DATA PRECISION 938 User Manual

DATA PRECISION 938

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42-5124

MODEL

938

3.5

MAKING

MEASUREMENTS

WITH

MODEL

938

CAUTION

A

maximum

excitation

voltage

of

2.8

volts

(peak)

is

applied

to

the

capacitor

under

test.

Observe

the

polarity

conventions

for

polarized

capacitors.

3.5.1

General

Follow

the

sequence

below

when

making

measurements.

1.

Select

the

range

switch

for

the

maximum

expected

capacitance.

2.

Turn

power

ON.

3.

Check

the

zero

display.

Before

connecting

the

capacitor

under

test

(but

after

inserting

any

test

leads

or

test

fixtures),

rotate

the

front-panel

thumbwheel

for

a

zero

display.

Note

that

the

maximum

offset

that

may

be

introduced

with

the

thumbwheel

is

about

20pF,

and

that

a

negative

value

is

possible

with

the

thumbwheel

use.

4.

Insert

the

capacitor

leads

into

the

spring

contact

connec



tors

(or

into

the

test

fixture,

or

connect

alligator

clips

to

the

capacitor

leads.)

Do

not

hold

the

capacitor.

5.

Read

the

display.

The

value

is

direct

reading

in

the

electrical

units

(picofarads,

nanofarads,

or

microfarads)

indicated

at

the

selected

range

switch.

If

the

three

less

significant

digits

are

blanked

(indicating

an

Out-of-Range

measurement),

select

the

next

higher

range

scale.

If

the

display

indicates

one

or

more

leading

zero

’

s,

shift

to

the

next

lower

range

scale

to

improve

the

resolution

of

the

measurement.

3.5.2

Powering

the

Model

938

a.

The

Model

938

is

normally

powered

from

a

standard

9V

carbon-zinc

battery

of

the

transistor

radio

type.

A

Data

Precision

Battery

Eliminator,

Model

BE9,

may

be

used

to

convert

the

in



strument

to

line

operation,

and

its

use

effectively

removes

the

battery

from

the

circuit.

b.

The

Model

938

incorporates

a

battery

(or

battery

eliminator)

voltage

sense

circuit.

A

low

voltage

output

from

these

power

sources

is

indicated

in

the

display

by

LO

BAT.

Appearance

of

the

LO

BAT

indication

when

using

the

battery

eliminator

is

in



dicative

of

a

low

ac

source

voltage.

When

using

internal

battery,

a

LO

BAT

display

indicates

a

nominal

battery

output

of

less

than

7V

and

that

approximately

20%

of

the

battery

life

remains.

The

3-6

MODEL

938

OPERATION

&

RECALIBRATION

battery

should

be

replaced

(or

shift

to

use

of

a

Battery

Eliminator)

as

soon

as

possible

after

the

appearance

of

the

LO

BAT

indication.

However,

in-spec

measurements

may

still

be

made

for

several

hours

after

the

LO

BAT

appears

before

the

in



strument

becomes

inaccurate.

A

weak

or

dead

battery

should

be

removed

promptly

to

prevent

possible

damage

because

of

leakage.

3.6

RECALIBRATING

THE

MODEL

938

3.6.1

General.

The

Model

938

requires

two

adjustments

to

obtain

in-spec

perfor



mance

on

all

range

scales.

One

of

the

adjustments

is

internal

gain

trimmer

R4.

It

calibrates

the

reference

voltage

that

is

developed

by

capacitor

discharge

for

the

dual

slope

integrating

A/D

converter.

The

second

adjustment

is

a

front-panel-

accessible

zero

potentiometer

R8,

by

which

up

to

20pF

offset

may

be

introduced

to

compensate

for

interal

and

lead

stray

cpacitances.

The

design

incorporates

sufficiently

high

quality,

precise

,

and

stable

components

to

assure

overall

specified

performance

with

these

two

adjustments.

The

Model

938

is

designed

to

remain

in

calibration

for

at

least

12

months.

Recalibration,

when

required,

should

be

performed

as

described

in

the

paragraphs

that

follow.

3.6.2

Access

to

Calibrating

Adjustment

R4.

Remove

the

back

cover

of

the

multimeter

and

locate

the

access

hole

in

the

inner

shield

cover.

See

Figure

3-2,

referenced

earlier

when

installing

the

battery.

The

adjusting

potentiometer

is

mounted

on

the

pcv

board,

and

its

shaft

is

accessible

through

the

hole

that

is

identified

by

the

label

R4

printed

on

the

shield

cover.

3.6.3

Recalibrating

Procedure.

(1)

Set

up

the

meter

with

the

lead

configuration

to

be

used

in

the

recalibration.

Insert

any

test

fixture

(Kelvin

clip,

e.g.)

or

con



nect

any

test

leads

to

be

used

later,

t

Remember

that

even

very

short

leads

may

have

a

significant

effect.)

(2)

Select

the

200pF

measurement

range.

(3)

Adjust

the

front-panel

control

R8

for

000

display.

Back

off

the

thumbwheel

position

one

half

turn

from

a

negative

display

(4)

Connect

a

standard

capacitor

whose

value

is

near

full

scale

on

one

of

the

higher

ranges,

and

is

known

to

within

±0.05%.

(5)

Select

the

appropriate

range

scale.

(6)

Adjust

R4

for

a

display

equal

to

the

known

capacitance.

(7)

Remove

the

standard

capacitor.

Replace

bottom

cover.

3-7

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DATA PRECISION 938 Specifications

General

Brand	DATA PRECISION
Model	938
Category	Measuring Instruments
Language	English