Heathkit IB-5281 - Page 41

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39

(eeguearuxrr]

standard

(Z,)

rather

than

a

calibrated

potentiometer

{as

used

in

the

Wheatstone

Bridge),

is

used

in

con-

junction

with

the

Range

switch,

SW2.

The

Range

switch

selects

the

multiplication

factor

and

a

certain

frequency

for

the

type

of

component

being

tested.

The

variable

function

is

provided

by

a

single

control,

which

is

divided

to

act

as

two

arms

of

the

bridge,

at

R1

and

R2.

Changing

both

of

these

values

on

the

bridge

provides

a

much

greater

range

than

a

single

control

would

provide.

When

you

test

an

unknown

component

value

(Zx),

the

known

component

(Zs)

must

be

the

same

type

as

the

unknown

component

(2x).

The

balance

control

at

R1

and

RZ

is

actually

matching

the

ratio

of

the

unknown

component

to

the

known

standard.

The

range

switch

marbaA

T

ma

OC

«cralaae)e

ta

cimntifts:

a

raad

is

illdg

i

ACU

in

R,

duy

ana

\

VdaiuieésS

to

Si

out.

Mmpiiry

the

read-

The

RLC

Bridge

circuit

uses

an

AC

source

rather

than

a

DC

type

since

capacitors

and

inductors

cannot

be

tested

with

DC.

The.

AC

required

to

operate

the

bridge

is

generated

by

a

Wien

bridge

oscillator.

This

oscil-

lator

consists

of

transistors

Qi

through

Q5.

It

provides

a

low

impedance

output

to

drive

bridge

transformer

T1.

Level

control

R24

adjusts

the

oscillator

output

level

to

keep

meter

M1

on

scale.

The

oscillator

output

voltage

is

rectified

by

diode

D3

and

is

used

as

a

con-

trol

voltage

at

the

gate

of

transistor

Q2.

Q2

acts

as

a

variable

source

resistance

for

transistor

Q1

and

con-

trols

its

gain.

The

oscillator

provides

three

output

frequencies,

which

are

selected

by

the

Range

switch

for

the

particular

type

of

component

being

tested.

The

frequency

used

for

each

range

is

as

follows:

RANGE

FREQUENCY

Zs

(external

standard)

1000Hz

Rx1

1000

Hz

Rx1i00

1000

Hz

Rx10k

1000

Hz

LxX1

pH

100

kHz

Lx.1

mH

10

kHz

LxX.61

H

1000

Hz

Cx1

pF

100

kHz

CXF

.0001

pF

10

kHz

Cx.01

pF

1000

Hz

PART

1

SOURCE

T

“NYT?

B

Ro

R4

Sw2

PART

2

reeo--

o-oo

1

Oz,

Ry

1

Q

AC

|

1,

°

t

”

0

S

—

-

ge,

_o)

|

|

21

9

M1

DINMTODTAT

=™

PEULVUVNIAL

/1

Meter

M1,

which

indicates

a

null

(

condition),

is

powered

by

imelifien

stages

O6

through

Q10.

DC

feedback

is

provided

via

R31

while

AC

feedback

is

provided

through

the

meter

circuit

via

capacitors

C14

and

C15.

Both

types

of

feedback

en-

hance

circuit

stabilization.

Power

is

supplied

by

two

9-volt

batteries

(see

“Intro-

duction”

for

the

type)

and/or

the

Heath

Model

IPA-

5280-1

Power

Supply.

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