This is only a preview of the September 1993 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Stereo Preamplifier With IR Remote Control; Pt.1":
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Do you have a boxful of unknown
transistors or a transistor circuit
that’s not working properly? This
simple tester will indicate whether
a transistor is working or not & tell
you whether it is an NPN or PNP
type.
By DARREN YATES
Build an in-circuit
transistor tester
I
F YOU’VE built a few projects,
then the odds are that you’ll have
a fair collection of transistors in
your junkbox. You will probably have
a good range of types as well, ranging from small signal to high power
devices.
After a while, it’s easy to forget
which ones are good and which are
suspect. And if you’ve bought one of
the semiconduc
tor “grab bags” that
some retailers offer, you’ll undoubtedly have trouble determining which are
NPN and which are PNP types –unless,
of course, you have the appropriate
data books.
That’s where this simple Transistor
Tester comes in handy. It can test both
56 Silicon Chip
small signal and power transistors
and will indicate whether the device
is an NPN or PNP type. Basically, it
tells you whether a device is “go” or
“no-go” and can indicate the nature
of a fault – it cannot determine the
lead configuration or tell you anything
about the gain.
In addition, the project can be used
to test transistors that are already in
circuit. So if you have an AM radio, an
amplifier or some other device that’s
not working, this project will prove
invaluable for troubleshooting. You
don’t even have to bother pulling the
transistors out of circuit to test them.
The test results are indicated by two
LEDs mounted side-by-side on the
front panel. If nothing is connected to
the test leads, both LEDs flash rapidly.
However, if a working device is connected, then one of the LEDs will go
out, depending on whether the device
is an NPN or a PNP type.
If the transistor is faulty, the result
will depend on the nature of the
fault. Both LEDs will flash if there is
a base-emitter short, while both LEDs
will go out if there is a short between
collector and emitter. A chart on the
front panel shows what the results
mean.
Circuit diagram
Let’s now take a look at the circuit
diagram - see Fig.1. It’s based on tran-
S1
1k
C
100k
1
16VW
100k
1
16VW
Q1
BC548
B
signals on the collectors of these
two transistors are complementary, their voltage levels will
be out-of-phase; ie, when one
is high, the other is low. This
causes both LEDs to flash alternately when power is applied,
provided no TUT is connected.
1k
9V
Q2
BC548
B
C
B
E
E
E
C
VIEWED FROM
ABOVE
LED1
A K
NPN test transistor
Let’s now see what happens
when
we connect a working
K
A
K
NPN
transistor
as the TUT.
A
There
are
two
conditions
to
D1
D3
consider. The first is when
4x1N4148
Q1’s collector is low and Q2’s
1k
D2
D4
collector is high. In this case,
the NPN TUT is biased on and
so current flows through D3, D4
and the collector-emitter juncE
B
C
tion of the TUT. This means that
TO T.U.T
there will be about 1.2V across
IN-CIRCUIT TRANSISTOR TESTER
D3 and D4, which is too low to
Fig.1: transistors Q1 & Q2 form a 5Hz
keep LED 2 on.
multivibrator which alternately switches
Thus, LED 2 will go out when
the collector & emitter terminals of the
the test transistor is conducting.
TUT high & low. If the device is good, one
LED 1 will also be off during
of the LEDs will alternately flash on & off.
this time, since it will be reverse
biased.
sistors Q1 and Q2 which are wired to
Now let’s consider what happens
operate as a standard astable multi when Q1’s collector goes high and
vibrator. The frequency of oscillation Q2’s collector goes low. In this case,
is set to about 5Hz by the associated the TUT will be biased off and so LED
100kΩ resistors and 1µF capacitors.
1 will be on. At the same time, LED
As a result, a 5Hz square-wave is 2 will be reverse biased and so will
produced at Q1’s collector while a sec- remain off.
ond 5Hz waveform of opposite phase
Thus, if a working NPN transistor
appears at Q2’s collector. Q1 drives is used as the TUT, LED 1 will flash
the emitter of the transistor under test on and off at a 5Hz rate, while LED 2
(TUT), while Q2 drives the base of the will be off at all times.
TUT via a 1kΩ resistor. The collector
of the TUT is driven via diode array PNP test transistor
D1-D4.
For a working PNP transistor, the
Note that these are universal inputs; opposite occurs. When Q1’s collector
ie, both NPN and PNP devices connect is low and Q2’s collector is high, the
to the same EBC test points without TUT will be biased off and LED 2 will
any need for switching.
light. LED 1 will be reverse biased
The two LEDs are connected in re- during this time and will be off.
verse-parallel between the collectors
When the collectors subsequently
of Q1 and Q2. Because the 5Hz output change state, the TUT will be biased
LED2
S1
1k
1uF
1k
K
Q2
D4
LED2
D2
C
100k
1k
100k
1uF
TO 9V
BATTERY
Q1
LED1
A
D1
D3
TO
B TEST
CLIPS
E
Fig.2: install the
parts on the PC
board as shown
here. The LEDs
are mounted
about 15mm
proud of the
board & clip into
two bezels on the
front panel.
PARTS LIST
1 plastic case, 82 x 54 x 30mm
1 PC board, code 04109931, 51
x 37mm
1 self-adhesive front panel label,
49 x 79mm
1 SPDT toggle switch (S1)
1 9V battery
1 9V battery clip lead
2 LED bezels
1 150mm length of black hookup wire
1 150mm length of yellow hookup wire
1 150mm length of blue hook-up
wire
3 small hook clips
Semiconductors
2 BC548 NPN transistors (Q1,Q2)
2 5mm green LEDs (LED1,LED2)
4 1N4148, 1N914 diodes (D1-D4)
Capacitors
2 1µF 16VW PC electrolytic
Resistors (0.25W, 1%)
2 100kΩ
3 1kΩ
on and current will flow through the
transistor, this time via diodes D1 and
D2. LED 2 will now be biased off, while
LED 1 will remain off due to the low
voltage across it. This voltage will be
equal to the voltage across the two
diodes plus the saturation voltage of
the transistor (ie, a little over 1.2V).
Thus, when a good PNP device is
used as the TUT, LED 1 goes out and
LED 2 flashes.
Crook devices
What if you connect a TUT with a
collector-emitter short? Regardless of
whether it’s an NPN or a PNP device,
neither LED will light because the
current will alternately flow through
each of the series diode pair. This
means that only about 1.2V will be
developed across the LEDs, which is
insufficient to turn them on.
If the base-emitter junction of the
TUT is shorted, then the transistor
will be unable to turn on and current
will flow through the 1kΩ base resistor.
Both LEDs will continue to flash since
the voltage developed across this 1kΩ
resistor is sufficient to allow them to
operate.
September 1993 57
C B E
+
+
TRANSISTOR TESTER
+
+
NPN
PNP
CE SHORT
BE SHORT
●
●
●
●
●
●
●
●
LEDON
LEDOFF
●
●
+
OFF
+
ON
+
Fig.4: this full-size artwork can be
used as a drilling template for the
front panel.
Make sure that all polarised parts are correctly oriented & note particularly
that D1 & D2 face in the opposite direction to D3 & D4. The battery clip must be
modified slightly to allow the battery assembly to fit inside the case – see text.
Power for the circuit is derived from
a 9V battery.
Construction
Since there are only a few devices
in the In-Circuit Tran
sistor Tester,
the construction is a breeze. All the
components are installed on a single
PC board measuring 51 x 37mm and
coded 04109931.
Fig.2 shows where the parts go on
the PC board. You can mount the parts
in any order but make sure that the diodes, LEDs, transistors and electrolytic
capacitors are the right way around.
The two LEDs should be mounted so
that their tops are about 15mm above
the surface of the board, so that they
later protrude through two bezels
mounted on the front panel.
You can easily identify the LED
leads since the anode lead will be the
longer of the two.
The board can now be mounted inside a small plastic utility case. First,
attach the adhesive label to the lid,
then use it as a template to drill out the
58 Silicon Chip
holes for the LED bezels and the on/off
switch. In each case, it’s best to drill a
small pilot hole first and then carefully
ream the hole out to the correct size.
Three small holes are also drilled in
one end of the case to take the flying
Base, Emitter and Collector leads for
the TUT.
This done, the on/off switch and
LED bezels can be mounted and the
Fig.3: this is the full-size etching
pattern for the PC board.
wiring to the PC board completed.
Use different colours for the test leads
and feed them through the holes in
the end of the case before making the
connections to the PC board. The PC
board is held in position by clipping
the two LEDs into the bezels.
The battery clip will have to be
modified to allow the battery assembly
to fit inside the case. This involves
removing the plastic cover from the
clip and soldering the leads onto the
sides of the clip eyelets.
Finally, the three test leads must
be fitted with hook-type test clips or
alligator clips. Alligator clips were
fitted to the prototype but you will find
that small hook clips are easier to use.
As soon as you switch on, you
should find that both LEDs flash at a
rapid rate. To test the circuit, you’ll
need two working transistors – one
an NPN device and the other a PNP.
Check that only the lefthand LED flashes when you connect the NPN device
and that the righthand LED flashes for
the PNP device.
If both LEDs stay on or both go out
and you are certain that the transistors
are OK, check that the two LEDs are
correctly oriented.
Finally, we should mention that the
In-Circuit Transistor Tester does not
work well with Darlington transistors.
This is because they have a higher saturation voltage than normal transistors
and so both LEDs will simply go dim
SC
for a working device.
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