Metra Electronics DAT Combo Bedienungsanleitung PDF herunterladen (Seite 67)

WIC

ammeter

for

experimenters

by James

A.

Fred

Measuring

the

current

drawn

by

an

AC-

powered

circuit

will often

pin-

point

those

obscure

defects

in

power

transformers

and

other

parts.

One

of

the

benchmarks

that

separates

the

tinkerer

from

the

serious

electronic

experi-

menter

is an

AC

ammeter.

Everyone

has

a

VOM

or

a VTVM,

but

very

few

tinkerers

ever

measure

AC

amperes.

There

are

many

times

when

the ability

to

measure

current

will

save

the

day

on a

repair

job

or an

elec-

tronic

design

project.

To keep

from

draining

the

bank account

an

ammeter

should

be

a

multi -range

job.

I

started

with a

0 -50

AC

milliammeter

simply

because

I had

acquired

one

in a trade.

You

can

usually

pick

up a good

used one

from

Bigelow

Electronics,

P.

O.

Box 71,

Bluffton,

Ohio

45817

or

buy

an inexpensive

new

one.

There

are

two

general

types

of

AC milli

-

ameters

in use

today. One

is called an

iron

vane

type

while the

other

is

simply a

DC

movement

with

a rectifier

to change

the

AC-

circuit

current

to

DC -meter

current.

The sec-

ond

is

referred

to as a rectifier

-type

ammeter.

For

the experimenters

the

inexpensive

iron -

vane

type

is

preferred

and

is

the

one used

here.

Since

the

meter

has a

full

-scale reading

of

50

ma

it

is best

to

make

it read three differ-

ent

values

of

current beginning

with

a five.

FEBRUARY-

MARCH,

1967

The

selected

ranges

were:

0 -50

AC

ma., 0-

500

AC

ma., and

0 -5 AC

amperes.

It's The

Shunt.

You can

make

any

range

current

meter

that

you

need

by following

these

directions.

To

make a

milliameter

read

higher

values

of current

it is necessary

to put

a

shunt

across

the

meter -you

put

a

lower

value

resistance

in parallel

with the

meter

-

coil

resistance.

To

make

the 0 -50

ma

meter

indicate

500

ma select

the shunt

so

that

50

ma

goes through

the

meter

and

450 ma goes

through

the

shunt. On

the 5 ampere

range

there

will be 50

ma flowing

through the

meter and

4.950

amperes through the shunt.

Internal

Resistance.

To

find the resist-

ance

of

the

shunt you

must

know the in-

ternal

resistance

of the

meter.

To find this

value connect

the

unknown

meter in series

with

a battery

and a

rheostat.

Adjust the

series rheostat

until the

meter reads

full scale.

Connect

a second

rheostat

in shunt

(across

the

meter)

and

adjust the

shunt

until the

meter

reads

half

scale. See

the

drawing for

this

circuit.

Disconnect

the

rheostat from

across

the

meter

and carefully

measure

its

resistance.

This

value of resistance

is equal

to

the

resistance

of the

meter.

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