HP 740B Operating And Service Manual page 31

Section IV
4-55. The dc output of the Operational Filter is applied
to the base of A4Q7. The base of A4Q7 is grounded
through A4 pin 8 and a deck of the Range and Function
Switchwhenever operation on the mV ranges in Stan-
dard mode is attempted, preventing operation of the
High Voltage Section. The Differential Amplifier con-
sists of A4Q7 through A4QIO. A4Q7/A4Q8 and A4Q91
A4QIO are Darlington Pairs which provide high gain
and sensitivity to the amplifier. Whenever a current
is drawn from the + and - OUTPUT terminals which
exceeds the setting of the CURRENT LIMIT control,
A4Q12 is forward biased by an input from A5, and
lowers the voltage at the em itters of A4Q8 and A4Q9,
reducing the output of the differential amplifier. The
dc output of the Differential Amplifier is further amp-
lified and inverted by A4Qll and fed through A4 pin 3
to the Pulse Width Converter. The signal at this point
varies from approximately +8.4 V (for a 0 V Main
Loop output) to + 6 V (for a 1000 V 20 mA Main Loop
output).
4-56. Pulse Width Converter and 20 kHz Clock,
plo
A5 (Figure 7-9). The dc input to the Pulse Width
Converter from the Differential Amplifier is a dc
level that varies from approximately +8.4 V (0 V
Main Loop output) to + 6 V (1000 V 20 mA Main Loop
output). The Pulse Width Converter converts this
input voltage to 20 kHz pulses that vary in width pro-
portional to the Main Loop output voltage and current.
The 20 kHz Clock controls the pulse frequency.
4-57. The 20 kHz Clock consists of A5Q1 and A5Q2
which form an astable (free -running) multivibrator.
A5Rl, A5C2 and A5R5 determine the 20 kHz switching
frequency. The multivibrator output is differentiated
byA5C1, A5C4and A5C3. A5CRlremovesthe nega-
tive spikes. The remaining 20 kHz positive spikes
are applied through A5CR2 to the Pulse Width Con-
verter at the collector of A5Q3.
4-58. A5Q3 and A5Q4 form a Schmitt-Trigger that
functions as a monostable (one-shot) multivibrator.
In the stable state, A5Q4 is conducting and A5Q3 is
cut off. Each positive pulse from the 20 kHz Clock
causes the multivibrator to switch to the unstable
state. The length of time the multivibrator stays in
the unstable state is determined by the voltage at the
base of A5Q3 from the Differential Amplifier. When
this voltage is + 8.4 V (representing a 0 V Main Loop
output) the unstable state is very short -- approxi-
mately the same length as the triggering pulse from
the 20 kHz Clock. As the voltage at the base of A5Q3
decreases toward + 6 V, the multivibrator stays in
the unstable state longer each time it is triggered.
When the A5Q3 base voltage reaches + 6 V (representing
maximum voltage and current output from the Main
Loop), the unstable state reaches a maximum dura-
tion of approximately 20 /lsec. The resultant output
of the Pulse Width Converter at the collector of A5Q4
is a series of negative 20 kHz pulses, varying in width
from approximately 2 /lsec to 20 /lsec, proportional
to the Main Loop output voltage and current. A5Q5,
an emitter follower, provides current gain for the
pulses. The pulses are then fed out A5 pin 19 and
through T3 to the Power Switch Driver, part of A7.
4-6
Model 740B
4-59. Power Switch Driver and Power Switch (Fig-
ure 7 -11). The Power Switch Driver and Power Switch
provide current gain and shaping to the variable-width
20 kHz pulses from the Pulse Width Converter.
4-60. The variable-width 20 kHz pulses from A5 pin
19 are transformer-coupled to A7 pin 1. From A7
pin 1 the pulses are applied through A7R1 and A7L1
to A7Q1. The leading edge of each pulse turns on
A7Q1. When A7Q1 conducts, negative voltage from
the emitter of A7Q1 turns on the Power Switch Tran-
sistors' Q1 and Q2. When Q1 and Q2 conduct, cur-
rent flows from the emitter of A7Q1 through A7R10
to the base of Q2. This base current through A7R10
keeps A7Q2 and A7Q3 cut off for the duration of the
pulse.
4-61. The trailing edge of the variable-width pulse
turns off A7Q1. The emitter of A7Qlis then clamped
to approximately +1.2 Vby A7CR1 and A7CR2. The
positive voltage turns off the Power Switch Transistors.
Q1and Q2, and turns on A7Q2 and A7Q3. Conduction
of A7Q2 and A7Q3 helps discharge the emitter-base
capacitance of the Power Switch Transistors, greatly
decreasing their turn-off time at the end of the vari-
able-width pulse. A7C8* (typically 0.0068
/IF)
is
factory selected to match the reactive characteristics
of the Power Switch Driver with the Power Switch
transistors, Q1 and Q2. Paragraph 5-95 explains
the A7C8* selection procedure.
4-62. Internal Current Limit,
plo
A7 (Figure 7-11).
The - 42 V Power Supply supplies collector current to
Q1 and Q2 through R4, F3 and the primary of T4.
At high output voltage and power levels, the voltage
pulses across R4 increase in amplitude. A7R14 and
A7C7 integrate these pulses and the resulting average
voltage is applied across A7CR3 and the emitter -base
junction of A7Q5. At excessive output voltage and
power levels, the voltage across R4 becomes
suI-
ficient to forward-bias A7Q5. The effect is further
amplified by A7Q4 which applies a positive voltage
to the base of A7Q1. A7Q1 then reduces or blocks
the variable width pulses from T3, resulting in lower
output voltage and current from the Main Loop.
4-63. High Voltage Pulse Transformer, T4, and High
Voltage Rectifier, plo All (Figure 7-3). The Power
Switch transistors, Q1 and Q2, function to store and
release energy in the High Voltage Transformer, T4.
The release of energy from the secondary of T4 is
converted to dc by the High Voltage Rectifier, AllCR1.
4-64. Q1 and Q2 conduct for the duration of each 20
kHz pulse from the Power Switch Driver. During this
conduction time, a magnetic field builds up around
T4 as a result of the large current drawn through the
primary winding. The voltage across the secondary
winding reverse biases the High Voltage Rectifier,
AllCR1, and no current flows in the secondary.
4-65. When the trailing edge of the 20 kHz pulse turns
off Q1 and Q2, the following events occur: The mag-
netic field around T4 begins to collapse attempting to
induce current flow in both the primary and secondary
windings. The primary circuit, however, is incom-
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