Keithley 244 Instruction Manual page 11

MODEL 244 HIGH VOLTAGE SUPPLY
CIRCUIT DESCRIPTION
3-3.
SERIES HIGH VOLTAGE REGULATOR. The series regu-
lator tube VlOl is biased by the collector voltage of
transitor QlOE to form a "cascade amplifier" with the
output load serving as the plate resistor. The Model
244 - OUTPUT is effectively the algebraic sum of the
unregulated high voltage (3200 V) and the drop across
"101 and QlOE. When the OUTPUT Switch SlOl is off
(e.g. no plate voltage is on "101) the voltage at pin
8 (Screen Voltage) of VlOl will be significantly less
than nominal + 95 volts since the tube current will
cause a voltage drop across R169.
3-4.
VOLTAGE CONTROL. The output voltage is divided
by a resistor divider composed of resistors R146
through R167 located an switches S102, 5103, and S104.
The sampling voltage is compared with the reference
supply voltage. (The reference supply is composed of
zener D119 and resistors Rl40 through R145). The
difference voltage is amplified by the error amplifier
(transistors QllO, Qlll, Q112, and Q113) which drives
cathode biasing transistor ~108. Diodes 0114 through
Dl17 and resistors R115, R117, R139, and R144 protec?
the comparator and zener reference from switching
transients when the output is changed.
Diode Dill
prevents a reverse bias on transistor QlOV beyond its
breakdown voltage.
3-5. CURRENT CONTROL,
a. The output current is sensed by measuring the
voleage drop across resistor R124. A current limit
amplifier consisting of Q107 controls the error am-
plifier through diode D112. When the current through
R124 exceeds approximately 11 milliamperes, transistor
Q107 forward biases D112 and turns off the error am-
plifier which reduces the Model 244 through the action
of VlOl.
b. Overload Indication.- The OVERLOAD Indicator
05102 is driven by transistors 4104, Q106, and Q107.
Transistor QlO5 is an emitter-follower which biases
9106 to insure that the OVERLOAD Indicator turns on
before the current limiting actually occurs. (The
OVERLOAD Indicator is designed to turn on at approx-
imately 11 milliamperes load current.)
3-6. LOW VOLTAGE SECTION. The low voltage for the
control circuitry is obtained from a full-wave rec-
tifier composed of diodes D109 and I1110 and filter
capacitor c115.
a. + 95 v d-c. This voltage is applied to a bias
screen on "101. The + 95 v is developed across R113
and D105.
b.
+ 20 v d-c. This voleage is regulaeed by zener
diode D105. A regulated current sowce (25~) supplies
a constant current far zener diodes D105 and D106.
The current source is composed of transistors QlOl,
9102, and Q103 and reference zener D107. The regulated
current is sampled at R116 and compared to zener 0107
by transistor Q102. The action of QlOl and Q102
regulates the currene through 0105, D106, and R113.
c.
- 10 v d-c. This voltage is regulated by zener
diode 0106.
NOTE
The a-c power to the high and low voltage
sections is controlled by the POWER ON
Switch ~102. The POWER ON Indicator
ix101 is driven by the low voltage d-c
SUPPlY.
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