Q&A
Figure 7
Dip Meter
On/Off
68pF D
G
MPF102
.001
+
9V
50pF
S
33k
L1
Sensitivity
1mH
500k
.05
0-50uA
found an NTE replacement, go to
the FindChips website at www.find
chips.com and enter the NTE
number.
Jameco and Mouser both have
in-depth stocks of these devices, but
they can be more expensive than a
generic replacement.
For example, the HEP724 cross
references to an NTE123A — which
is the equivalent of a 2N2222 that
sells for one-third the price.
The NTE website has data
sheets that you can use to find
a cheaper generic. Follow the
guidelines in the question below
(“Transistor Selection”) to find a
suitable replacement from a data
sheet. And — yes — the MPF102 is
an acceptable substitute for the
TIS59, according to their data
sheets.
Specialty
Transistors
Q. I am an electronics hobbyist and
I was wondering if
know of any websites
that I could visit
for semiconductor
equivalents. I have
been looking for a
transistor whose
reference is H8N80FI
and can’t find it in any
transistor catalog.
Elimane Bathily
via Internet
Transistor Selection
Q. My question is what the determining factor is in transistor
choice? How interchangeable are the
different types? I have a circuit that
calls for a 2N3707, which I have
never even heard of.
B. Brown
via Internet
A. Not all transistors have a cross
reference — especially those
which are privately labeled like the
H8N80FI. In cases like this, I do a
Google search ( www.google.com).
As a rule, I hit nothing, but I got
lucky this time and came up with a
2SK1363 — an N-FET power transistor. Unfortunately, this transistor is
only available overseas and not
stateside, as are many specialty
transistors and ICs. Try Donberg
Electronics at www.donberg.ie
(Before you send that letter, let
me say that the 2SK1363 was also
used in an Apple CRT monitor and
an Apple repair center may still have
one or two on a dusty shelf.)
A. From an engineering point
of view, there are four critical
parameters when it comes to
choosing transistors: voltage,
current, gain, and frequency.
Supposedly, each transistor is
custom fit (as in a specific
2N number) to fill a niche for
performance and cost. That is, you
don’t want to pay for performance
you don’t need, but you do not want
to be caught short with a marginal
part, either. Hence, all the various
numbers.
However, like most electronic
devices, a transistor with better
specs can always replace one with
lesser expectations.
As for the 2N3707 — which is
now obsolete — it’s an NPN general-purpose audio transistor rated at 30
volts, 200 mA, and a gain of 100 to
400.
To me, that sure sounds like a
2N2222, which is rated at 60 volts,
800 mA, with a gain of 300 at 150
mA. Both are of the same sex —
NPN as opposed to PNP — both are
silicon (not germanium), and both
have an upper frequency limit of 250
MHz. So, why does your design
specify the 2N3707? Probably
because the designer got a good
deal (price break) on this transistor
at a time when he needed those
parameters.
NUTS & VOLTS
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Figure 8
L1
68pF D
G
MPF102
50pF
S
10k
33k
Dip Meter with
Counter Out
1mH
.05
On/Off 9V
+
.001 10k
1uF
D
G
MPF102
S
Freq. Out
1M
+
2.4k 22uF
Sensitivity
500k
0-50uA
Radioactive Peanuts
18
A couple of years ago —
January 2003, to be exact — I published a circuit for a Geiger counter
using an NE-2 neon lamp. I found
the design in an old copy of
Experimente mit Strahlenquellen.
JANUARY 2005
