Epson L90P - TM Two-color Thermal Line Printer Manual de usuario descargar pdf (Pagina 49)

THE UBIQUITOUS VOM

one

of the

handiest pieces of test equipment

available to

the

electronics

experiment-

er. Although useful in a thousand differ-

ent ways, there are

times when a

VOM

can

be the

cause of damage to

the equipment

being tested -by

applying excessive

cur-

rent

low-

resistance devices,

for in-

stance.

This means

that you can't use

conventional

VOM to test D'Arsonval

meter

movements,

meter

fuses,

or tran-

sistors, to

name

few items

that

are cur-

rent sensitive.

Another

limitation on the use of the

VOM,

the

poor

accuracy obtainable on

the

lowest

resistance range

( usually

Rx1). The

VOM

range selector

switch,

battery

clips,

and

test lead

terminations

often

become

slightly

resistive

with time

and

use

and

interfere with the readings

for very

low

resistances. Of

course you

can

clean

clips and

lead

ends but

it's

April, 1969

Constant-

Current

Ohmmeter

BY ALVIN

B. KAUFMAN

Build

unusual

test

equtpleHt

project

little difficult

get

the

contacts

the selector

switch.

The constant

-current

ohmmeter

de-

scribed

here

eliminates

these problems

and, in addition,

does

not

require a

zero

adjustment

for

resistance

measurements.

Although

this

new ohmmeter

has

its

own

meter, an

external

d.c.

voltmeter

can be

used

if desired.

Construction.

The

author

built

his me-

ter

a conventional

í" x

4" x

metal case with

a sloping

front, although

any other approach

can be used.

The

two

external

meter jacks (J1 and

J2) , switch

Si, and

meter

Ml are mounted

the

front panel.

The two

pairs of testlead

jacks

-J4

and J5 -J6)

are mounted on

the

top. The battery

is secured

in the case

by a

mounting clip.

The circuit, shown

in Fig. 1,

is wired

point -to- point.

When

making the

connec-

1 2 ... 44 45 46 47 48 49 50 51 52 53 54 ... 115 116

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Epson L90P - TM Two-color Thermal Line Printer Manual de usuario Pagina 49

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