Tektronix PS 503A, Инструкция по эксплуатации

Страница: 28 / 57

Section 5— PS 503A
CIRCUIT
DESCRIPTION
Introduction
This section of the manual contains a description of the
circuitry used in the PS 503A Dual Power Supply. In­
dividual descriptions are separated into the following
parts: Dual Tracking, Reference Supply, Automatic
Crossover, Constant Voltage/Current Limited Output,
Load Regulation, Power Line Regulation, Power Supplies,
Over-voltage Protection, and Output Current Selection.
Refer to the appropriate diagrams in the Diagrams section
of this manual while reading the circuit description.
NOTE
The negative and positive supplies operate iden­
tically. The two supplies are internally connected
with a common front-panel VOLTS DUAL
TRACKING control for varying the output of both
supplies at a constant ratio. Therefore, except for
Dual Tracking operation, this discussion will cover
only the positive supply.
Dual Tracking
When the VOLTS DUAL TRACKING knob is pulled out,
S30 disconnects the input to the reference buffers (U35
and U135) from the fixed voltage at the collector of Q30
and connects it to the potentiometer wiper arm of the
VOLTS DUAL TRACKING control, R30.
voltage, while U55 controls output current. For any value
of load resistance, the power supply acts either as a
constant-voltage source or a current source—but never
both. Automatic crossover is accomplished by combining
the outputs from pin 6 of U45 and U55 in a negative-true,
"OR" gate configuration (see Fig. 5-1). The amplifier with
the lower output voltage at pin 6 causes its associated
diode (CR45 or CR48) to conduct, which eventually
reverse biases the other diode. A portion of the current
available from R87 is then diverted away from the base of
Q85 and to the amplifier with the lower output. The
remainder of the current drives the base of Q85, which in
turn controls the current through the series-pass tran­
sistor in the Power Module to the load.
Constant Voltage/Current Limited Output
Output current or voltage varies when an imbalance
occurs between the comparison voltage at pin 2 and 3 of
either U45 or U55. This imbalance is caused by a change in
the load resistance or in the reference voltage selected by
CURRENT LIMIT control R70 or by + VOLTS coarse
control R50A. Fig. 5-2 illustrates the output characteristics
of the power supply with various currents, voltage and
loads.
Reference Supply
The reference supply consists of temperature-
compensated, constant-current circuit Q30-Q25-VR24.
This circuit furnishes constant voltage to voltage-follower
buffer U35 and to the current limiting circuitry. Voltage­
follower buffer U35 supplies voltage to inverter-follower
buffer U135. The U135 buffer then supplies reference
voltage to + voltage error amplifier U45. The constant
current circuit
supplies current to the current-limiting
circuitry via Q80.
Automatic Crossover
The positive supply employs two operational
amplifiers; U45 and U55. Amplifier U45 controls output
CURRENT LIMIT control R70 establishes the non­
inverting reference input to pin 3 of U55, the current­
controlling amplifier.
With no load connected to the output terminals, R - «>,
I = 0 and E — S
vl
, the front-panel voltage limit setting (see
Fig. 5-2). When a load is connected to the power supply
output terminal, output current increases, which results in
an
increase in the voltage drop across current-sensing
resistor R67. This drop provides the inverting inputtopin 2
of U55. If the voltage at pin 2 exceeds the reference voltage
at pin 3, then the voltage at pin 6 drops to a level where
CR48 turns on and CR45 is reverse biased; thus, control
crosses over to U55, the current controlling amplifier. U55
holds the voltage at pin 2 to that set at pin 3 by R70, the
CURRENT LIMIT control (S
cl
). The supply is now in
current-limit operation (see Fig. 5-2).
5-1