February 1960 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged.
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Integrated circuits are the de facto standard of today, but
40+ years ago when this article was written, individual transistors
were all that was available to both hobbyists and professional
engineers. Believe it or not, there are still a lot of applications
in modern products that use discrete transistors for output
stage drivers, buffers, and where parts costs might save a penny
or two in high volume production. There are also, of course,
millions of circuits in existence and in daily use that include
transistors. This transistor tester will allow you to do a simple
check to determine whether a particular device is still working,
or whether some newer transistor might be a suitable replacement
for one gone bad. The truth is, though, that unless you just
really like to build circuits, you can buy a DMM with a built
in transistor tester for $20- $30.
Build a Dual-Meter Transistor Tester
You can check both audio and power transistors with one easy-to-operate
By R. J. Shaughnessy
Sometimes you'll finish building a transistorized project
and find that it doesn't work. It's easy enough to recheck your
wiring, but if you do and the unit still doesn't work, then
what? Were the transistors good before you put them in the circuit?
Were they burned out accidentally? It's obvious that you need
a transistor tester to check the transistors before you wire
them into the circuit and to check them again if the circuit
This tester measures the two important characteristics of
almost all audio and power transistors: current gain (Beta)
and collector-to-base leakage (Ico). Only transistors which
have a 5-ma. maximum collector current cannot be tested with
this unit; see the manufacturer's data for special testing techniques
for these low current jobs.
Two meters are built into the tester to allow the base current
and the collector current to be monitored simultaneously under
various bias settings. This monitoring feature enables a transistor
to be tested under actual circuit load conditions.
Transistor tester base current control R2
should be wired so that maximum resistance is obtained when
ganged switch S1 is open.
For maximum flexibility, no sockets were incorporated in
the tester proper. The transistor under test is simply connected
by its leads to the tester terminals. An adapter which plugs
into the tester's binding posts can be built which will accommodate
the various types of power and audio transistor sockets. Parts
used in the tester and optional adapter are not critical. With
all new components, cost of the tester is about $15.
of the tester is begun by mounting all the components directly
on the cabinet. Before mounting function switch S2, crimp all
jumper leads to the switch terminals. After the switch is mounted,
connect and solder the remaining leads to it.
Observe polarity of diodes and capacitors
in tester power supply detail (left).
Two-wafer ganged function switch is used
in tester as shown in pictorial diagram above. Both wafers are
identical. Note that pins two and eight are not used.
Power cord of tester is led through grommet
in mating half of Minibox before soldering it in place.
The transistor tester adapter can be built into the smallest
Minibox that will accommodate a standard three-lead transistor
socket (in-line or circular type) and a power transistor socket.
When a transistor is being tested, the adapter's banana plugs
(which are connected to the appropriate pins on the transistor
sockets) plug into the tester's universal binding posts.
TESTER PARTS LIST
CI, C2, C3, C4 - I60-μf., 15-volt capacitor
germanium diode (Sylvania) F1 , F2 -1/2-amp. 3AG fuse (to fit
MI-0-1 ma. meter (Shurite 950-9300Z)
meter (Shurite 950-9307)
PL1-Fuse plug (El-Menco EL-32)
R1-6800-ohm, 1/2-watt resistor
R3-1000-ohm, 1-watt resistor
S1-On-off switch mounted on rear of R2
S2-Four-pole, four-position rotary switch (Centralab
T1-6.3-volt filament transformer (Triad F-13X)
1-7" x 5" x 3" Minibox (Bud CU-2108A)
2-Six-lug terminal strips
Testing for leakage is simple. Rotate function switch
S2 to Leakage N-P-N or Leakage P-N-P, depending on the transistor
in question. Connect the transistor base lead to the tester's
emitter binding post. Then connect the transistor collector
to the collector binding post. Leave the transistor emitter
lead unconnected. (The transistor emitter is left unconnected
for all leakage measurements.) Now turn on the tester by advancing
the Base Current potentiometer (R2). If the 0-100 ma. collector
current meter (M2) is not deflected, the leakage current is
within acceptable limits.
You can safely measure the
exact leakage current on the more sensitive 0-1 ma. base current
meter (M1). Turn off the tester and reconnect the transistor
base and collector leads to the corresponding tester binding
posts. Do not connect the emitter lead; keep the function switch
in the "leakage" position. When you turn on the power, you'll
find that most transistors will give little - if any - deflection
of the 0-1 ma. base current meter. Some lowleakage silicon
units will give no perceptible deflection at all.
ADAPTER PARTS LIST
1-2 3/4" x 2 1/8" x 15/8" Minibox
(Bud CU 2100A)
1-Three-lead transistor socket
transistor socket (Motorola MK-10 or equivalent)
Tester adapter has two sockets accommodating
power transistors and smaller audio transistors.
If the transistor passes the leakage test, you can safely
perform the current gain (Beta) test. Current gain cannot be
read directly on the tester, but Beta is very easily found by
dividing the collector current reading by the base current reading.
The Beta test is made by setting S1 to Beta N-P-N or Beta
P-N-P. Make sure the power is off. Connect the transistor base,
emitter, and collector leads to the corresponding binding posts.
Check the manufacturer's specifications for the maximum collector
current for the transistor under test and never exceed this
value as read on the 0-100 ma. collector current meter. Now
switch on the tester, but leave the Base Current pot full counterclockwise.
Record the base current and collector current meter readings.
Dividing the collector current by the base current will give
you one value for the Beta (current gain) of the transistor
Current gain ( Beta) test for n-p-n transistors
is identical to p-n-p test shown in simplified schematic but
polarities of meters and power source are reversed by switching
Now increase the base bias current with the Base Current
potentiometer. This will cause an increase in the collector
current. Once more, record the meter readings and compute the
current gain. Continue this process until you have several values
for current gain.
Note that Beta is constant except at the higher collector
currents; this is a normal transistor characteristic. Check
your computed values for the current gain against the manufacturer's
specs to see if the transistor is up to snuff.
You'll soon find that you'll have more confidence in the
circuits you build and trouble-shoot. Using the tester, you'll
be able to give transistors a rapid checkout and use them to
Leakage test effectively puts two meters
in series with transistor as shown in simplified schematic.
Polarities for n-p-n transistors are reversed as in Beta