Section Iv Principles Of Operation; Overall Description; Detailed Circuit Description - HP 6236A Operating And Service Manual

SECTION IV
PRINCIPLES OF OPERATION
4-1 OVERALL DESCRIPTION
4-2 This section presents the principles of operation of
the Models 6236A and 6237 A Triple Output Power Supply.
Throughout this section refer to the combined schematic
diagram of Figure 7-1.
NOTE
All information in this section applies
to
both
models unless otherwise indicated.
4-3 The two primary windings of the power transformer
are connected in one of four different ways by setting the
two slide switches mounted on the circuit board. These
switches select one of the nominal ac input voltages for
which the supply is designed: 100V, 120V, 220V, or 240V.
4-4 The transformer secondaries, together with recti-
fiers and capacitor filters, provide raw dc for the three out-
put regulator circuits and for another regulator which pro-
vides reference and bias voltages to the output regulators.
4-5 By comparing its output to a high-stability refer-
ence, the 0 to +6-volt regulator (6236A) or 0 to +18-volt
regulator (6237A) holds its output voltage at the value
determined by a front panel control. Any error in the
actual output as compared to the desired output is
am-
plified by an operational amplifier and applied as fe~dback
to control the conduction of a series regulator transistor.
As a result, the voltage across the series transistor varies so
as to hold the output voltage constant at the desired level.
The high gain of the voltage comparison amplifier and the
stability of the reference voltage ensure that input voltage
or load current variations have Iittle effect on the output
voltage.
4-6 The 0 to +6-volt output in the Model 6236A is
protected by a current foldback limiter which minimizes
dissipation in the series regulator transistor during overloads.
In a current foldback circuit, the current limit depends on
the output terminal voltage and in this regulator ranges
from 2. 75A ±5% at 6 volts to 1A ±15% with the output short-
ed. (An output of 2. 75A is 110% of the rated maximum of
2.5A at 6 volts.) The operating region of the +6-volt regu-
lator output is enclosed by a heavy line in Figure 3-3. If
the operating point reaches the diagonal current limit line,
a decrease in load resistance moves the operating point
4-1
down the line, reducing the output voltage and current.
Current foldback is controlled by a second operational
amplifier in the regulator which monitors the dc output
current. This current comparison amplifier takes control
of the 'output away from the voltage comparison amplifier
when the current reaches the design limit. Removing the
overload restores constant voltage operation automatically.
4-7 The +20-volt regulator has a fixed current limit at
110% of its 0.5 amp maximum rated output but is otherwise
similar to the +6-volt regulator.
4-8 The 0 to -20-volt regulator is, in turn, similar to the
+20-volt regulator except that it resembles a complementary
mirror image of the latter. The output voltages of the +20-
volt and -20-volt supplies are both set by the same front
panel control and track each other within 1%. Precise track-
ing of the two outputs is achieved by controlling the positive
output conventionally and using that output as the reference
voltage for the negative output.
4-9 The 0 to +18-volt regulator in the Model 6237 A
is similar to the +20-volt regulator. It has a fixed current
limit at 110% of its 1.0 amp output.
4-10 The reference and bias supply provides reference
and bias voltages for the output regulators.
4-11 The turn-onlturn-off control circuit prevents out-
put transients when the supply is turned on or off. It does
this by delaying the application of certain bias and reference
voltages at turn-on and removing them shortly after turn-off.
4-12 A three-position meter switch selects which of the
supplies has its output voltage and current indicated on the
front panel meters. The proper range of the dual-range
meters is selected automatically.
4-13 DETAILED CIRCUIT DESCRIPTION
4-14 0 To +20-Volt Regulator
4-15 Voltage Comparison Amplifier. The voltage com-
parison amplifier in the +20-volt supply controls the conduc-
tion of series regulator transistor Q1 so that the voltages at
the two inputs of the amplifier remain equal. A fixed volt-
age divider holds its inverting input (U1-2) at -16mV. Its
non-inverting input (U1-3) monitors the output voltage in