Tektronix 7613 User Manual

LOW-VOLTAGE POWER SUPPLY
The Low-Voltage Power Supply circuit provides the
operating power for this instrument from six regulated
supplies. Electronic regulation is used to provide stable,
low-ripple output voltages. Each supply (except the +130 V
supply, which is fused) contains a short-protection circuit
to prevent instrument damage if a supply is inadvertently
over-loaded or shorted to ground. Fig. 3-22 shows a
detailed block diagram of the Low-Voltage Power Supply
circuit. A schematic of this circuit is shown on diagram 9 at
the rear of this manual.
Power Input
Power is applied to the primary of transformer T801
through line fuse F1000, thermal cutout $1000, and
POWER switch $1001. The Voltage-Selector Jumper,
P1001, connects the two halves of the primary of T801 in
parallel for 110-volt (nominal) operation. Voltage-Selector
Jumper P1002 connects the two halves of the primary in
series for 220-volt (nominal) operation. The line fuse,
F1000, must be changed to provide the correct protection
for 220-volt nominal operation.
Each half of the primary of 1801 has taps above and
below the 110-volt (220-volt) nominal point. When the
Voltage Selector Jumper is moved from LOW to MED to
HI, more turns are effectively added to the primary winding
and the turns ratio is decreased to compensate for the
increased primary voltage. This configuration extends the
regulating range of the 7613.
A fan provides forced-air cooling. The fan is connected
in parallel with one half of the primary winding of T801.
Therefore, it always has the same voltage applied regardless
of the position of the Voltage-Selector Jumper.
Thermal cutout $1000 provides thermal protection for
this instrument. If the internal temperature of the instru-
ment exceeds a safe operating level, S1000 opens to
interrupt the applied power. When the temperature returns
to a safe level, $1000 automatically closes to re-apply the
power.
—50-Volt Supply
The following discussion includes the description of the
50 V Rectifier, —50 V Series Regulator, —-50 V Feedback
Amplifier, —50 V Reference, and —50 V Current Limiting
stages. Since these stages are closely related in the operation
of the -50-volt regulated output, their performance is most
easily understood when discussed as a unit.
Circuit Description—7613/R7613 Service
The 50 V Rectifier assembly CR808 rectifies the output
at the secondary of T801 to provide the unregulated
voltage source for both the —50- and +50-volt supplies.
CR808 is connected as a bridge rectifier and its output is
filtered by C808-C809. Transistors Q886, 0896, Q900
operate as a feedback-stabilized regulator circuit to main-
tain a constant —50-volt output level. Q886 is connected as
a differential amplifier to compare the feedback voltage at
the base of O886B against the reference voltage at the base
of Q886A. The error output at the collector of O886B
reflects the difference, if any, between these two inputs.
The change in error-output level at the collector of Q886B
is always opposite in direction to the change in the
feedback input at the base of O886B (out of phase).
Zener diode VR890 sets a reference level of about —9
volts at the base of Q886A. A feedback sample of the
output voltage from this supply is connected to the base of
Q886B through divider R880-R881-R882. R881 in this
divider is adjustable to set the output level of this supply.
Notice that the feedback voltage to this divider is obtained
from a line labeled —50 V Sense. Fig. 3-23 illustrates the
reason for this configuration. The inherent resistance of the
interconnecting wire between the output of the —50-Volt
Supply and the load produces a voltage drop which is equal
to the output current multiplied by the resistance of the
interconnecting wire. Even though the resistance of the
wire is small, it results in a substantial voltage drop due to
the high output current of this supply. Therefore, if the
feedback voltage were obtained ahead of this drop, the
voltage at the load might not maintain close regulation.
However, the —50 V Sense feedback configuration over-
comes this problem since it obtains the feedback voltage
from a point as close as practical to the load. Since the
current in the —50 V Sense fine is small and constant, the
feedback voltage is an accurate sample of the voltage
applied to the load.
Regulation occurs as follows: If the output level of this
supply decreases (less negative) due to an increase in load,
or a decrease in input voltage (as a result of line voltage
changes or ripple), the voltage across divider R880-
R881-R882 decreases also. This results in a more positive
feedback level at the base of O8868 than that established
by the —50 V Reference stage at the base of Q886A. Since
the transistor with the more positive base controls the
conduction of the differential amplifier, the output current
at the collector of O886B increases. This increase in output
from O886B allows more current to flow through Q896
and Q900 to result in increased conduction of —50 V Series
Regulator Q903. The load current increases and the output
voltage of this supply also increases (more negative). As a
result, the feedback voltage from the —50 V Sense line
increases and the base of Q886B returns to the same level as
the base of Q886A. Similarly, if the output level of this
supply increases (more negative), the output current of
Q886B decreases. The feedback through O896 and Q900
reduces the conduction of the —50 V Series Regulator to
decrease the output voltage of this supply.
3-27
7613

Tektronix 7613 Specifications

General

Brand	Tektronix
Model	7613
Category	Test Equipment
Language	English