On Apr 26, 2010, at 12:40 AM, Garey Barrell wrote:
The Fairchild data sheet I looked at for the LM78XX series showed a
0.33 uF on the output. Drake seems to have used a 1 uF tantalum
(solid) on ALL the "ins" and "outs". The tantalum cap should be ok
for 3-30 MHz, but is that TOO much capacitance. My experience says
that a 0.33 disc ceramic would be less expensive than the tantalums,
so I would assume that Drake thought it would be better.
I remember when the consumer magazines starting writing about the
LM78xx regulators. They all wrote that one MUST have a 1uF tantalum
capacitor bypassing the input and output. I had never heard of a
tantalum capacitor, and had never seen a 1uF capacitor that small.
I don't know where the concept came from, but my data books had them
in the application notes.
It's quite likely that someone decided that a 1uF tantalum capacitor
was what someone thought would be "bulletproof", i.e. it would never
be too little capacitance, and it was better to specify something much
too big, than to let people figure it out on their own and get it wrong.
As far as I can remember, circa 1975, the concept of making things as
small and as cheaply as possible with as few components as close to
their limits as you could specify them had not yet arrived in consumer
electronics. After all consider the parts that were replaced by one,
and the extra cost of the capacitors was trivial.
Geoff.
--
geoffrey mendelson N3OWJ/4X1GM
Jerusalem Israel [email protected]
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge
or understanding, as in he has a sub-wikipedia understanding of the
situation. i.e possessing less facts or information than can be found
in the Wikipedia.
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