Hi all,
I am trying to rake together the components to build a metal detector.
The plans are from the UK, and I'm US. The plans call for BC183B
transistors, and I was wondering if there was an easier-to-get
equivalent.
The BC183 is a plain vanilla NPN silicon small-signal transistor (a bit
out of date, although I think I have one), and the "B" gain group is the
middle of the range gain (hFE 240-500 at 2mA Ic and Vce=5; typically
330). Not especially high voltage, not especially low noise. So quite a
few transistors should fit in there. Unfortunately, unless you have the
-A, -B and -C (or -16, -25 and -40) gain range specification, which many
Pro-Electron (BC... - most European small signal transistors) you may
have to go through transistors like PN100 or 2N3569 with a gain tester,
as others have suggested. The best idea is to hunt down something like
the BC549B (see below).
For what it's worth, the background to that family of transistors goes
something like this: the "BC" at the start identifies a silicon audio
transistor (using the Pro-electron naming scheme, which I think is more
helpful than JEDEC), and is reasonably easy to find explained on the
internet, but there isn't exactly a formal system to the number part...
what is NPN or PNP, for instance. But back around 1966 a family of
TO-18 silicon transistors came out, BC107 to BC109, that not only became
very popular, their number system became popular and so was copied by
many other transistors, up to the present day. This is how it works:
...7 means high voltage, ..9 means high/gain and low noise, and ..8 means
cheap - not especially great in any way. This was followed by BC177 to
BC179 for PNP counterparts. Then BC147-9 and BC157-9 for "lockfit"
plastic casing versions, and various other plastic case versions with
the middle digit EVEN for NPN and ODD for PNP. A lot (but beware: not
all) BC... transistors use the ..7/8/9 tradition, a common pinout that
goes e-b-c (dating back to the AC126 and OC71), and odd/even middle
digits indicating PNP/NPN... exceptions being ones like BC338, BC640,
etc. Standardisation is a wonderful thing in the electronics industry;
it is just a pity there are so many different standards to choose from!
An even more well-used tradition/standard is the gain group letters at
the end: A for low gain, B for middle gain, and C for high gain. While
the BC..7/8/9 family was becoming popular, Texas Instruments had
Pro-Electron transistors BC182/3/4 (NPN) and BC212/3/4 (PNP) with high
voltage/cheap/high gain signified by the last digit a bit like the 7/8/9
family. But they had letters like "L" at the end giving the pinout
(e.g. b-c-e), not the gain. Quite a few US manufacturers (including
Motorola) made transistors with Pro-Electron names, many making the
BC182/3/4 series. It is interesting to see them swing, over time, to
follow the -A/B/C suffix giving gain ranges (which is nice), while some
European brands eventually moved away from that to use -25, -40 etc to
show 250+ gain, 400+ gain, etc!
Back to the problem at hand: Many transistors popular in the US (e.g.
the 2N4124, and 2N930 - is that still popular?) *may* have the
appropriate gain, but they often lack the restricted gain range marking
that ensures a particular transistor will have enough gain. You may have
to try a few on an hFE tester to get one that is suitable. Or you could
try something like a 2N5088 and know you have enough gain, but it has
less current capability and base-emitter reverse voltage rating - which
might not matter in your circuit, but I cannot be sure. Some of the
good 2N.... equivalents, that are pretty likely to have a high enough
minimum gain, are somewhat rare (e.g. TO18 can). There is, however, a
really good answer...
The best idea, as Eeyore suggested, is to get one of those many
transistors like BC547B, BC549B, etc that have the gain range specified;
these are pretty common at mail-order parts suppliers like futurelec,
jaycar and digikey, and pretty much the transistor-of-choice for general
circuits from Australia to England so not likely to go obsolete any time
soon. But not everyone stocks the restricted gain range versions, or
specifies them in the catalogue (e.g. Jaycar simply says "BC549" but you
are likely to get BC549B). Shop around for a supplier that advertises a
BC549B or whatever - they are plentiful.
Other examples: BC109B, BC168B,BC169B,BC547B,BC548B,BC549B,BC546B, or
BC550B. The -C gain group is even better. If you have to get a BC54-
transistor without the gain group specified choose the BC..9 or BC550,
since they pretty much always don't include much below a gain of 250.
Mark A
