is found
on drawings
237-166 and 236-126
(page
2).
For bw
voltage,
the
output stage
for
the Model
237 func
tions the
same as
the one for the
Model
236
(see
expiana
tion
for Model
236
output
stage). However,
additional
circuitry
is used
to switch
over
to the
1200V supplies
needed
for programmed
source
voltages
exceeding
150V. The ±1200V
supplies for
the
Model
237
are derived
from a resonant
converter
circuit
(see
paragraph
4.2.4).
For voltage
outputs
of
150V,
Q21
and diode
CR6 is
for
ward biased
providing
source output
from
the bw volt-
age
(+5OV),
high current
(l2OmA)
supply. Transistor
Q16
is reverse-biased (off)
and therefore
disconnects
the
1200V supply
from
the output.
As
the voltage
output
ap
proaches
150V, CR2
conducts
via
the 15V
zener
(VR1O)
and Q16
turns on.
At
150V, Q21
is
effectively
saturated,
As the output
voltage
exceeds
150V, CR6
becomes
re
verse-biased
and hirns
offdisconnectingthe
150V
supply
from
the output.
For >150V,
only
the
high voltage
(1200V),
bw current
(limA)
suppiy is
connected
to
the
output,
Power
supply switching
occurs
in
a
similar man-
ner for
negative
output
voltage.
4=5

Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English