HP 6236A Operating And Service Manual page 12

series with the voltage across R 1. Since R2 is connected
between the -6.2V reference supply and a point which feed-
back action holds near -16mV, its current remains constant.
This current flows through R1 to produce a voltage drop
across R1 proportional to its resistance setting, thus the
output voltage of the supply is proportional to the resistance
setting of R1. At the output of the voltage comparison
amplifier (U1-1), a positive voltage change corresponds to a
decrease in the conduction of Q1.
4-16 CR2 and CR3 protect the input of the amplifier
against transient overloads, C2 and R4 speed up loop re,.
sponse time, and C4 and R12 stabilize the supply's high
frequency characteristics.
4-17 OR-Gate. To permit either the voltage comparison
amplifier or the current comparison amplifier to control the
series regulator transistor, the outputs of both amplifiers
are connected to the base of driver Q2 through an OR-gate
composed of CR5 and CR6. CR5 is normally reverse
biased by a negative output from the current comparison
amplifier, permitting the voltage comparison amplifier to
drive Q2 through CR6. An overload drives the output of
the current comparison amplifier positive, forward biasing
CR5 and reducing the supply output. When the overload is
removed, CR5 is reverse biased again and the voltage com-
parison amplifier resumes control of the output.
4-18 Driver and Series Regulator. The -12.4 V output
of the bias supply provides the turn-on bias for series regu-
lator transistor Q1. Its complete current path includes Q15,
CR59, R14, and Q1, and returns to common through current
monitoring resistor R8. (It is because this bias current flows
through R8 that the output ammeter requires the zero off-
set bias circuit described in paragraph 4-43.) Through the
OR-gate, either the voltage or the current comparison ampli-
fier controls the conduction of driver Q2, which regulates
the flow of turn-off bias through Q1's base-emitter circuit.
The algebraic sum of the nearly constant turn-on bias through
R14 and the variable turn-off bias through Q2 controls the
conduction of series regulator transistor Q1.
4-19 Current limit Circuit. In the +20-volt regulator,
the current comparison amplifier compares the voltage
across current monitoring resistor R8 to the fixed voltage
across part of current limit adjust potentiometer R6. The
current limit adjustment is set so that the input voltage to
the current comparison amplifier is negative in the normal
operating region, but becomes zero when the output current
increases to 0.55 amps. When the amplifier's input voltage
reaches zero, it takes control of the regulator output voltage
and reduces it as necessary to keep the output current from
exceeding 0.55 amps. When the overload is removed, the
output of the current comparison amplifier goes negative,
reverse biasing CR5 and returning control to the voltage
4-2
comparison amplifier.
4-20 Turn-On/Turn-Off Control. When the power supply
is turned on or off, Q15 in the turn-on control circuit with-
holds turn-on bias from Q1 while the regulator bias voltages
are too low. This prevents an output voltage transient from
occurring before the amplifiers are properly biased. The
output of the -6.2V reference supply is also temporarily
held at a low voltage by Q14, which conducts to short that
output.
4-21 Circuit Protection Components. Diodes C R1,
CR7, and CR9 each protect the +20-volt supply from spe-
cific hazards. Output diode CR1 protects the supply com-
ponents if a reverse voltage is applied to the output terminals.
A common way for this to occur is for an unenergized supply
to be connected in series with another that is energized. If
the output voltage is turned down quickly while a large
capacitor is connected across the output, CR7 protects
driver Q2 from excessive dissipation by shunting some of its
base current to common. The series regulator diode, CR9
protects the series regulator transistor from reverse voltage.
Series regulator voltage could occur if a deenergized sup-
ply were connected in parallel with an energized one.
4-22 0 To -20-Volt Regulator
4-23 Instead of using an
NPN
driver and a
PNP
series
regulator in the negative output line as in the +20-volt regu-
lator, the -20-volt regulator uses a PNP driver and an NPN
series regulator in the positive output line. The -20-volt
regulator circuit is the complementary equivalent of the
+20-volt circuit in other respects, as well. Their current limit
circuits operate similarly. At the outputs of the current and
voltage comparison amplifiers in the -20-volt circuit, a neg-
ative voltage change corresponds to a decrease in series regu-
lator conduction. The turn-on bias for its series regulator
transistor, Q3, is supplied from a positive voltage source,
the +7.5V bias supply, and is switched on and off by Q13
in the turn-on control circuit.
4-24 The -20-volt supply uses the output of the +20-volt
supply as its reference voltage. As a result, both outputs are
set by a single front panel control and track each other with-
in 1 %. Two resistors in resistor network Z1 are connected
in series between the +20-volt and -20-volt outputs. These
resistors are closely matched in resistance and temperature
coefficient so that the voltage across each is exactly half of
the total. The midpoint of this divider is connected to the
non-inverting input of the -20-volt supply's voltage com-
parison amplifier. The amplifier's inverting input is connect-
ed to common through R32 to hold it at zero volts. The
amplifier keeps its differential input voltage at zero by match-
ing the output voltage of the -20-volt supply to that of the
+20-volt supply.