Keithley 236 Service Manual

SECTION
4
Principles
of
Operation
Table 4-1.
Volts Range
Relays
236/237
Relays
Range
Energized
1.1V
K6,K7
liv
K6,K7
110V
K7
1100V
None
424 Resonant Converter
(Model 237)
The ±1200V
supplies required
for
the
Model 237 are
pro
vided by
the resonant converter. A
simplified
schematic
diagram
of the
resonant
converter is
shown
in
Figure
4-7.
The
detailed
schematic is found on
drawing
237-176.
The±1200V switching supply
is basically alow
noise,low
EMI/RFI
dc-to-dc converter.
This high
voltage
supply is
derived
from
the
+50V
power supply.
In
general, an
LC
resonant
circuit
is used
to
produce a
current
that resembies
a
sinusoidal
ac
waveform. As a
re
sult, an
alternating
voltage waveform
is applied
to
a step
up transformer. The
output
voltage
of
the
transformer is
then
fed
to
an eight
stage
voltage
multiplier
where
volt-
age
is
increased
to
±1200V.
The
resonant
tank
circuit
(LR,
T2 and
CR)
is
controlled
by
the
voltage
controlled oscillator (VCO),
and switching
FETs
Q1 and
Q2.
The VCO
generates pulse
pairs
to
drive
Q2
and
Q1.
During
each pulse pair, the
two
FETs
are al
ternately
driven
on and
off.
During
each pulse pair, FET switch
Q2
turns
on
and a
cur
rent that
resembles
the
positive portion
of
an ac
sinusoidalwaveform
is generated in the
tank
circuit.
FET
Q2
hirns off and, after
a
short period of tinte,
FET
Q1
turns
on.
Current flows through the tank
circuit in
the
op
posite direction
producing
the
negative
portion
of
the ac
waveform,
As
a
result, an
alternating voltage
waveform
is applied to
the primary of transformer T2.
The
speed
of
the oscfflator
(VCO)
varies.
Its
ftequency
increases (up
to
50kHz)
as
the bad
requires more current.
For
simplicity, only two
stages
of the
voltage
multiplier
are
shown in Figure
4-7.
As shown
the,
circuit
on
the
sec-
ondary of T2 is configured as a
voltage doubler
for
+V
and
—V
outputs. When
voltage
on
the
secondary
of
T2 is
negative, current
wffl
flow
through diode
CR19 and
charge capacitor
C28.
For
example,
with the
secondary of
T2
at
—245V, C28
wffl
charge
to
+245V. When
the
voltage
on
T2
reverses
direction
to
+245V,
the
result
is
two
volt-
age
sources (T2
and
C28)
in
series.
The
voltage at +V
is
+490V (VT2 +
VC28).
The
circuit
for —V
output
operates
in a
similar
manner.
That
is,
C27
is charged to
—245V
and
when
T2
reverses direction
to
—245V,
—490V is
present at
-V.
In
actual circuit
implementation,
six more
stages
are
used;
three for
positive voltage
and
three for
negative
voltage.
For the
positive output,
capacitors C28,
C32,
C36
and
C40
are all
charged by
T2
resulting with a
+1200V
output.
On the
negative side,
capacitors C27,
C31, C35
and C39
are
charged by
T2
resulting
with
a
—1200V
out
put.
4.3
A/D
CONVERTER
The
Source
MeasureUnit
uses
a
constant frequency,
van-
able
pulse
width,
analog-to-digital
conventer.
The
sche
matic
diagnam for
the A/D
conventen is located
on
page 3
of
drawing
236-106.
Reading
conversions start
with a
charge
balance
phase
and
end with
a
single
slope
phase, thus
pnoducing
two
sets
of counts
from U47
back to the
pre-scale
counten
(U21)
and VIA
of the
source/measune
controller.
The A/D
input
is applied
to
curnent
offset nesistons
R63
and
R64. The
input curnent is
applied to
the gate
of Q4
via
FET switch
Q2
conventing the
bipolan ±10V
input
to
a
unipolan
cunrent.
Operational
amplifien U50
and
Q4
form
an
integraton
with
capacitor
C63.
This integnaton
ramps in
a
positive direction
(since
the current
is
negatively
offset)
until it
reaches
a
threshold, or
until a
timebase intenval
occurs
and switches
on
a
balance
current.
This
continues
fon
alength of
time that is
detenmined
by
the
value
in reg
isten
U44 which is
pnognammed by
the
source/measune
contnoller.
Device U45
is
an 8
bit
magnitude
companator
which con
tinuously
companes
the
set value to
the
count
value in
counten
U46.
When
these
two
numbens
match,
the output
on
pin 19 of
U45
stops
the
A/D
converten, thus
signaling
the
end
of the
charge-balance
phase.
After
detenmining
that
the charge
balance
has
stopped
(by
monitoning
counts
or
timing
out),
the
software
sets
the
final-siope
balance
line
high.
This
clears
U46 making
it
ready
fon
a
4-8

Keithley 236 Specifications

General

Brand	Keithley
Model	236
Category	Measuring Instruments
Language	English