TI
R3
R4 R5
TOOK
470 IM
Fig. 1. Basic circuit
ends at
J1
and
J¿.
The
dread to the
right is
used
to test tran.sistO)'s.
PARTS LIST
J1
to
J5-
Banana or tip jack (J3, J4, J5
optional
-see text)
Pl to P5- Banana or tip plug to match
jacks (P3, P4, P5- optional -see text)
RI,R2-
4700-
ohm,1/2-watt, I%
resistor
R3- 100,000 -ohm, 1/2-watt, l% resistor
R4-
470,000
-ohm, 1/2-watt, I% resistor
R5- l- megohm,
1/2
-watt, l% resistor
reverse
biased and will seem to be an
open circuit.
Hence, the trace
ob-
served on
the
scope's CRT
will form
a
right
angle with
one
leg horizontal
and
the
other
vertical. The intersection
of
the two
legs
will be
centered on
the
screen.
Few things in nature
are perfect, the
diode is no
exception.
A
silicon
diode
requires
about 0.6 volt forward bias
before it
begins
to
conduct,
ger-
manium about
0.2
volt. Illustrated in
Fig.
2A are the differences in forward
conduction voltage
as "offsets"
in
ex-
panded horizontal
traces.
Hence,
the
tester
can
identify
a
diode under test
as
either silicon or
germanium.
If you
connect a
5 -volt
zener diode
between
the
test probes connected to
J1 and J2,
the resultant
waveform will
appear as
shown
in
Fig. 2B. Note that
the zener's impedance
and break-
down voltage
can be obtained
from
an
expanded horizontal trace. The zener
impedance is
equal
to the
change
in
voltage
divided
by
the
change
in
cur-
rent
within the linear
breakdown re-
gion
of the trace.
Snap-
recovery
and
tunnel
diodes also produce their
own
characteristic traces.
In
testing
transistors,
it is useful
to
visualize them as
variable
resistors
whose emitter
-to-
collector
resist-
ances are controlled
by a
minute base
current.
Within
a transistor's
operat-
ing range, the collector current
will
approximately
double
if base current
is doubled.
The
"resistance"
of
the
transistor,
as
"seen"
by
its associated
circuit components,
will decrease
by a
factor
of
approximately
two.
To use the tester
as a
transistor
checker,
the SO/ portion of
the circuit
60
*OPTIONAL
(SEE
TEXT)
r
TO
SCOPE
HORIZ.
r TO
SCOPE GND
SI-Normally-open
pushbutton switch
S2-
4- position, nonshorting
rotary switch
SOI -
Transistor socket (chassis
mount)
TI-
12.6
-volt filament transformer
Misc. -Line cord
with plug; flexible test -
lead cable; control knob;
test probes (2);
jacks for scope
input
(3); case (such
as
Radio Shack No. 270B098 -61/4" x 33/4"
x
2 "); lettering kit; machine hardware;
rubber grommets; hookup
wire; solder;
etc.
(A)
SLOPE DUE TO
LEAKAGE
CURRENT
WITH BASE
OPEN
BASE TO
EMITTER
REVERSE
/VOLTAGE
BREAKDOWN
FORWARD
VOLTAGE
BREAKDOWN
(C)
trace will be displayed as shown
in Fig.
2D.
For most transistors, ß can
be de-
termined by rotating S2
until the trace
is
close
to 45'. For the component
val-
ues shown
in Fig.1,
and
a trace slope
of
450, the
ß
is
approximately
21
in
position
2
of S2,
approximately
100
in
position 3,
and approximately
213
in
position
4.
Other Uses.
If you
connect
a good
capacitor
across
the test leads
con-
nected to
J1
and
J2,
an
elliptical trace
will be displayed.
This trace will
result
because of
the phase shift at 60
Hz
between
the vertical and
horizontal
inputs
of
the scope due
to the RC net-
work. A trace shape approaching
a
circle
is
obtained
when the
capacitive
value under test
is
approximately 0.5
µF. Inductors will produce
similar
ZENER
(.-
BREAKDOWN
VOLTAGE
(
(B)
in Fig. 1 must be
plugged
into
J1
and
J2
or the separate
test leads
must
be
plugged into the
jacks.
Assuming the
former,
if
a transistor
is
plugged
into
SO/ and S2
is
set
to
position
1, the
trace
observed
should be
a
continu-
ous
horizontal line.
(With
no
base cur-
rent, the transistor
should
have an
in-
finite
emitter
-to-
collector
resistance.)
The trace will be
off
the
horizontal
only
if there is leakage current within
the
transistor or
the transistor's break-
down voltage
is
exceeded.
These
traces are depicted
in Fig. 2C.
To
perform
a beta (ß)
measurement
of a
transistor, start with
no transistor
plugged
into
SO
/. Depress S/ and
ad-
just the scope's controls
for
a
45°
slope
to the trace.
Insert the transistor
in the test
socket,
observing
the
proper
lead
basing,
and set
S2
to posi-
tion 3.
If the transistor's ß
is
100, a
45"
(D)
Fig. ¿. Typical
scope
traces:
(A)
ide ii tifïcatiou of
silicon
or
germ niinni .janctiurt;
(B)
zener diode
curve; (C)
transistor
with
leakage,
forward
breakdown voltage,
reverse breakdown;
(D)
transistor
with a gain
a /'100.
traces, and
the
tester can also be used
to match
inductances
of
home -made
toroidal transformer coils.
Since
the
current
through
the
probes
is limited to less than 4 mA, the
tester
can be
used to check
most
in-
circuit components.
Connect
the test
leads
across
an
in-
circuit semicon-
ductor, and the
resultant trace shape
and slope
will be determined
by
the
various resistances and
reactances
associated
with the semiconductor.
Although
the waveform
displayed can
be greatly distorted,
you can check
junction operation
by looking
for the
telltale
junction
discontinuity
that
is
typical at
the zero axis.
Not
only
can
you check
semiconductor
junctions,
you can
also check
the qualities of
other componants
such
as
resistors,
capacitors,
and
inductors that are
common
to the transistor stage.
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