Epson L90P - TM Two-color Thermal Line Printer Manuel d'utilisateur télécharger pdf (Page 51)

CALIBRATION OF OHMMETER

X1 SCALE X100 SCALE

Resistance

Meter

Reading

Meter

Resistance Reading

102 .16 500U

.095

20 .275

1000 .165

30 .365 2000 .29

40

.44 3000 .38

50 .50

4000 .45

60 .54

5000 .51

70 .58

6000

.56

80

.62

7000 .60

90 .65 8000 .64

100 .68

10,000 .695

200 .83 20,000 .84

300

.90 30,000 .91

400 .93

40,000

.95

900

1.00

50,000 .97

HOW

IT

WORKS

The

circuit

uses a new

sewiomductur device-

the constant -current diode.

This diode

maintains

a

constant current

through an unknown

resistance

regardless

of the ohmic

value, up to

sonic

speci-

fied resistance.

Since

the voltage developed

across

the unknown resistance

is

being

measured,

no

balancing

or current

adjustment

controls are re-

quired.

The

constant- current

diode is basically

a junc-

tion

field-

effect

transistor

IilE'II

with

its gate

and

source

electrodes connected

together

inside

the

case.

The

constant

current

is accurate pro-

vided

the applied

voltage is

bet )Veen 1 and

100

volts

(depending

on the diode

selected).

There

are

32 diode types

available

with con-

stant currents

ranging

from 220

microamperes

to 4.7 milliamperes (1

\3253

through

1X3314).

'Che current value selected

determines

three other

measurement

parameters.

These are

the ohms/

volt value, the

voltage sensitivity

required

of the

meter, and the

high resistance

range

of

the test

set.

In the circuit

shown

in

Tig. 1, 1)1 is a I -nl:A

constant -current

diode.

The

ohmmeter range

could

not exceed 20,000

ohms if the diode pinch

-

off rating

was one volt, because

the drop

across

the

unknown

resistor

would then

be

20

volts. If

the

unknown

resistor

were

zero

ohms, the

full sup-

ply voltage would

be placed

across the diode.

Thus the diode must be

selected to

withstand

the

voltage

and power

dissipation

encountered

in the

operational condition.

If

the readout

meter is to indicate

ohms on a

linear

scale,

it should (for

the range selected),

have

a

resistance twenty

or

more

times

the value

it is to indicate.

The

effect

of

lead

-wire

resistance is

eliminated

by

the

use of a

four -wire arrangement.

The cur-

rent carrying

leads are

connected to the

alligator

clips at

the same points as the

voltage

measuring

leads. Thus the

voltage drop is

read

directly

across

the unknown

resistance.

April,

1969

Operation. With

switch

Si

on OFF,

connect the test clips to an unknown

re-

sistance.

If

the unknown

is 900 ohms or

less,

place SI

in

the

`,1 position and

de-

termine the resistance by using the cali-

bration table. If

the

unknown

is

above

900 ohms, use the

x100

position

of

Si.

To

use an external d.c.

meter,

be

sure

that it has

at

least 20,000 ohms per volt

and

connect it

to

J1

and

J:2

with

SI in

the EXT position. Using a

1 -mA

con-

stant- current diode for Di, divide the

meter

reading by 0.001 to get the

value

of

the unknown

resistance in

ohms.

For

example,

a 0.1

-volt indication

would

mean

100 ohms,

a

1 -volt indication, 1000 ohms,

etc.

Almost

any 0 -1

-mA meter can be used

for Ml,

provided

both

range resistors

(R1

and

R2)

are adjusted

for

correct

dial

reading.

You can use an accurate

resistor

decade box for

the

unknown re-

sistor

and adjust the

range resistors

to

get

the

proper indications. For maxi-

mum accuracy, use 1%

resistors

and

a

meter

with a comparable

tolerance.

In

any

case,

do

not turn

the

constant

-

current ohmmeter on

unless the test

leads

are

connected

to a

resistor. An open

circuit applies the

full

30 volts to

the

meter.

For very high

accuracy, a

d.c.

oscil-

loscope

or a

low

-range sensitive d.c. volt-

meter can be used

for

the external

me-

ter.

400-

0/

v

qck

HMOY

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