HP 6235A Service manual

TRIPLE OUTPUT
POWER SUPPLY
MODEL 6235A
OPERATING AND SERVICE MANUAL
FOR SERIALS 1752A-00101 AND ABOVE
* For Serials above 1752A-00101,
a change page may be included.

HEWLETT M PACKARD
m
CERTIFICATION
Hewlett-Packard Company certifies that this instrument met its published
specifications at the time of shipment from the factory. Hewlett-Packard
Company further certifies that its calibration measurements are traceable to
United States National Bureau of Standards, to the extent allowed by the
Bureau s calibration facility, and to the calibration facilities of other Inter¬
national Standards Organization members.
WARRANTY AND ASSISTANCE
This Hewlett-Packard product is warranted against defects in materials and
workmanship for a period of one year from the date of shipment [,except
that in the case of certain components listed in Section I of this manual the
warranty shall be for the specified period], Hewlett-Packard will, at its op-
lon, repair or replace products which prove to be defective during the war¬
ranty period provided they are returned to Hewlett-Packard, and provided the
preventive maintenance procedures in this manual are followed. Repairs neces-
^ b y misuse °f the product are not covered by this warranty. NO
01 HER WARRANTIES ARE EXPRESSED OR IMPLIED INCLUDING
BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANT¬
ABILITY AND FITNESS, FOR A PARTICULAR PURPOSE. HEWLETT-
PACKARD IS NOT LIABLE FOR CONSEQUENTIAL DAMAGES.
Service contracts or customer assistance agreements are available for Hewlett-
Packard products.
For any assistance, contact your nearest Hewlett-Packard Sales and Service
Office. Addresses are provided at the back of this manual.

TABLE OF CONTENTS
Section Page
I GENERAL INFORMATION. 1-1
1-1 DESCRIPTION. 1-1
1-7 SPECIFICATIONS AND SUPPLE¬
MENTAL CHARACTERISTICS. 1-1
1-9 OPTIONS.. 1-1
1-12 SAFETY CONSIDERATIONS .... 1-1
1-14 ACCESSORIES.. 1-1
1-16 INSTRUMENT AND MANUAL
IDENTIFICATION. 1-2
1- 1 8 ORDERING ADDITIONAL
MANUALS. 1-2
II INSTALLATION. 2-1
2- 1 INITIAL INSPECTION . 2-1
2-3 Mechanical Check. 2-1
2-5 Electrical Check.; . 2-1
2-7 INSTALLATION DATA. 2-1
2-9 Location. 2-1
2-11 Outline Diagram. 2-1
2-13 Rack Mounting. 2-1
2-15 Input Power Requirements. 2-1
2-18 Power Cable. 2-2
2- 2 2 REPACKING FOR SHIPMENT ... 2-2
III OPERATING INSTRUCTIONS ... 3-1
3- 1 INTRODUCTION. 3-1
3-3 CONTROLS AND INDICATORS. . 3-1
3-4 Line Switch. 3-1
3-5 Voltage and Current Metering .... 3-1
3-8 Voltage Controls. 3-1
3-10 TURN-ON CHECKOUT
PROCEDURES. 3-1
3-13 LINE VOLTAGE OPTION
CONVERSION. 3-2
3-15 OPERATION. 3-2
3-17 Tracking Ratio. 3-3
3-19 Overcurrent Protection. 3-3
3-21 Operation Beyond Rated Output . . 3-3
3-23 Connecting Loads. 3-3
3-26 Parallel Operation.. . 3-3
3-28 Special Operating Considerations . . 3-3
Section Page
IV PRINCIPLES OF OPERATION ... 4-1
4-1 OVERALL OPERATION. 4-1
4-5 0 TO+18V REGULATED
SUPPLY. 4-1
4-8 Constant Voltage Operation. 4-1
4-12 Current Limit Operation. 4-2
4-14 Circuit Protection Components . . . 4-2
4-16 0 TO -18V REGULATED
SUPPLY. 4-2
4- 20 0 TO +6V REGULATED
SUPPLY . 4-2
V MAINTENANCE. 5-1
5- 1 INTRODUCTION. 5-1
5-4 TEST EQUIPMENT REQUIRED . . 5-1
5-6 PERFORMANCE TEST. 5-1
5-8 General Measurement Techniques. . 5-2
5-15 Rated Output, Tracking, Meter
Accuracy, and Current Limit . . 5-2
5-17 Load Effect (Load Regulation) ... 5-3
5-19 Source Effect (Line Regulation). . . 5-4
5-21 PARD (Ripple and Noise). 5-4
5-29 Load Transient Recovery Time . . . 5-5
5-32 TROUBLESHOOTING. 5-6
5-35 Troubleshooting Procedure. 5-6
5-38 Open Fuse Troubleshooting ..... 5-6
5-40 REPAIR AND REPLACEMENT. . . 5-7
5-41 Series Regulator Replacement .... 5-7
5-43 Semiconductor Replacement. 5-7
5-46 ADJUSTMENT AND
CALIBRATION . 5-7
5-47 Current Limit Adjustment . 5-7
5-50 Meter Calibration . 5-7
VI REPLACEABLE PARTS. 6-1
VII CIRCUIT DIAGRAMS . 7-1
n

SECTION I
GENERAL INFORMATION
1-1 DESCRIPTION
1-2 The Model 6235A Triple Output Power Supply is a
compact general purpose bench supply particularly useful
for powering developmental !C circuits, both linear and
digital. This constant-voltage/current-limiting supply com¬
bines two 0* to ±18V tracking outputs rated at 0.2A with
a single 0* to +6V output rated at 1 A. The +18V and —18V
tracking outputs can also be used in-series as a single 0 to
36V, 0.2A output. Connections to the outputs are made
to binding posts on the front panel. The +6V and ±18V
outputs share a common output terminal which is isolated
from chassis ground. The chassis ground terminal is located
on the supply's rear heat sink assembly.
1-3 Each output is protected against overload or short-
circuit damage by separate fixed current limit circuits. The
+18V and —18V outputs are each limited to 0.255A ±15%
and the +6V output is limited to 1.275A ±1 5%.
1-4 Voltage controls, output terminals, and a combin¬
ation voltmeter/ammeter are located on the front panel.
One voltage control sets the 0 to +6V output and another
simultaneously adjusts the 0 to +18V and 0 to -18V dual
tracking outputs. A tracking ratio control sets the ratio
between the +18V and -18V outputs. It can set the nega¬
tive supply's output to any value between a minimum that
is less than 0.5V and a maximum that is equal to the positive
supply's output. A 1:1 ratio is useful when powering opera¬
tional amplifiers and other circuits that require balanced
positive and negative voltages. Once the tracking control
has established a voltage ratio between the positive and neg¬
ative outputs, the ratio remains constant as the ±18V volt¬
age control varies both outputs.
1-5 The front panel also contains a line pushbutton
switch with a simulated (non-electrical) on-off indicator,
and four meter select pushbutton switches. One meter push¬
button selects either voltage or current monitoring and
the remaining three pushbuttons select the output to be
monitored and the proper meter range.
1-6 In addition to the standard 104-to-127Vac, 47-to-
63Hz input, a 240Vac nominal line voltage option is avail-
*Each output has a minimum operating voltage of < 50m V.
able. 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.
1-7 SPECIFICATIONS AND SUPPLEMENTAL
CHARACTERISTICS
1-8 Table 1-1 provides specifications and supplemental
characteristics for Triple Output Power Supply, Model
6235A. Specifications describe the supply's warranted
performance characteristics. The supplemental characteristics
are typical, but non-warranted, performance characteristics
and are intended to provide additional information useful
in applying the power supply.
1-9 OPTIONS
1-10 Options are factory modifications of a standard
instrument that are requested by the customer. The follow¬
ing options are available with this instrument.
OPTION NO. DESCRIPTION
028 Input power: 208-250Vac, 47-63Hz,
single phase.
910 One additional operating and service
manual shipped with the power supply.
1-11 Before the supply is shipped from the factory, an
internal line 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.
The user can convert an instrument from one line voltage
option to the other by following the instructions in para¬
graph 3-13.
1-12 SAFETY CONSIDERATIONS
1-13 This product is a Safety Class 1 insturment (pro¬
vided with a protective earth terminal). The instrument
and manual should be reviewed for safety markings and
instructions before operation.
1-14 ACCESSORIES
1-15 The accessory listed below may be ordered from
your local Hewlett-Packard field sales office either with the
1-1

power supply or separately. (Refer to the list at the rear of
the manual for addresses.)
.HP PART NO . DESCRIPTION
14522A Rack Mounting Tray for mounting
one or two 6235A supplies in a
standard 19" relay rack.
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 manufac¬
ture. 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 manufac¬
ture. The second part is the power supply serial number. A
different sequential number is assigned to each power sup¬
ply, starting with 00101.
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 ORDERING ADDITIONAL MANUALS
1-20 One manual is shipped with each power supply.
(Option 910 is ordered for each extra manual, see paragraph
1-9.) Additional manuals may also be purchased separately
from your local Hewlett-Packard 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 pro¬
vided on the title page.
Table 1-1. Specifications and Supplemental Characteristics
SPECIFICATIONS:
The following specifications describe the 6235A's
warranted performance characteristics.
Source Effect (Line Regulation);
Given for any line voltage change within rating.
0 to +6V Output: 8m V
DC Output:
0 to ±18V Outputs: 15mV
Voltage span over which output may be varied using
front panel controls.
* Minimum operating voltage for each output is < 50
millivolts.
PARD (Ripple and Noise):
All Outputs: Less than 1mV rms and 5mV p-p (20Hz to
20MHz),
0 to +6V Output: Maximum rated output current is
1 A. Short circuit output current is 1.275 ±15% and a fixed
current limit circuit limits the output to this maximum at
any output voltage setting.
Load Transient Recovery Time:
All Outputs: Less than 50qsec for output recovery to
within 15m V of nominal output voltage following a load
change from a full load to half load (or vice versa).
0* to ±18V Output: Maximum rated current is 0.2A for
each output. Short circuit output current is 0.255A ±15%
and fixed current limit circuits limit the output of each sup¬
ply to this maximum at any output voltage setting. Unbal¬
anced loads within current rating are permitted.
Temperature Ranges:
Operating 0 to +40°C ambient. From 40°C to 55°C, out¬
put current is derated linearly to 50% at 55°C.
Storage: -40°C to +75°C.
Meter Ranges:
Input Power:
Standard Option: 104-127Vac (120Vac nominal), 47-
63Hz, single phase. (240Vac line voltage option available,
see paragraph 1-9.)
0 to +6V Output: 0 to 7V, 0 to 1,2A
0 to +18V Output: 0 to 21V, 0 to 0.24A
0 to -18V Output: 0 to 21V, 0 to 0.24A
Meter Accuracy:
Load Effect (Load Regulation):
Voltage load effect is given for a load current change
equal to the current rating of the supply.
0 to +6V Output: 8mV
0 to ±18V Outputs: lOmV
Voltmeter: All Outputs: ±3% of full scale
Ammeter: All Outputs: ±4% of full scale
Weight:
Net: 5 lb (2.3 kg)
Shipping: 7 lb (3.2 kg)
1-2

Table 1 - 1 . Specifications and Supplemental Characteristics (Continued)
SUPPLEMENTAL CHARACTERISTICS:
The following characteristics are intended to provide
information useful in 6235A applications by giving typical,
but non-warranted, performance parameters.
Regulation and Ripple (Typical performance parameters):
The maximum load regulation, line regulation, and ripple
specifications listed on page 1-2 are given for 0 to 40°C
temperature range. At an ambient temperature of 25°C,
ty pical performance characteristics are as follows:
Load Regulation:
0 to +6V Output: 4mV
0 to ±18V Output: 6mV
Line Regulation:
0 to+6V Output: 4mV
0 to ±18V Output: 6mV
Ripple:
All outputs: 0.2mV rms.
Tracking Accuracy (±18V Outputs):
The output voltage tracking ratio remains constant
(within 1 %) over the voltage range from 1 to 18 volts for
any TRACK control setting.
Temperature Coefficient:
All Outputs: Less than 0.04% + 2mV voltage change per
degree Celsius over the operating range from 0 to 40°C
after 30 minutes warm-up.
Drift (Stability):
All Outputs: Less than 0.1%+10mV (dc to 20Hz) during
8 hours at constant line, load, and ambient after an initial
warm-up time of 30 minutes.
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 Hewlett-Packard 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.
2-7 INSTALLATION DATA
2-8 The instrument is shipped ready for bench opera¬
tion. Before applying power to the instrument, see the
CAUTION notice in paragraph 3-11.
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 dimen¬
sions of this supply.
2-13 Rack Mounting
2-14 One or two 6235A's may be mounted in a standard
19-inch rack panel using rack mounting tray HP Part No.
14522A. Installation consists of bolting the rack mounting
tray to the 19-inch rack and sliding the power supply(s) Into
the slot(s) provided in the tray. The power supply's rubber
feet are seated in holes in the bottom of the tray.
2-15 Input Power Requirements
2-16 The supply may be operated continuously from
a nominal 1 20V or 240V (47-63Hz) single phase power
source. The supply is shipped from the factory ready to
be operated from one of these power sources. A label on
the rear heatsink identifies the line voltage option of your
supply. The input voltage range and input current required
for each of the nominal inputs are listed below. The maxi¬
mum input power (high line, full load conditions) required
for either input is 35 watts.
Line Voltage
Maximum
Option
Range
Input Current
Standard (1 20Vac) 104-127Vac .26A
028 (240Vac) 208-250Vac
.14A
_
2- 1 7 If desired, the user can convert the unit from one
option to another by following the instructions in paragraph
3- 1 3 . A unit is converted by resetting an internal line volt¬
age selector switch, replacing the fuse and line cord plug,
and changing the line voltage label.
TOP
TERMINAL
Figure 2-1. Outline Diagram
2-1

2-18 Power Cable
2-22 REPACKAGING FOR SHIPMENT
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 instru¬
ment is grounded. The offset pin on the power cable three-
prong connector is the ground connection. In no event
shall this instrument be operated without an adequate cabi¬
net ground connection.
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 6235A is equipped at the factory with a
power cord plug appropriate for the user's location. Figure
2-2 illustrates the standard configuration of power cord
plugs used by HP. Below each drawing is the HP Part Num¬
ber for a replacement power cord equipped with a plug of
that configuration. Notify the nearest HP Sales Office if
you require a different power cord.
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 instru¬
ment 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 instrumenfwhich
specifies the owner, model number, full serial number, and
service required, or a brief description of the trouble.
Figure 2-2. Power Cord Plug Configurations
2-2

SECTION III
OPERATING INSTRUCTIONS
3-1 INTRODUCTION
3-2 This section describes the operating controls and
indicators, turn-on checkout procedures, and other opera¬
ting considerations for the Model 6235A Triple Output
Power Supply.
WARNING
Before the instrument is switched on, all protective
earth terminals, extension cords, auto-transformers
and devices connected to it should be connected
to a protective earth grounded socket Any inter¬
ruption of the protective earth grounding will cause
a potential shock hazard that could result in
personal injury.
Only fuses with the required rated current and
specified type should be used. Do not use repaired
fuses or short circuited fuseholders. To do so
could cause a shock or fire hazard.
3-3 CONTROLS AND INDICATORS
3-4 Line Switch
3-5 The LINE pushbutton switch (item (?) , Figure
3-1) is pushed-in to turn the supply ON and released (out
position) to turn the supply OFF. A simulated (non-electri¬
cal) ON indicator (within the pushbutton) "lights" when the
button is pushed-in whether or not power is applied to the
unit.
3-6 Voltage and Current Metering
3-7 Four meter select pushbutton switches (items (5)
and (3) ) permit the output voltage or current of any one
supply (+6V, +1 8V, or —18V) to be monitored on the
VOLTS/AMPS meter (?) . The V/A pushbutton (2)
selects either voltage (out position) or current (in position).
The +6, +18, and -18 output select pushbuttons (s)
connect the desired output to the metering circuit when the
applicable button is pushed-in. The three output select
pushbuttons are mechanically interlocked so that only one
can be pushed-in at a time. The voltmeter and ammeter
ranges selected by the +6, +18, and -18 pushbuttons are
listed below. The shaded areas on the meter scales indicate
the amount of output voltage or current that may be avail¬
able in excess of the normal rating.
OUTPUT
METER RANGES
VOLTS
AMPS
+6V
0-7
0 -1.2
+18V 0-21 0-0.24
-18V 0-21 0-0.24
3-8 Voltage Controls
3-9 The+6 VOLTAGE control ( 5 ) sets the output
voltage level of the +6V supply and the ±18 control (6)
sets the output voltage levels of the dual tracking ±1 8V sup¬
plies. Precise tracking of the +18V and —18V outputs is
achieved by controlling the positive output and using it as a
reference voltage for the negative supply. The TRACK
VOLTAGE control (?) sets the ratio between the —18V
and +18V output voltages. Once the ratio is set, the +18
VOLTAGE control will control both outputs with a constant
ratio maintained between the two outputs. The voltage
controls (cermet potentiometers) have infinite resolution;
thus, the resolution obtained depends only upon the user's
care in setting the controls. Greater accuracy in setting the
controls can be achieved by using a DVM to measure the
outputs.
3-10 TURN-ON CHECKOUT PROCEDURES
3-11 The following steps describe the use of the Mode!
6235A front panel controls and indicators illustrated in
Figure 3-1 and serve as a brief check that the supply is opera¬
tional. 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 performance test beginning in
paragraph 5-6 if any difficulties are encountered.
—— CAUTION-
Before the supply is switched on, check
the label on the heat sink to make certain that
the supply's line voltage option agrees with the
line voltage to be used. The supply will be damaged
if its internal switch is set for a 120Vac input and
240 Vac input power is applied.
a. Connect line cord to power source and push
LINE switch (?) in.
b. Set meter select switches (2) , (3) to monitor
3-1

i
Figure 3-1. Controls and Indicators
+6V supply s output voltage. With no load connected, vary
the +6 VOLTAGE control © over its range and check
that the voltmeter 0 responds to the control setting.
c. Monitor the +18V supply's output voltage. With no
load connected, vary the ±18 VOLTAGE control © over
its range and check that the voltmeter responds to the
control setting.
d. Monitor the —18V supply's output voltage. Turn
TRACK VOLTAGE © control fully clockwise. With no
load connected, vary the ±18 VOLTAGE control © over
its range and check that the voltmeter responds to the con¬
trol setting,
e. Monitor the ±1 8V output and adjust ±18 VOLTAGE
control for +18V indication on voltmeter. Monitor the
-18V output and check the effect of the TRACK VOLTAGE
control on the voltage of the -18V output. The -18V out¬
put should be adjustable from less than 0.5 volts to a maxi¬
mum of 1 8 to 21 volts as the TRACK control is rotated
from its fully CCW to its fully CW position.
f. Turn the +6, ±18, and TRACK VOLTAGE controls
fully CW. Connect ammeter (e. g. Simpson Multimeter,
Model 269), in turn, between each output and COM. The
internal resistance of the meter is low enough to overload
the supply so that the output will current limit. Verify that
the current limit circuit in each supply is limiting the output
current to:
Supply
Current Limit
+6 V
1.275A ±15%
+18V
0.255A ±15%
-18V
0.255A ±15%
g. R e m o ve meter and connect loads to the output ter¬
minals (see paragraph 3-23).
3-1 2 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.
3-13 LINE VOLTAGE OPTION CONVERSION
3-14 To convert the supply from one line voltage option
to the other, proceed as follows:
T Disconnect line cord from the power source.
2 . Remove top cover from supply by removing
2 screws in rear of supply and sliding cover to the rear. The
line voltage selector switch (S2) is mounted on the circuit
board behind the meter.
3. Set S2 to the desired position (see Figure 3-2).
The forward switch position (toward front panel) selects
120 V ac input and the rear switch position (toward heat
sink) selects the 240Vac input.
4. Check the rating of the installed fuse and
replace it with correct value, if necessary. For 120Vac
input, use a 0.4A slo-blow fuse (HP Part No. 2110-0340).
For 240Vac input, use a 0.2A slo-blow fuse (HP Part No
2110-0235).
5. Install proper line cord and plug (see paragraph
2 - 2 1 ) .
6. Mark the supply clearly with a tag or label
indicating the correct line voltage to be used.
3-15 OPERATION
3-16 This power supply can be operated individually or
in parallel with another supply (see paragraph 3-27). All
output terminals are isolated from chassis ground. The ±6V
and ±1 8V outputs use a single common output terminal ©
This common (COM) terminal or any one of the other out¬
put terminals may be grounded to the chassis ground termi¬
nal which is located on the supply's rear heat sink. All out¬
puts may also be left floating. Loads can be connected
separately between each of the 0 to 18V output term inals
and the COM terminal, or between the +18V and —1 8V
terminals for a 0 to 36V output. A single load can also be
connected between the ±6V and -18V terminals for a 0
to 24V output.
Figure 3-2. Line Voltage Selector (Set For 120Vac)
3-2

3-17 Tracking Ratio
3-18 The TRACK VOLTAGE control can be used to
set a 1:1 ratio so that the voltage of the —18V supply tracks
that of the +1 8V supply within 1 % for convenience in vary¬
ing the symmetrical voltages needed by operational ampli¬
fiers and other circuits using balanced positive and negative
inputs. The TRACK control can also be used to set the
negative supply's output from a minimum of less than
0.5 volts to a maximum equal to the +18V supply's output.
Once the ratio is set, the ±18 VOLTAGE control will con¬
trol both output and maintain a constant ratio between their
voltages. To set the ratio between the —18V and +18V out¬
puts, proceed as follows:
NOTE
Any accidental movement or mechanical
vibration can vary the tracking ratio setting.
Greater accuracy in setting the controls can
be achieved by using a D VM to measure the
±18V outputs.
a. Set the 0 to +18V supply's output to the desired
value using the ±18 VOLTAGE control.
b. Set the 0 to —18V supply's output to the desired
value (equal to or lower than the magnitude of the +18V
supply's output) using the TRACK VOLTAGE control.
3-19 Overcurrent Protection
3-20 All three outputs are individually protected against
overload or short-circuit damage by separate current limit¬
ing circuits. The circuits for the +18V and —18V supplies
are factory adjusted to limit the output current to 0.255A
±1 5%. The circuit for the +6V supply is factory adjusted
to limit the output current to 1.275A ±15%. The current
limits are set by adjusting resistors R3 in the +1 8V, R13 in
the —1 8V and R23 in the +6V supply. (See paragraph 5-47
for current limit calibration procedures). No deterioration
of a supply's performance occurs if the output current
remains below the current limit setting. If a single load is
connected between the +18V and —18V outputs, the circuit
set for the lesser current limit will limit the output.
3-21 Operation Beyond Rated Output
3-22 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 ex¬
tend into the shaded areas on the meter faces without dam¬
age to the supply, but performance specifications cannot
be guaranteed.
3-23 Connecting Loads
3-24 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 satis¬
factory regulation at the load. Make each pair of connecting
wires as short as possible and twist or shield 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. The 6235A's chassis ground term¬
inal is located on the rear of the supply.
3-25 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.
3-26 Parallel Operation
3-27 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 neces¬
sary to fulfill the total current demand.
3-28 Special Operating Considerations
3-29 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.
3-30 Output Capacitance. An internal capacitor across
the output terminals of the power supply helps to supply
high-current pulses of short duration during constant-volt¬
age operation. Any capacitance added externally will im¬
prove the pulse current capability, but will decrease the
3-3

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.
3-31 Reverse Current Loading. An active load connec¬
ted to the power supply may actually deliver a reverse cur¬
rent 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 poss¬
ible 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.
3-32 Reverse Voltage Protection. Internal diodes con¬
nected 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.
3-33 Output Voltage Overshoot. During turn-on or
turn-off of ac power, output plus overshoot will not exceed
1V if the output is set for less than IV. If the control is set
for 1V or higher, there is no overshoot.
3-4