Metra Electronics DAT Combo Manuel d'utilisateur télécharger pdf (Page 68)

gmammeter

Shunt -Resistance

Value.

Using the

fol-

lowing

formula

with

the

meter

resistance just

found

you

can determine

the values

of the

shunt

resistors.

R n:

R

=(n

-1)

In this

formula

R equals

the shunt resist-

ance.

Rn:

is

the meter resistance

and n is

the

scale multiplication

factor. For

example let's

convert

the 0 -50

ma meter

to read

0 -5 am-

peres.

The

scale

multiplying

factor

is 100.

If

the meter

resistance

is 100

ohms then:

R = (100

-1)

= 1.01

ohms.

The 1.01

-ohm

shunt will

have

to

carry

4.950

amperes

at the full

scale reading.

By

using

the power

equation

P=1R

we find

that we

need a 27

-watt resistor.

A 50-

watt

adjustable

resistor will

work

nicely

here -if

you

actually intend

to

measure

5

amperes. (1f

all

your

work will

be

in

the 2

ampere

neighborhood

a 25 -watt, 1 -ohm

re-

sistor

will be

adequate.) If the

shunt

resist-

SHUNT

RHEOSTAT

SERIES

RHEOSTAT

íIll

To find internal

resistance

of meter set series

rheostat for

full

scale on M1 then

connect

shunt rheostat

and set

it for

a half -scale in-

dication

on Ml. Next measure resistance

of

shunt rheostat -it

equals meter resistance.

Schematic diagram below is

a practical multi -

range

ammeter -circuit is for

AC or DC meter.

METER FUSE

ance

figures

out

to less

than

one ohm it

may

be

necessary

to make

it

from

copper

or

nichrome

wire.

Remember

it will be

neces-

sary

to multiply

the

scale readings

by 100

when

using

this

shunt.

You can

use the

same

formula

to figure

other values

of

shunts.

Calibration.

When

you

get

ready to

check

the

calibration

of your

meter

it would

be

wise

to

have

another

meter

to use

as

a

standard.

Connect

the

standard

ammeter,

your meter,

and

a load

of

the proper

size in

series.

(See

the

circuit

diagram

for

this

step.)

Adjust

the load

for

2.5

amperes

through

the

standard

meter

and

adjust

the

shunt's

re-

sistance

until

your

meter

reads

2.5

amperes.

Recheck

the

meter

with

a 5-

ampere

load.

A

four

-position

single

-pole

switch

is wired,

as

shown,

to

select

the

different

meter

ranges.

A

meter

short

-circuit

position

is

included

be-

cause

it is

good

practice

to

always

short

out

the meter

until

you

are sure

of your

circuit.

A phenolic

board

is used

to

mount

the

switch

and resistor.

The

meter

is

mounted

first

in

the

aluminum

box

and

then

the phenolic

board

is

mounted

to

the

meter

by

the meter

studs.

The

photographs

show

how

everything

goes

together.

Easier

Testing.

Not

only

is

an ammeter

useful

to

an

electronics

experimenter,

but

many

radio

-TV

repairmen

are

finding

set

* SEE TEXT

-74

PARTS

LIST

FOR

AC

AMMETER

J1,

J2 -5

-way

binding

posts;

1 red,

1

black

(Lafayette

99C6233

or

equiv.)

J3,

J4-

banana

jack

(to

attach

ammeter

to

adapter

-optional)

M1 -AC

milliammeter

(see

text)

R1,

R2-

wirewound

shunt

resistors

(see

text)

51-

Selector

switch

(Mallory

13131;

Allied

56A4253

or equiv.)

1-

Chassis

box,

5

x

4 x

3 -in.

(Bud

CU2105A;

Allied

42D7621

or

equiv.)

PARTS

LIST

FOR

AMMETER

ADAPTER

F1

-Fuse

of

proper

rating

for

range

in

use

(see

text)

J5,

J6 -5

-way

binding

posts;

1 red,

1

black

(Lafayette

99C6233

or equiv.)

J7-

Chassis

-mount

AC

receptacle

( Amphenol

61 -F;

Allied

40H677

or

equiv.)

P1, P2-

Banana

plug

(to

attach

adapter

to

am-

meter-

optional)

P3-

Chassis

-mount

AC

plug

(Amphenol

61 -M;

Allied

40H675

or equiv.)

I-

Recessed

steel

shell

for

P3

(Amphenol

61-

61; Allied

40H086

or equiv.)

1-

Chassis

box, 4

x

2 x

23/4

-in.

(Bud

CU-

2115A;

Allied

42D7631

or

equiv.)

Misc.-

machine

screws, nuts,

wire,

solder,

phenolic

board,

wire,

etc.

Estimated

cost: $14.00

Construction time:

2 hours

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