HP 6960A Operating And Service Manual page 17

Model 6960 A
Section IV
,
,
R 3 and R 4 allow a current flow through the rectifier bridges even when no output
current is drawn from the instrument. Series regulators Q 1 and Q 2 act as variable
resistors determining output voltage and current. The output current from one regulator
is monitored by the resistor combinations R 39 B, R 9 and R 8, while R 39 A, R 9 and R 10
monitor the current from the other regulator. The voltage drop across these resistors is
applied to the emitter of
Q
5. When this potential exceeds a dist inet value determined
by R 39 A and B and the load current, transistor
Q
5 is caused to clip. From then on
Q 5 holds the base voltage and hence the emitter voltage of Q 1 through CR 13 and
voltage amplifier
Q
3, thus preventing any further increase in the load current. Voltage
divider R 5, R 6 adjust the base-emitter voltage of transistor Q 2 if the power supply is
driven in the 36 V / 300 mA range. The ammeter (M 1) indicates the output current by
measuring the voltage across R 22 and the voltmeter M 2 indicates directly the output
voltage of the power supply.
The voltage error amplifier consists of the two transistors Q 7 and Q 8 besides Q 3. In
the constant voltage mode changes in the output voltage due to changes in load or line
voltage are amplified and applied to the base of
Q
1 in such a way as to keep the output
voltage constant. R 23 is used to adjust the positive feedback from the collector of
Q
8
to the base of
Q
7. A proper adjustment of this feedback will result in no output voltage
change when the load current is changed from no load to full load. It can even be
adjusted to yield a negative output resistance,
i.
e., when a load is applied, the output
voltage increases rather than decreases. Normally, this is adjusted to give zero output
resistance and not a negative resistance.
CR 14 and CR 13 are disconnecting diodes. In the constant voltage mode CR 13 is
closed, CR 14 open, so that the current error amplifier is out of circuit. In the constant
current operation CR 13 will be automatically opened and CR 14 will be closed to isolate
the voltage error amplifier.
The reference supply consists of a complete feedback amplifier, Q 4 and Q 6. The
reference voltage itself is controlled by zener diodes CR 20 and CR 21, which also
stabilizes the negative auxiliary voltages. R 41 helps to supply the thermal stability of the
base potential of transistor
Q
5. Manufacturing tolerances of CR 21 are compensated by
factory adjustment of R 27 by means of shunt resistor R 38. The value of R 17 allows
current changes proportional to line voltage changes. These current changes compensate
possible variations of the current through CR 21 with line voltage variation and minimize
changes in output voltage.
4-2
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