HP 740B Operating And Service Manual page 32

Model 740B
plete since Q1 and Q2 are in the non-conducting state.
The collapse of the field around the secondary winding
forward biases the High Voltage Rectifier, AllCRl,
and a large current flows in the secondary circuit.
The secondary current charges several capacitors in
the output circuit. These capacitors charge to a value
determined by the energy in the collapsing magnetic
field around T4.
4-66. The energy transferred to the secondary of T4
depends on several factors. Ql and Q2 turn off very
rapidly at the end of each 20 kHz pulse, partly due to
the action of A7Q2 and A7Q3 (Paragraph 4-61). This
extremely sudden interruption of primary current in
T4 results in a very high voltage across the secondary
winding. The power delivering capability of the
secondary winding depends on how much energy is
available in the collapsing magnetic field. The energy
content of the magnetic field is determined mainly by
the length of time the Power Switch allows current to
flow through the primary of T4 each time a 20 kHz
pulse is applied. A wide pulse applied to the Power
Switch allows a large magnetic field to be developed
and a large amount of power to be delivered by the
secondary. Conversely, a narrow pulse results in
a small amount of power delivered by the secondary
of T4.
4-67. The output of High Voltage Rectifier, AllCR1,
is fed through AllR1 to the output circuits.
4-68. Output Circuits (Figure 7-3). The positive spikes
from the High Voltage Rectifier are filtered in several
places. The filtering is provided by A5C4, C4 and
AllL1 on the 1 V range and below; A5C5, A5C8, C4
and AllL1 on the 10 V range; A5C8, C3, AllL1 and
C4 on the 100 V range; and C3, AllL1 and C4 on the
1000 V range.
4-69. The filtered output voltage is fed through the
Output Rotary Switch, K1, to the
+
OUTPUT terminal
whenK1 is energized. K1 is a two-position solenoid-
actuated rotary switch that energizes when the Output
Pushbutton Switch, Sll, is depressed. - 42 V is
supplied to one lead of the switch solenoid. Depressing
Sll connects the other lead of the solenoid to ground,
completing the path for current through the solenoid
and energizing K1.
In
the energized position, current
through the solenoid is limited by R3 to prevent over-
heating.
4-70. An interlock from pin 1 to pin 5 in the Output
Cable Assembly prevents K1 from energizing when
the Output Cable Assembly is not connected to the
Output Jack, J2. When Sll is turned on, SllDS1 lights.
When Sll is depressed a second time the ground is re-
movedfrom K1; K1 de-energizes and SllDS1 goes
out. The operation of SllDS1 is not affected by the
interlock for K1. Cycling Sll without the Output Cable
Assembly connected to the Output Jack will cause
SllDS1 to alternately light and go out even though K1
will not energize.
4-71. The Floating 12 V Power Supply (p/o A5) re-
ferences the - Output terminal to + 12 Vwith respect to
the negative lead of the High Voltage Pulse Trans-
former, T4. This condition causes the High Voltage
01794-1
Section IV
Section to generate an output in seri es -opposition to
the
+
12 V from the floating supply. The High Voltage
Section then must generate a + 12 V output just to
maintain 0 V across the + and - Output terminals.
This action is necessary in order to keep the Pulse
Width Converter and Power Switch circuits in the
linear region of their dynamic range when the Main
Loop is generating an output near 0 V.
4-72. High Voltage Section Feedback Divider (Fig-
ure 7-3). The closed loop gain of the High Voltage
Section is controlled by degnerative feedback from
the output stage to the Differential Amplifier input
stage (p/o A4). Feedback is controlled by the High
Voltage Section Feedback Divider, SlRll through
SlR14.
On
the 1 V range and below, the feedback
factor ({3) is 1 (unity) resulting in a closed loop gain
of 1. On the 10 V, 100 V and 1000 V ranges,
f3
de-
creases to 0.1, 0.01, and 0.001 resulting in closed
loop gains of 10, 10 2 and 103 respectively.
Precise
closed loop gain characteristics in t)1e High Voltage
Section are not essential since the overall Main Loop
feedback controls the Main Loop accuracy and will
overcome small gain errors in both the Low Voltage
and High Voltage sections.
4-73. External Current Limit, plo A5 (Figure 7-9h
The External Current Limit Circuit functions to pro-
tect the Model 740B from excessive loads connected
to the Output terminals. The External Current Limit
Circuit is comprised of the front Panel CURRENT
LIM:IT Control, the Current Limit Adjustment (A10R8)
and the Current Limit Detector, plo A5.
4-74. R12 (CURRENT LIMIT Control), A10R1 and
A10R8 are in series with the - Output from the Main
Loop. The voltage drop across this series combina-
tion appears across A5 pin 17 and A5 pin 6 and is
directly proportional to th~ current delivered by the
Output terminals. Under no-load conditions, the
voltage at both pin 17 and pin 6 is approximately - 12 V,
caused by the operation of the Floating 12 V Supply.
Under these conditions, A5Q6 is turned off and the
voltage level coupled from A5Q6 to the base of A5Q7
keeps A5Q7 from conducting. The collector of A5Q7
is connected through A5 pin 15and A4 pin 20to a volt-
age divider consisting of A4R35, A4CRll and A4CR12
(Figure 7-8). The voltage divider clamps the collector
of A4Q7 and the bases of A4Q6 and A4Q12 to approxi-
mately -1 V. A4Q6 and A4Q12 function as diodes and
do not conduct unless an overload condition exists.
4-75. When excessive current is delivered by the Main
Loop Output, a significant voltage drop occurs across
R12, A10R1 and A10R8 causing A5Q6 to conduct. This
causes A5Q7 to conduct and the collector of A5Q7 goes
positive, overcoming the -1 V from A4R35, A4CRll
and A4CR12. The positive level at the collector of
A5Q7 is coupled through A5 pin 15 and A4 pin 20 to
the bases of A4Q6 and A4Q12. A4Q6 and A4Q12 are
then forward biased, reducing the gain of the Oper-
ational Filter and Differential Am plifier, and the Main
Loop output voltage decreases to a safe level.
4-76. The CURRENT LIMIT Control (R12) varies the
sensitivity of the Current Limit Detector by changing
the resistance in series with the - Output. When the
4-7