Allied Radio Knight-Kit 83 Y 140 Manuale di montaggio - Pagina 7

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A-C

voltage measurements

can

be made

at any

frequency

from 25 cps through the audio range.

higher frequencies,

shunting capacitance

in the rec-

tifier causes readings to become increasingly low.

How

to Measure

D-C

Resistances

Caution:

Before making any resistance meas-

urements in a circuit, make certain that the power

is turned off.

It is also advisable to discharge

any capacitors in the part of the circuits in which

resistance measurements

are to be made.

Turn the Function

Switch to one of the "' 1 "

positions.

Plug the black test lead into the ''COM''

jack and the red lead into the ""V. ©..A'' jack.

Con-

nect the prod ends of these two test leads together.

Turn the '' 1. ADJ'' knob until meter reads full scale

deflection, at which the pointer lines up with zero on

the ohms scale.

Turn Function Switch to such '* (L"' position that

the meter will read somewhere in, or as near as pos-

sible

to,

the

center

portion

of the

scale

when

the

measurement is made.

Recheck if the meter deflects

full scale when the two test lead probes are connected

together.

Connect

the test prods respectively to the termi-

nals of the resistance to be measured and observe the

meter

reading.

After

any

change

of ranges,

the

"<0. ADJ'? should be rechecked.

The value of the

measured resistance can be determined by multiplying

the ohms scale meter reading by the factor indicated

by the Function Switch knob,

The following paragraph shows the d-c resistance

range

covered,

and the center-scale

reading for each

Function Switch setting.

If the Function

Switch is in "'fl X1" position,

the 0-1 K ohm range is covered; center scale reading

is 12 ohms.

Read scale directly.

If the Function

Switch is in the "'{L X100" position, the 100 ohms -

100 K ohms range is covered.

Center scale reading is

1200 ohms.

Multiply scale yeading by 100.

I£ the

Function Switch is in the "0

X10 K'' position, the

10 K - 10 Meg range is covered.

Center scale reading

is 120 K ohms.

Multiply scale reading by 10,000.

It always

should

be kept in mind

that in the

measurement

of resistance a current is made to flow

through the unknown resistance.

Usually this current

is so smal] that it can be neglected.

However on the

lowest ohms range (X1) the halfscale reading is 12

ohms.

That means

a current of 65 milliamperes is

flowing through

the unknown

reaistor.

If lower re-

sistances

than

12 ohms

are

measured,

the current

through them may be as high as 130 ma.

Therefore it

is good practice

to consider

the current

flow first

when measuring the d-c resistance of a device which

can safely pass only low currents without burning out.

For all other cases no damage will result as long as

the

ohmmeter

current

does

not

exceed

the current

rating of the unknown resistance.

How to Measure

Direct Current

Never connect the test leads across any source of

voltage directly when the VOM is used as a current

meter, otherwise the meter will be damaged.

Instead,

always

connect

the meter

in series

with the load.

Plug the black test lead into the

"'COM"' jack and the

red lead into the "V. 1. .A'' jack.

Tum

the Function Switch to any "MA" or "A''

position appropriate.

It is good practice to start with

highest range and then reset switch to obtain a con-

venient reading.

li currents in the order of 1 to 10 amperes are to

be measured, the red lead has to be plugged into the

"10 A"? jack.

Open the circuit in which the current

is to be measured.

Connect the black test lead probe

to the negative side of the circuit break, and the red

test lead probe to the positive side.

Connect the circuit under test to its power source.

The resulting meter deflection is in a direct proportion

to the unknown current.

To find the magnitude of the

current, the meter reading has to be multiplied by the

factor indicated by the Function Switch.

The listing

below gives the different ranges covered, the scale on

which the meter reading is done, and the multiplication

factor of each current range.

For

0-0.1

range

use

the ''DC

10" scale.

Divide meter scale reading by 100.

Each minor scale

division equals 0.002 ma.

For 0-10 ma range use the ""DC 10" scale.

Read

meter

scale

directly.

Each

minor

scale division

equals 0.2 ma.

For the 0-100 ma range use the 'DC

10" scale.

Multiply meter scale reading by 10.

Each minor scale

division equals 2 ma.

For

the 0-1 a range use

the "'DC

10' scale.

Divide meter scale reading by 10.

Each minor scale

division equals 0.02 a.

For the 0-10 a range use "'DC 10"? scale.

Read

scale

directly.

Each

minor

scale

division

equals

0.2 a.

If the meter pointer is deflected to the left of the

scale, the test probes are connected in wrong polarity.

Turn

off the power,

reverse

the meter

lead connec-

tions, and turn on the power again.

How to Use the VOM

as an Output Meter

Sometimes

it is necessary

to measure

an a-c

voltage

which

is superimposed

a d-c voltage.

Correct measurement

of the a-c component in such a

case is possible by use of the OUTPUT circuit of the

VOM, in which a d-c blocking capacitor is employed

to keep

direct

current

from

the meter,

which then

indicates only the a-c voltage.

To measure voltages of this nature, plug the black

test lead into the ''COM'' jack and the red test lead

into the "OUTPUT" jack. Then proceed as indicated

under

''How to Measure

A-C Voltages''.

Since the

5000 v a-c range would require a change in the red

lead

connection,

the blocking

capacitor,

necessary

for output measurement would not be connected in the

meter circuit, therefore this a-c range can not be used

for output measurements, unless d-c is kept out by an

external capacitor or isolating transformer.

When

a-c voltages

are measured

with the output

meter,

the impedance

of the d-c blocking capacitor

will

have

effect

on the accuracy

of the meter

reading.

The

error which

occurs

varies

with the

frequency of the applied voltage, and becomes negli-

gible on the 50 v scale and higher. The actual voltage

will then be higher than the measured

value.

The

higher the frequency the smaller this error will become.

How

to Measure

Output

Voltages

in DB

(Decibels)

A-C output voltages are often measured

in units

called

decibels,

which

are used to indicate power

levels in amplifiers or general telephone work.

The

DB scale (bottom meter scale) is based on the voltage

developed

across

a 500-ohm

line when 0.006 watts

are dissipated.

This voltage is a reference taken as

0 db.

Such a voltage deflects the pointer to 1.73 v