Power Supply - Tektronix PG502 Instruction Manual

emitters of Q516 and Q520 also go negative due to the
an operational amplifier. Conduction is
C1520 and decreased in Q516. Currenf flow is
increased in Q504 and decreased in 0508. This action
causes an absolute magnitude current increase in the load
resistance (collectors of Cl504 and Q508 move in the
positive direction). This current change is sensed through
R502 and R508 and fed back to pin 2 of U480A. causing a
stable condition at its input. The output current (pulse high
level) is proportional to the Setting of the HIGH LEVEL
control.
The output amplifier is on when the pulse output is in
the low state. The output voltage is proportional to the sum
of the offset current and the witched output current.
Manual control of the pulse low level occurs by varying
the voltage at pin 5 of U48OB with the front panel LOW
LEVEL control. Turning the LOW LEVEL control CW
cau?es pin 7 of U4808 to go more positive. This increases
the curmnt through Q470 and consequently the pulse
amplitude. The output high level is unchanged as the low
level goes lower. Pin 6 of U48OB. connected to the emitter
of 0470 through R493, also goes more positive, until the
dtage between pins 5 and 6 of U48OB is zero.
If the pulse high level is raised, more current rnu~ flow
in the output amplifier to keep the pulse low lwel at the
same voltage. When the collectors of Q504 and Q508 go
positive, their emitters 90 negative. This change is coupled
through R495 to pin 6 of lJ480B. Pin 7 of U48OB goes
positive. increasing current flow through the Output Am-
plifier, and effectivel" increasing pulse amplitude. The pulse
low level is unaffected by adjustment of the pulse HIGH
LEVEL control. The Low Level Bal, infernal adjustment, is
provided to optimize the tracking of the output level
programming circuitry.
Diodes CR445, VR445, CR446, CR448, CR449, and
VR449 protect U400 against voltage reflections from
reactive loads.
Power Supply
U640 is a precision voltage regulator for the +5 V
supply. R645 sets the value of the o"tp"f voltage howwer,
it is adjusted for accuracy of the +I5 V supply, as this is the
most critical voltage in the PG 502. Internal reference
voltage from U640 is available at pin 6. Operating voltage at
pin 12 is pre-regulated by VR616. for reduced ripple.
5 V DC is applied to the collectors of Q650. The +5 V is
Theory of Operation-PG 502
taken from the emitter. If the current to the PG 502 is
increased. pin 4 of U640. connected to the emitter of
Q650, goes negative. This causes pin 10 to go positive,
increasing current flow through Q650, and restoring the
voltage To is prew value. Should the current supplied
increase excessively, the voltage drop across R650 causes
Q650 to decrease conduction through infernal action in
U640. limiting the current to a safe value.
Q606 and Q610 serve as a comparator for the +I5 V
supply. If the +15 V supply goes negative, due to increased
load, Q6OE will decrease conduction. Its collector will go
positive increasing conduction in Q600. This will increaw
cuwnf flow in emitfer follower Q612, which is connected
to the Series pass transistor in the main frame. The series
pass transistor will increase current available to the load,
and the voltage is restored to the correct value. R615 sets
the maximum current available from the supply. If the
+15 V is shorted, Q600 Saturates with its collector approx-
imately 3 V (due to VR600) below the unregulated supply.
This drop is reduced, by the bare emitter drops of Q612,
and the series pass transistor in the main frame, to about
1.8 V across R615. This drop limits the maximum current
available from the supply. F600 additionally protects the
components from overcurrent.
The -20 V supply is connected To the base of 0660. If
the -20V goes more negative, conduction increaws in
Q660. This reduces conduction in Q670, and (1674
connected to the base of the series pass transistor in the
main frame. The series pass transistor reduces conduction,
restoring the -20 V to its preset level. Current is limited
through R676. If the supply is shorted, Q660 reduces
conduction, saturating Q670 with its collector approx,
imately 3 V from the unregulated volage. The drop across
the base emifter junction of Q674 , and the series pass
transistor in the main frame, sets rhe voltage across R676.
at which the series pass transistor limits the current
available. Fuse F670 further protects components from
abnormal currents. CR675 prevetits The -20 V supply from
going positive with respect to ground.
Additional protection for the PG 602 is provided by
Q626. and its associated components. If the +I5 V supplv
goes to about 17 V. 0620 conducts. causing the gafe of
Q626 to move in the positive direction. When the gate of
Q626 is about 1 V positive with respect to the cathode, the
diode conducts, shorting the +15 V, +5 V and the -20 V
supplies together. If the -20 V supply goes several volts
more negative, Q632 conducts, causing the sam'e action in
Q626, and shorting the +15 V, and -20 V and +5 supplies.
If the fault remains, the current limits in The +I5 V, -20 V
and +5 V supplies protect the circuitry. Should tha current
limits fail, fuses F600 and F670 will open.
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