HP 721A Owner's manual

721A

SERIALS

PREFIXED:

024

OPERATING

AND

SERVICING

MANUAL

PRINTED

1/61

00010-2

MODEL

721

A

POWER

SUPPLY

SERIALS

PREFIXED:

024

-

i

«

Copyright

HEWLETT-PACKARD

COMPANY

1959

1501

PAGE

MILL

ROAD,

PALO

ALTO,

CALIFORNIA,

U.S.A

0

printed

1/61

00010-2

Model

721A

SPECIFICATIONS

REGULATED

OUTPUT

VOLTAGE:

0

to

30

volts

dc,

continuously

variable

FULL

LOAD

OUTPUT

CURRENT:

150

ma

over

entire

voltage

range.

LOAD

REGULATION:

With

the

meter

monitoring

voltage,

the

change

in

output

voltage

from

no

load

to

full

load

is

less

than

0.

3%

or

30mv

whichever

is

the

greater.

LINE

REGULATION:

The

change

in

output

voltage

for

a

change

from

nominal

line

voltage

of

±10%

is

less

than

±0.

3%

or

±15

mv

whichever

is

greater.

RIPPLE

AND

NOISE:

Less

than

150

^j.v

rms.

OUTPUT

IMPEDANCE:

Less

than

0.

2

ohm

in

series

with

less

than

30

;ih

with

meter

monitoring

voltage.

METER

RANGES:

Full

scale

indications

of:

10

ma,

30

ma,

100

ma,

300

ma,

10

v

and

30

v.

OVERLOAD

PROTECTION:

Maximum

current

selected

by

switch

in

four

steps,

25

ma,

50

ma.

100

ma,

225

ma.

OUTPUT

TERMINALS:

Three

banana

jacks

spaced

3/4"

apart.

Positive

and

negative

terminals

are

isolated

from

chassis.

A

maximum

of

400

volts

may

be

connected

between

ground

and

either

output

terminal.

POWER:

115/230

volts

±10%,

50

to

60

cps,

16

watts.

WEIGHT:

Net

4

lbs.,

shipping

7

lbs.

DIMENSIONS:

7”

wide,

4-3/8”

high,

5-1/4”

deep.

Model

721A

SECTION

I

SECTION

II

SECTION

III

SECTION

IV

SECTION

V

CONTENTS

GENERAL

INFORMATION

Page

I

-

1

Manual

Content.

!

"

|

1-2

General

Description.

1.-3

Inspection.

1-4

Power

Cable.

r

1

-

5

230

Volt

Operation.

1

"

4

OPERATING

INSTRUCTIONS

2-1

Operating

Controls

..

2-2

Meter

Range

Switch.

2-3

Short

Circuit

Current

Switch

.

2-4

Series

Operation

of

Supplies

.

2-5

Parallel

Operation

of

Supplies

THEORY

OF

OPERATION

.Ill

-

1

3-1

3

-

2

.

...

Ill

-

1

........

Ill

-

1

O

"

D

.Ill

-

1

3-4

.Ill

-

1

3

•“

5

.Ill

-

2

3-0

3

-

7

Frequency

Response

Control.

.Ill

-

2

MAINTENANCE

4-1

Contents.

4-2

General

Maintenance

Information.

4-3

Trouble

Localization.

4-4

Checking

Voltage

Regulation

and

Ripple

.

4-5

Measuring

AC

Internal

Impedance.

4-6

Meter

Calibration.

4-7

Setting

Maximum

Output

Voltage.

4-8

Calibrating

the

Short

Circuit

Current

Circuit

4-9

Replacing

the

Power

Transistor.

4-10

Replacing

Diodes

CR5,

CR6

and

CR7.

TABLE

OF

REPLACEABLE

PARTS

5-1

Table

of

Replaceable

Parts

IV

-

1

IV

-

1

IV

-

1

IV

-

3

IV

-

3

IV

-

3

IV

-

5

IV

-5

IV

-5

IV

-5

V

-

1

Model

721A

Sect.

I

Page

1

SECTION

I

GENERAL

INFORMATION

1-1

MANUAL

CONTENT

The

material

for

this

instruction

manual

is

written

in

five

sections:

Section

I

contains

material

of

a

general

nature.

Section

II

explains

how

to

operate

the

power

supply.

Section

III

explains

how

the

circuit

operates.

Section

IV

covers

maintenance

and

trouble

shooting

procedures.

Section

V

is

a

table

of

replaceable

parts.

1-2

GENERAL

DESCRIPTION

The

Model

721A

Power

Supply

produces

a

dc

regulated

voltage

adjustable

from

0

to

30

volts.

The

supply

makes

load

circuit

performance

inde¬

pendent

of

external

power

supply

influences.

The

supply

has

very

low

source

impedance

and

ex¬

cellent

regulation

against

change

in

line

and/or

load.

This

supply

is

especially

useful

as

a

source

of

power

for

transistor

circuits.

A

circuit

is

provided

which

electronically

limits

the

maximum

output

cur¬

rent

that

can

be

supplied

to

four

nominal

values

selected

by

a

front

panel

switch.

This

feature

helps

prevent

the

accidental

destruction

of

an

expensive

transistor

should

an

accident

occur

that

would

nor¬

mally

allow

excessive

current

to

flow

through

it.

The

SHORT

CIRCUIT

CURRENT

switch

can

be

set

to

the

value

which

is

closest

above

the

normal

operating

current.

The

supply

will

automatically

limit

the

peak

current

flow

to

this

nominal

value

regardless

of

the

load

resistance.

Built-In

Metering

A

built-in

meter

allows

either

output

voltage

or

current

to

be

monitored

as

selected

by

the

METER

RANGE

switch.

Isolated

Output

The

power

supply

has

both

output

terminals

in¬

sulated

from

chassis

ground.

Either

terminal

may

be

grounded

or

a

number

of

supplies

may

be

con¬

nected

in

series

to

obtain

higher

voltages.

Insula¬

tion

is

such

that

the

supply

may

be

operated

as

high

as

400

volts

off

of

ground.

Parallel

Operation

Parallel

operation

of

two

or

more

supplies

is

pos¬

sible

due

to

the

unique

electronic

current

limiting

switch.

The

supplies

will

each

contribute

only

the

number

of

milliamperes

selected

by

the

SHORT

CIRCUIT

CURRENT

switch.

The

individual

supplies

may

be

loaded

to

approximately

225

ma

with

some

reduction

in

ripple

and

regulation

characteristics.

Reliability

The

Model

72XA

Power

Supply

is

very

compact,

and

has

low

internal

losses,

which

are

made

possible

by

fully

transistorized

circuitry.

The

trouble

free

characteristics

of

transistors

together

with

the

use

of

high

quality

components

throughout,

will

result

in

a

minimum

of

maintenance.

1-3

INSPECTION

When

the

Model

721A

is

received,

inspect

it

for

damage

received

in

transit.

Operate

the

instrument

to

make

certain

that

it

is

functioning

satisfactorily.

If

damage

is

evident,

follow

the

procedures

outlined

in

the

“CLAIM

FOR

DAMAGE

IN

SHIPMENT”

page

of

this

manual.

1-4

POWER

CABLE

The

power

cable

consists

of

three

conductors

and

is

terminated

in

a

three-prong

male

connector

rec¬

ommended

by

the

National

Electrical

Manufac-

Sect,

I

Page

2

00010-2

Model

72iA

turers’

Association.

The

third

contact

is

an

offset

round

added

to

a

standard

two-blade

connector

which

grounds

the

instrument

chassis

when

used

with

an

appropriate

receptacle.

To

use

this

NEMA

connector

in

a

two-contact

receptacle,

a

three-

prong

to

two-prong

adapter

should

be

used.

When

the

adapter

is

used,

the

third

contact

is

terminated

in

a

short

lead

from

the

adapter

which

can

be

con¬

nected

to

the

outlet

mounting

box

in

order

to

ground

the

instrument

cabinet.

1-5

230

VOLT

OPERATION

This

instrument

may

be

easily

converted

from

115

to

230

volt

operation

by

removing

two

jumpers

and

installing

one

jumper.

This

changes

the

dual

115

volt

primary

windings

from

a

parallel

to

a

series

connection.

Refer

to

the

schematic

diagram

and

Figure

4-6

for

details.

The

main

fuse

(FI)

should

be

changed

(see

Table

of

Replaceable

Parts

for

the

correct

type

of

fuse).

Model

721A

Sect.

II

Page

1

SECTION

II

OPERATING

INSTRUCTIONS

2-1

OPERATING

CONTROLS

Figure

2-1

shows

the

function

o'Peach

of

the

controls

and

is

normally

self

explanatory.

There

are

a

few

additional

facts

to

be

considered

which

may

be

im¬

portant

in

some

applications.

2-2

METER

RANGE

SWITCH

This

switch

connects

precision

internal

resistors

into

the

meter

circuit

to

obtain

the

various

voltage

and

current

ranges.

When

measuring

current,

the

meter

shunt

resistor

is

in

series

with

the

output

terminals.

The

meter

shunt

resistance

adds

to

the

source

impedance

of

the

supply

which

is

normally

less

than

0.2

ohm

in

series

with

less

than

30p,h.

The

Table

2-1

lists

the

additional

resistance

for

each

current

range.

Where

minimum

source

im¬

pedance

is

important,

the

METER

RANGE

switch

should

be

left

on

one

of

the

voltage

ranges

except

when

actually

measuring

load

current.

An

accidental

short

circuit

can

damage

the

meter

if

it

is

on

one

of

the

lower

ranges

and

the

SHORT

CIRCUIT

CUR¬

RENT

switch

is

set

to

a

high

value.

Short

time

overloads

of

two

times

full

scale

will

not

damage

the

meter

movement.

2-3

SHORT

CIRCUIT

CURRENT

SWITCH

This

switch

controls

a

circuit

which

adjusts

the

peak

current

output

capability

of

the

supply.

The

cali¬

bration

is

nominal.

The

actual

value

may

be

read

by

shorting

the

supply

and

reading

the

value

on

the

monitor

meter.

The

clipping

action

is

gradual.

Consideration

should

be

given

to

this

characteristic

if

pulse

type

circuits

are

being

supplied.

The

average

current

may

be

within

the

supply

rating

(150

ma)

but

peak

currents

may

be

high

enough

to

cause

the

supply

to

clip.

If

the

switch

is

set

to

a

low

peak

value

this

situation

can

occur

at

low

average

current

levels.

When

pulse

circuits

are

being

supplied,

this

switch

must

be

set

to

a

value

which

is

greater

than

the

peak

cur¬

rent

requirements

of

the

circuit.

The

output

circuit

has

24

jif

capacity

shunting

it,

which

helps

supply

high

current

peaks,.providing

they

are

of

extremely

short

duration.

The

value

of

any

external

capacity

added

will

improve

the

peak

current

capability

but

will

decrease

the

safety

provided

by

the

SHORT

CIRCUIT

CURRENT

switch,

since

the

external

capacity

will

provide

high

surge

currents.

The

surge

currents

may

destroy

external

components

before

the

average

current

increases

sufficiently

inside

the

supply

to

cause

the

limiting

circuits

to

operate.

TABLE

2-1.

ADDITIONAL

INTERNAL

RESISTANCE

Monitor

Meter

Current

Added

Internal

Range

(f.s.)

Resistance

10

ma

10

ohms

30

ma

3.33

ohms

100

ma

1

ohm

300

ma

0.33

ohm

2-4

SERIES

OPERATION

OF

SUPPLIES

When

operating

the

Model

721A

at

a

voltage

more

than

a

few

volts

off

of

ground

be

careful

not

to

accidentally

short

circuit

the

external

circuits

so

the

Model

721A

is

subjected

to

voltages

of

reverse

polarity

or

high

voltages

of

the

same

polarity.

To

do

so

will

instantly

destroy

the

electrolytic

capa¬

citors

in

the

power

supply

and

possibly

the

transis¬

tors.

When

a

number

of

supplies

are

operated

in

series,

be

sure

the

SHORT

CIRCUIT

CURRENT

switch

on

each

supply

is

set

to

the

same

(or

higher)

value

than

maximum

peak

current

desired.

Sect.

II

Page

2

Model

721A

METER

range

SHORT

CIRCUIT

CURRENT

VOLTAGE.

ADJUST.

METER

READS

OUTPUT

VOLTS

OR

CURRENT

AS

SELECTED

BY

METER

RANGE

SWITCH

SELECT

MAXIMUM

CURRENT

OUTPUT

SELECT

METER

FUNC¬

TION

AND

RANGE

Note:

Meter

shunt

resistance

is

in

series

with

load

on

ma.

Leave

on

volts

for

lowest

(rated)

internal

impedance.

POWER

Output

increases

with

clockwise

rotation

CONNECT

LOAD

TO

(+)

AND

(-)

TERMINALS

Either

terminal

may

be

connected

to

grounded

cabinet

terminal.

(+)

and

(-)

terminals

may

be

operated

up

to

±400

volts

dc

from

cabinet

ground

ON/OFF

ADJUST

OUTPUT

VOLT¬

AGE

OR

CURRENT

CABINET

GROUND

CAUTION

DO

NOT

SUBJECT

POWER

SUPPLY

OUTPUT

TERMINALS

TO

A

VOLTAGE

SOURCE

OF

REVERSE

POLARITY.

TO

DO

SO

WILL

DE¬

STROY

A

TANTALUM

CAPACITOR

CONNEC¬

TED

ACROSS

THESE

TERMINALS

MP

-S-445

Figure

2-1

model

721A

Power

Supply

Operating

Controls

Model

7

21A

Sect.n

Page

3

2-5

PARALLEL

OPERATION

QF

SUPPLIES

Two

supplies

may

be

operated

in

parallel

to

supply

loads

in

excess

of

150

ma.

Set

the

SHORT

CIR¬

CUIT

CURRENT

switch

of

the

first

unit

to

maximum

(225

ma).

The

first

supply

becomes

a

225

ma

con¬

stant

current

source.

The

supply

will

furnish

up

to

this

amount

of

current

without

harm,

however

the

regulation

and

ripple

specifications

can

no

longer

be

guaranteed.

Adjust

the

voltage

of"

the

second

supply

to

be

the

same

as

the

first

unit

before

con¬

necting

them

together.

The

second

supply

can

be

made

to

share

load

by

advancing

the

VOLTAGE

ADJUST

control

slightly

clockwise.

The

second

supply

will

furnish

the

regulation

action

up

to

the

limit

of

its

capacity

(150

ma).

The

second

supply

may

be

loaded

beyond

the

150

ma

point

up

to

a

maximum

of

225

ma,

making

a

total

of

450

ma

available.

The

usefulness

of

this

extra

out¬

put

capacity

depends

on

the

performance

required

from

the

supplies.

The

supplies

become

constant

current

sources

when

the

SHORT

CIRCUIT

CUR¬

RENT

limiting

circuit

is

operating.

At

this

point

the

ripple,

source

impedance

and

voltage

regulation

specifications

cannot

apply.

Operation

of

more

than

two

instruments

in

parallel

is

not

recommended

as

the

total

current

capacity

becomes

greater

and

accidental

gross

misadjusted

controls

may

result

in

instability.

THEORY

OF

OPERATION

3-1

GENERAL

CIRCUIT

DESCRIPTION

The

regulation

is

accomplished

in

a

manner

which

is

similar

to

vacuum

tube

type

circuits

.

A

power

type

transistor

in

series

with

the

rectified

output

and

the

load,

acts

like

a

variable

resistor

which

maintains

a

constant

output

voltage

or

current

as

selected

by

the

controls.

The

power

transistor

is

controlled

by

a

two

stage

amplifier

which

amplifies

any

changes

in

the

relative

amplitudes

of

the

output

voltage

and

the

reference

voltage.

The

electronic

short

circuit

current

limiting

switch

is

a

unique

feature

which

is

not

normally

found

in

the

vacuum

tube

counterpart

of

the

supply.

This

circuit

senses

any

increase

in

current

above

a

pre-selected

value

and

in

turn

controls

the

conduction

of

the

power

transistor

to

limit

the

peak

current

to

the

pre-selected

value.

A

detailed

description

of

the

Power

Supply

and

its

operation

follows.

Refer

to

the

schematic

diagram

to

identify

the

various

com¬

ponents.

3-2

MAIN

AND

AUXILIARY

SUPPLY

DESCRIPTION

Transformer

T1

supplies

ac

voltages

to

the

main

and

auxiliary

supplies.

The

main

supply

consists

of

silicon

rectifiers

CR1

and

CR2

and

capacitor

C2.

This

supply

furnishes

about

43

volts

to

the

regulator

circuit.

Silicon

rectifier

CR3,

CR4

and

capacitors

C3,

C4

and

C5

supply

-20

volts,

which

is

required

for

operation

of

the

control

circuits.

Regulator

transistor

Q1

acts

as

a

variable

series

resistance

to

lower

the

voltage

to

the

desired

value,

as

set

by

the

front

panel

VOLTAGE

ADJUST

control

R19.

Q1

conducts

more

current

when

the

base

volt¬

age

goes

more

negative

with

respect

to

the

emitter.

3-3

REFERENCE

VOLTAGE

CR7

is

a

reversed

biased

diode

operating

in

the

break-down

condition.

The

diode

maintains

a

con¬

stant

nominal

7

volts

across

itself,

establishing

a

constant

reference

voltage

between

the

negative

out¬

put

lead

and

the

base

of

Q4.

Q4

is

an

emitter

follower

which

repeats

the

reference

voltage

at

its

emitter

terminal,

less

a

constant

internal

base-

emitter

drop

of

about

0.2

volt.

The

voltage

at

the

emitter

has

a

low

source

impedance,

making

it

insensitive

to

normal

variations

in

current

flow.

The

output

voltage

from

the

(+)

output

bus

is

sampled

by

VOLTAGE

ADJUST

control

R19

which

causes

a

current

flow

through

R20

and

R21.

The

regulator

action

maintains

a

constant

current

through

R19

and

R20.

3-4

REGULATION

CYCLE

DESCRIPTION

Assume

the

output

level

has

been

set

with

R19

and

some

change

has

occurred

which

causes

the

output

voltage

to

rise.

The

voltage

at

the

base

of

Q3

is

that

which

would

appear

across

a

forward

biased

diode

and

is

essentially

constant.

The

electron

current

flow

through

R20

is

constant.

When

the

output

voltage

rises,

part

of

the

normal

electron

flow

into

the

base

of

Q3

is

diverted

through

R19.

The

reduced

base-emitter

electron

current

of

Q3

reduces

the

collector-emitter

electron

current

flow

from

R17

by

a

factor

of

approximately

100.

Since

fewer

electrons

flow

into

the

collector

of

Q3

from

R17

and

thte

-

16

volt

bus,

the

voltage

at

R16

goes

in

a

negative

direction.

This

causes

more

electrons

to

flow

through

R16

into

the

base

of

Q2.

Increased

Q2

base

to

emitter

current

causes

much

higher

collector-emitter

current.

Increased

Q2

collector

current

raises

the

voltage

at

R5

which

reduces

the

base

to

emitter

current

of

Ql.

The

reduced

base-emitter

current

of

Ql

increases

its

collector-emitter

resistance

to

electron

current

flow,

hence

increases

its

collector

to

emitter

volt¬

age

drop.

That

voltage

drop

increases

just

enough

to

compensate

for

the

initial

output

voltage

rise,

maintaining

the

output

voltage

at

a

constant

level.

3-5

SHORT

CIRCUIT

CURRENT

LIMITING

CIRCUIT

The

current

flow

to

the

load

is

sensed

by

a

voltage

drop

across

Rll

ABCD.

Silicon

diode

CR5

is

HP 721A Owner's manual

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