HP 6236a, 6237b schematic

TRIPLE OUTPUT POWER SUPPLY

MODELS 6236B AND 6237B

OPERATING AND SERVICE MANUAL FOR:
MODEL 6236B, SERIALS 1705A-00101 AND ABOVE
MODEL 6237B, SERIALS 1706A-00101 AND ABOVE

* For instruments with serial numbers above those listed, a change

page may be included. Refer to Appendix A for manual back­dating changes applying to Model 6236A and 6237 A supplies.

HP Part No. 5950-1782

Hewlett-Packard

Printed: February 1977.

SECTION I
GENERAL INFORMATION

1-1 INTRODUCTION

1-2 This manual covers two triple output power supply
models, the 6236B and 6237B. Both models are com­pact general purpose bench supplies that are particularly
useful for powering developmental IC circuits, both linear
and digital. Unless one model or the other is specifically
identified, all information in this manual applies to both
the 6236B and the 6237B.

——— CAUTION ———

Carefully read Sections II and III of this
manual before attempting to operate the
power supply.

1-3 DESCRIPTION

1-4 These constant-voltage/current-limiting triple
output supplies combine two 0 to ±20V tracking outputs
rated at 0.5 amps with an additional single output that in
the Model 6236B is rated at 0 to+6 volts and up to 2.5
amps, and in the Model 6237B is rated at 0 to +18 volts
and 1 amps. The +20V and -20V tracking outputs can
also be used in series as a single 0 to 40V 0.5-amp output.
Connections to the supply's output and to chassis ground
are made to binding posts on the front panel. The supply's
three outputs share a common output terminal, which is
isolated from chassis ground so that anyone output term­inal can be grounded.

1-5 All outputs are protected against overload and
short-circuit damage.The+18V output in the 6237B and
the ±20V outputs in both models are protected by circuits
that limit the output current to 110% of its nominal maxi­mum.The overload protection circuit for the +6V output
in the 6236B has a current foldback characteristic that
reduces the output current as an overload increases until
only 1 amp flows through a short circuit. The 6V output's
current limit depends on the output terminal voltage and
varies linearly between 2.75 amps at 6 volts and 1 amp
at zero volts.

located on the front panel. One voltage control sets the
0 to+6V (or+18V) output and another sets the voltages
of the 0 to +20V and 0 to -20V outputs simultaneously.
These dual tracking outputs are made more versatile by the
provision of a tracking ratio control in addition to the usual
voltage control. With the tracking ratio control turned fully
clockwise to its "fixed" position, the dual outputs have a
fixed 1:1 tracking ratio. As the ±20V voltage control is
adjusted, the voltage of the negative supply tracks the posi­tive output within ±1%. Turning the tracking ratio control
away from its fully-clockwise position switches the dual
tracking outputs into a variable tracking ratio mode. In this
mode the voltage of the negative output can be set lower
than that of the positive output. The tracking ratio control
allows the negative supply's output to be set to any value
between a maximum that is within 10% of the positive
supply's output and a minimum that is less than 0.5 volts.
Once a ratio is established by the tracking ratio control, the
ratio of the positive output voltage to the negative output
voltage remains constant as the ±20V voltage control

varies the 0 to+20V output over its range.

1-7 The front panel also contains a line switch and a
pilot light, a voltmeter and an ammeter, and a meter
switch that selects one of the supply's three outputs to be
monitored on both dual-range meters.

1-8 In addition to the standard 104-to-127Vac 47-to­63Hz input, three other line voltage options are available for
nominal inputs of 100, 220, and 240Vac. The supply is
furnished with a permanently attached 5-foot 3-wire ground­ing type line cord. The ac line fuse is in an extractor type
fuseholder on the rear heatsink.

1-9 SPECIFICATIONS

1-10 Table 1-1 lists detailed specifications for the
power supply.

1-11

1-12 Options are factory modifications of a standard
instrument that are requested by the customer. The follow­ing options are available for the instruments covered by
this manual.

OPTIONS

1-6 All controls, meters, and output terminals are

1-1

OPTION NO. DESCRIPTION

100

220

240

910

Input Power: 87-106Vac,47-63Hz,

single-phase.

Input Power: 191-233Vac,47-63Hz,

single-phase.

Input Power: 208-250Vac,47-63Hz,

single-phase.
One additional operating and service
manual shipped with the power supply.

The user can convert an instrument from one line
voltage option to another by following the instructions
in paragraph 3-4.

1-14 ACCESSORIES

1-15 The accessories listed below may be ordered
from your local Hewlett-Packard field sales office either
with the power supply or separately. (Refer to the list at
the rear of the manual for addresses.)

1-13 Before the supply is shipped from the factory, an
internalline voltage selector switch is set and the proper
fuse installed for the line voltage specified on the order. A
label on the rear heatsink identifies this line voltage option.

———

Before applying power to the supply, make certain
that its line voltage selector switch (S3) is set for
the line voltage to be used. (See CAUTION notice
in paragraph 3-2 for additional information.)

CAUTION

———

Table 1-1. Specifications, Model 6236B and 6237B

NOTE

Specifications apply to both models
unless otherwise indicated.

INPUT POWER:

Standard: 104-127Vac (120Vac nominal), 47-63Hz,
single-phase, 112W, 140VA (Other line voltage
options are listed in paragraph 1-12).

HP PART NO.

14513A

14523A

Model 6237 B
0 to +18V Output: Maximum rated output current is

1.0A. Short circuit output current is 1.1A ±5% and a fixed
current limit circuit limits the output to this maximum at
any output voltage setting.

TRACKING ACCURACY:
The +20V and -20V outputs track within 1% with the
TRACKING RATIO control in the FIXED position.

Rack Mounting Kit for mounting one
3 1/2" high supply in a standard 19"
relay rack.

Rack Mounting Kit for mounting two
3 1/2" high supplies side by side in a
standard 19" relay rack.

DESCRIPTION

DC OUTPUT AND OVERLOAD PROTECTION:

0 to ±20V Outputs: Maximum rated output current is

0.5A. Short circuit output current is 0.55A ±5% and a
fixed current limit circuit limits the output of each supply
to this maximum at any output voltage setting. Unbal­anced loads within current rating are permitted.
(Switching to the variable tracking ratio mode allows
the -20V output to be varied from less than -0.5V to
within ±10% of the voltage setting of the +20V output.)

Model 6236B

0 to +6V Output: Maximum rated output current is 2.5A
at 6V. The maximum available output current decreases
with the output voltage setting. A current foldback circuit
limits the output to 2.75A ±5% at 6 volts and, with
decreasing voltage, reduces the current limit linearly to 1A
±15% at zero volts (short circuited).

LOAD EFFECT (Load Regulation):

All Outputs: Less than 0.01% plus 2mV for a full load

to no load change in output current.

SOURCE EFFECT (Line Regulation):

All Outputs: Less than 0.01% plus 2mV for any line

voltage change within rating.

PARD (Ripple and Noise):

All Outputs: Less than 0.35mV rms and 1.5mV p-p

(20 Hz to 20 MHz).

DRIFT (Stability):

All Outputs: Less than 0.1% plus 5mV (dc to 20Hz)
during 8 hours at constant line, load, and ambient after
an initial warm-up time of 30 minutes.

1-2

1-16

INSTRUMENT AND MANUAL

IDENTIFICATION

1-17 Hewlett-Packard power supplies are identified by
a two part serial number. The first part is the serial number
prefix, a number-letter combination that denotes the date

of a significant design change and the country of manufac­ture. The first two digits indicate the year (10 = 1970,
11= 1971, etc.) the second two digits indicate the week,
and the letter "A" designates the U.S.A. as the country of
manufacture. The second part is the power supply serial
number. A different sequential number is assigned to each
power supply, starting with 00101.

Table 1-1. Specifications, Models 6236B and 6237B (Continued)

1-18 If the serial number on your instrument does not
agree with those on the title page of the manual, Change
Sheets supplied with the manual or Manual Backdating
Changes define the difference between your instrument
and the instrument described by this manual.

1-19

1-20 One manual is shipped with each power supply
unless Option 910 is ordered for each extra manual. Addi­tional manuals may be purchased from your local Hewlett­Packard field office (see the list at the rear of this manual
for addresses). Specify the model number, serial number
prefix, and the HP Part Number provided on the title page.

ORDERING ADDITIONAL MANUALS

LOAD EFFECT TRANSIENT RECOVERY TIME:

All Outputs: Less than 50µsec for output recovery to
within 15mV of nominal output voltage following a load
change from full load to half load (or vice versa).

OUTPUT VOLTAGEOVERSHOOT:

All Outputs: During turn-on or turn-off of ac power,
output plus overshoot will not exceed 1 V if the output
control is set for less than 1V. If the control is set for 1V
or higher, there is no overshoot.

TEMPERATURE COEFFICIENT:

All Outputs: Less than 0.02% plus 1 mV voltage
change per degree Celsius over the operating range from
0 to 40°C after 30 minutes warm-up.

*OUTPUT IMPEDANCE (typical):

Ω

0 to +20V Output: 0.5m

0 to -20V Output: 0.5mΩ

Model 6236B

0 to +6V Output: 0.3mΩ

Model 6237B

0 to +18V Output: 0.3Ω plus 1.5µH

plus 1.5µH
plus 1.5µH

plus 1µH

RESOLUTION:

(Minimum output voltage change obtainable using front

panel voltage control)

0 to ±20V Outputs: 70mV
Model 6236B
0 to +6V Output: 20mV

Model 6237B
0 to +18V Output: 70mV

TEMPERATURE RATINGS:

Operating: 0 to +40°C ambient. At higher
temperatures, output current is derated linearly to 50% at
55°C maximum temperature.

Storage:-40°C to+75 °C.

METER RANGES:

0 to +20V Output: 0-25V, 0-0.6A

0 to -20V Output: 0-25V, 0-0.6A

Model 6236B
0 to +6V Output: 0-7V, 0-3A

Model 6237B
0 to +18V Output: 0-21V, 0-1.2A

* Operating characteristics listed as typical are provided
for the user's information only and are not warranteed
specifications.

METER ACCURACY:

±4% of full scale

DIMENSIONS:

3.47 in. H x 8.22 in. W x 12.56 in. D
(88mm H x 209mm W x 319mm D)

WEIGHT:

9.5 lb (4.3kg)

1-3

SECTION II

INSTALLATION

2-1 INITIAL INSPECTION

2-2 Before shipment, this instrument was inspected
and found to be free of mechanical and electrical defects.
As soon as the instrument is unpacked, inspect for any
damage that may have occurred in transit. Save all packing
materials until the inspection is completed. If damage is
found, file claim with carrier immediately. The Hewlett­Packard Sales and Service office should be notified as soon
as possible.

2-3 Mechanical Check

2-4 This check should confirm that there are no broken
knobs or connectors, that the cabinet and panel surfaces
are free of dents and scratches, and that the meter is not
scratched or cracked.

2-5 Electrical Check

2-6 The instrument should be checked against its elec
trical specifications. Section V includes an "in-cabinet" per­formance check to verify proper instrument operation.

Figure 2-1. Outline Diagram

2-7 INSTALLATION DATA

2-8 The instrument is shipped ready for bench
operation. Before applying power to the instrument, see
the CAUTION notice in paragraph 3-2.

2-9 Location

2-10 This instrument is air cooled. Sufficient space
should be allotted so that a free flow of cooling air can
reach the rear of the instrument when it is in operation. It
should be used in an area where the ambient temperature
does not exceed 40°C (up to 55°C with derating).

2-11 Outline Diagram

2-12 Figure 2-1 illustrates the outline shape and dimen­sions of this supply.

2-13 Rack Mounting

2-14 This instrument may be rack mounted in a standard
19-inch rack panel either by itself or alongside a similar
unit. Figures 2-2 and 2-3 show the components of the rack
mounting kits available for this power supply. Ordering
information for rack mounting accessories is given in para­graph 1-14.

Figure 2-2. Rack Mounting, One Unit

Figure 2-3. Rack Mounting, Two Units

2-1

2-15 Input Power Requirements

2-16 Depending on the line voltage option ordered, the
supply is ready to be operated from one of the power
sources listed in Table 2-1. The input voltage range, and
the input current and power at high line voltage and full
load is listed for each option. A label on the rear heat sink
identifies the line voltage option of your supply. All options
of this model operate from a 47-63 Hz single-phase line.

2-17 If desired, the user can easily convert the unit from
any of these options to another by following the instructions
in paragraph 3-4. A unit is converted by resetting an internal
line voltage selector switch, replacing the fuse, and changing
the line voltage tag.

2-20 To preserve the protection feature when operating
the instrument from a two-contact outlet, use a three-prong
to two-prong adapter (if permitted by local regulations) and
connect the green lead on the adapter to ground.

2-21 Model 6236B and 6237B supplies are equipped
at the factory with a power cord plug appropriate for the
user's location. Figure 2-4 illustrates the standard configu­rations of power cord plugs used by HP. Above each draw­ing is the HP option number for that configuration of power
connector pins. Below each drawing is the HP Part Number
for a replacement power cord equipped with a plug of that
configuration. Notify the nearest HP Sales and Service
Office if the appropriate power cord is not included with
the instrument.

2-22 Repackaging for Shipment

———

If the supply might possibly have been
converted to a line voltage option other
than the one marked on its identifying label
without being relabeled in some way, check
the setting of the line voltage selector
switch and the fuse rating before applying

power. (See CAUTION in paragraph 3-2.)

CAUTION

———

2-18 Power Cable

2-19 To protect operating personnel, the National
Electrical Manufacturers Association (NEMA) recommends
that the instrument panel and cabinet be grounded. This
instrument is equipped with a three conductor power cable.
The third conductor is the ground conductor and when the
cable is plugged into an appropriate receptacle, the instru­ment is grounded. The offset pin on the power cable three­prong connector is the ground connection. In no event shall
this instrument be operated without an adequate cabinet
ground connection.

2-23 To insure safe shipment of the instrument, it is
recommended that the package designed for the instrument
be used. The original packaging material is reusable. If it is
not available, contact your local Hewlett-Packard field office
to obtain the materials. This office will also furnish the
address of the nearest service office to which the instrument
can be shipped and provide the Authorized Return label
necessary to expedite the handling of your instrument return.
Be sure to attach a tag to the instrument which specifies the
owner, model number, full serial number, and service
required, or a brief description of the trouble.

Figure 2-4. Power Cord Configurations

Table 2-1. Input Power Requirements

Option Line Voltage Range Input Current Input Power

100 (100 Vac) 87-106 Vac 1.3A 140 VA
Standard (120 Vac) 104-127 Vac 1.1A 140 VA
220 (220 Vac) 191-233 Vac 0.6A 140 VA
240 (240 Vac) 208-250 Vac 0.55A 140 VA

2-2

SECTION III

—

OPERATING INSTRUCTIONS

a. Connect line cord to power source and turn LINE
switch { on. LINE ON indicator | will light.

Figure 3-1. Controls and Indicators

3-1 TURN-ON CHECKOUT PROCEDURE

3-2 The following steps describe the use of the Model
6236B or 6237B front panel controls and indicators illus­trated in Figure 3-1 and serve as a brief check that the sup­ply is operational. Follow this checkout procedure or the
more detailed performance test of paragraph 5-6 when the
instrument is received and before it is connected to any
load equipment. Proceed to the more detailed procedures
beginning in paragraph 5-6 if any difficulties are
encountered.

CAUTION

——

Before applying power to the supply, make certain
that its line voltage selector switch (S3) is set for
the line voltage to be used. This switch is mounted
on the circuit board behind the voltmeter and is
visible through the perforations in the top cover.
The positions of the two white marks on the
switch indicate the switch setting (see Figure 3-2).
If the switch setting does not correspond to the
intended power source, proceed to paragraph 3-4
before applying power.

———

b. Set METER switch }

no load connected, vary +6V VOLTAGE control ~ over
its range and check that the voltmeter responds to the
control setting and the ammeter indicates zero.

c. Set the +6V VOLTAGE control for a 6-volt meter
indication and short the +6V output terminal to COM
(common) terminal
ammeter should indicate a short-circuit output current of
approximately 1.0A (1.1 A in the 6237B). Remove the
short from the output terminals.

d. Set the METER switch to the +20V position and

turn TRACKING RATIO control
FIXED position. With no load connected, vary ±20V VOL­TAGE control
meter responds to the control setting and the ammeter
indicates zero.

e. Set the ±20V VOLTAGE control for a 20-volt meter
indication and short the +20V output terminal to the com­mon terminal with an insulated test lead. The ammeter
should indicate a short-circuit output current of 0.55A
±5%. Remove the short from the output terminals.

f. Repeat steps (d) and (e), but substitute the -20V
position of the METER switch and the -20V output ter­minal.

g. Adjust the +20V output for a 20V meter indication.
Then set the METER switch to the -20V position and
check the effect of the TRACKING RATIO control on the
voltage of the -20V output. The -20V output should be
adjustable from less than 0.5 volts to a maximum of 18 to
22 volts.

3-3 If this brief checkout procedure or later use of the
supply reveals a possible malfunction, see Section V of this
manual for detailed test, troubleshooting, and adjustment
procedures.





over its range and check that the volt-

to the +6V position and, with

with an insulated test lead. The



fully clockwise to the

NOTE

For the Model 6237B, substitute+18V
for +6V in the following steps.

3-4 LINE VOLTAGE OPTION CONVERSION

3-5 To convert the supply from one line voltage option
to another, the following three steps are necessary:

3-1

Figure 3-2. line Voltage Selector (Set for 120 Vac)

1. After making certain that the line cord is discon­nected from a source of power, remove the top cover from
the supply and set the two sections of the line voltage
selector switch for the desired line voltage (see Figure 3-2).

2. Check the rating of the installed fuse and repla­ce it with the correct value, if necessary. For options 100
or 120, use a normal time-constant 2-amp fuse (HP Part No.
2110-0002). For Options 220 or 240, use a normal time­constant 1-amp fuse (HP Part No. 2110-0001).

3. Mark the instrument clearly with a tag or label
indicating the correct line voltage to be used.

3-6 OPERATION

3-10 Overload Protection Circuits

3-11 ±20-Volt Current Limit. The +20V and -20V

outputs are individually protected against overload or short­circuit damage by separate current limit circuits adjusted
at the factory to limit the output current to 0.55A ±5%.
(This is 110% of the maximum rated output.) The current
limits can be set by adjusting resistor R6 for the +20V
output and R26 for the -20V output. (See paragraph 5-47
for current limit calibration instructions.) No deterioration of
supply performance occurs if the output current remains
below the current limit setting. If a single load is
connected between the +20V and -20V outputs, the circuit
set for the lesser current limit will limit the output.

3-12 +6V Current Foldback (Model 6236B). The over­load and short-circuit protection circuit for the +6V output
of the Model 6236B reduces the output current limit as the
output terminal voltage decreases. (The operating region of
the +6V output is enclosed by heavy lines in Figure 3-3).
The maximum rated output current is 2.5A and the current
limit is factory-adjusted to operate at 2.75A ±5% when
the output is 6 volts. At lower output voltages, the circuit
reduces the maximum obtainable output current linearly
until 1A ±15% flows when the output is shorted.
The shortcircuit current cannot be adjusted, but R46 can be
set to limit the maximum current at 6V to 2.75A ±5%.
(See paragraph 5-47 for current limit calibration instruction.)

3-7 This power supply can be operated individually
or in parallel with another supply (see paragraph 3-19). All
output terminals are isolated from ground. The ±20V and
+6V or +18V outputs use a single common output
terminal. This common (COM) terminal or anyone of the
other output terminals may be grounded to the chassis at



the front panel ground terminal (
outputs may be left floating. Loads can be connected
separately between each of the 0 to 20V output terminals
and the COM terminal, or between the -20V and the +20V
terminals for a 0 to 40V output.

in Figure 3-1 ), or all

3-8 Tracking Ratio Control

3-9 With the TRACKING RATIO control in the FIXED
position, the voltage of the -20V supply tracks that of the
+20V supply within 1% for convenience in varying the
symmetrical voltages needed by operational amplifiers
and other circuits using balanced positive and negative
inputs. Turn the TRACKING RATIO control counterclock­wise out of the FIXED position to set the voltage of the ­20V supply lower than that of the +20V supply. The
negative supply can be set from a minimum of less than

0.5 volts to a maximum within 10% of the +20V
supply's output. Once this is done, the ±20V VOLTAGE
control still controls both outputs and maintains a
constant ratio between their voltages.

3-13 +18Volt Current Limit (Model 6237B). The +18­volt output of the Model 6237B is protected by a fixed cur­rent limit circuit that operates at 1.1 A (110% of its maxi­mum rated output). The circuit is similar to the ones in the
±20-volt supplies. (See paragraph 5-47 for calibration
instructions.)

3-14 Operation Beyond Rated Output

3-15 The supply may be able to provide voltages and
currents greater than its rated maximum outputs if the line
voltage is at or above its nominal value. Operation can ex­tend into the shaded areas on the meter faces without
damage to the supply, but performance cannot be
guaranteed to meet specifications. If the line voltage is
maintained in the upper end of the input voltage range,
however, the supply probably will operate within its
specifications.

3-17 Connect each load to the power supply output
terminals using separate pairs of connecting wires. This
minimizes mutual coupling between loads and takes full
advantage of the low output impedance of the supply.
Load wires must be of adequately heavy gauge to maintain
satisfactory regulation at the load. Make each pair of
connecting wires as short as possible and twist or shield

3-2

them to reduce noise pick-up. If shielded wire is used,
connect one end of the shield to the power supply
ground terminal and leave the other end unconnected.

3-18 If load considerations require locating output
power distribution terminals at a distance from the power
supply, then the power supply output terminals should be
connected to the remote distribution terminals by a pair of
twisted or shielded wires and each load should be
connected to the remote distribution terminals separately.

3-19 Parallel Operation

3-20 Two or more power supplies can be connected in
parallel to obtain a total output current greater than that
available from one supply. The total output current is the
sum of the output currents of the individual supplies. The
output voltage controls of one power supply should be set
to the desired output voltage, and the other supply set for
a slightly larger output voltage. The supply set to the
lower output voltage will act as a constant voltage source,
while the supply set to the higher output will act as a
current-limited source, dropping its output voltage until it
equals that of the other supply. The constant voltage
source will deliver only that fraction of its rated output
current necessary to fulfill the total current demand.

3-24 Reverse Current Loading. An active load connected
to the power supply may actually deliver a reverse current to
the supply during a portion of its operating cycle. An external
source cannot be allowed to pump current into the supply
without risking loss of regulation and possible damage to the
output capacitor. To avoid these effects, it is necessary to
preload the supply with a dummy load resistor so that the
power supply delivers current through the entire operating
cycle of the load device.

3-25 Reverse Voltage Protection. Internal diodes con­nected with reverse polarity across the output terminals
protect the output electrolytic capacitors and the driver
transistors from the effects of a reverse voltage applied
across a supply output. Since series regulator transistors
cannot withstand reverse voltage either, diodes are also
connected across them. When operating supplies in
parallel, these diodes protect an unenergized supply that is
in parallel with an energized supply.

3-21 Special Operating Considerations

3-22 Pulse Loading. The power supply will automatically

cross over from constant-voltage to current-limit operation

in response to an increase in the output current over the
preset limit. Although the preset limit may be set higher
than the average output current, high peak currents as
occur in pulse loading may exceed the preset current limit
and cause crossover to occur and degrade performance.

3-23 Output Capacitance. An internal capacitor across
the output terminals of the power supply helps to supply
high-current pulses of short duration during constant-volt­age operation. Any capacitance added externally will im­prove the pulse current capability, but will decrease the
load protection provided by the current limiting circuit. A
high-current output pulse may damage load components
before the average output current is large enough to cause
the current limiting circuit to operate.

Figure 3-3. Current Limit Characteristics of the 6V Supply

(Model 6236B)

3-3

SECTION IV

PRINCIPLES OF OPERATION

4-1 OVERALL DESCRIPTION

4-2 This section presents the principles of operation
of the Models 6236B and 6237B 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
rectifiers and capacitor filters, provide raw dc for the three
output regulator circuits and for another regulator that pro­vides reference and bias voltages to the output regulators.

4-5 By comparing its output to a high-stability refer­ence, the 0 to +6-volt regulator (6236B) or 0 to +18­volt regulator (6237B) 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
amplified by an operational amplifier and applied as
feedback 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 little effect on the output voltage.

4-6 The 0 to +6-volt output in the Model 6236B is
protected by a current foldback limiter to minimize
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 shorted. (An output of 2.75A is 110% of the 2.5A
rated maximum at 6 volts.) The operating region of the
+6-volt regulator 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 down the line, reducing the output voltage

and current. Current foldback is controlled by a second
operational amplifier in the regulator that 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 +20volt and -20-volt supplies are both set
by the same front panel control and track each other within
1% in the fixed tracking ratio mode. Precise tracking 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 6237B
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-on/turn-off control circuit prevents output
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 dual­range meters is selected automatically

4-13 DETAILED CIRCUIT DESCRIPTION

4-14 0 To +20-Volt Regulator

4-15 Voltage Comparison Amplifier. The voltage com­parison amplifier in the +20-volt supply controls the conduc­tion of series regulator transistor Q1 so that the voltages at
the two inputs of the amplifier remain equal. A fixed voltage
divider holds its inverting input (U1-2) at -16mV. Its non­inverting input (U1-3) monitors the output voltage in series

4-1

with the voltage across R1. Since R2 is connected between
the -6.2V reference supply and a point that feedback 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.

and returning control to the voltage 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 with­holds 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.

4-16 CR2 and CR3 protect the input of the amplifier
against transient overloads, C2 and R4 speed up loop
response 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 com­parison amplifier resumes control of the output.

4-18 Driver and Series Regulator. The -12.4V output of
the bias supply provides the turn-on bias for series
regulator transistor Q1. Its complete current path includes
Q15, CR59, R 14, 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 offset bias circuit described in paragraph
4-43.) Through the OR-gate, either the voltage or the
current comparison amplifier 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

4-21 Circuit Protection Components. Diodes CR1, CR7,
and CR9 each protect the +20-volt supply from specific
hazards. Output diode CR1 protects the supply components
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.
Reverse series regulator voltage could occur if a deenergized
supply 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
regulator, the -20-volt regulator uses a PNP driver and an
NPN series regulator in the positive output line. The -20­volt 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 -20­volt circuit, a negative voltage change corresponds to a
decrease in series regulator 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 +20­volt 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% in the fixed tracking ratio mode.
Two resistors in resistor network Z1 are connected in
series between the +20volt 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
comparison amplifier. The amplifier's inverting input is
connected to common through R32 to hold it at zero volts.
The amplifier keeps its differential input voltage at zero by
matching the output voltage of the -20-volt supply to that
of the +20-volt supply.

4-2