PET composite video adapter

Holger Veit holger.veit at iais.fraunhofer.de
Thu Oct 16 09:07:08 CDT 2008


Tony Duell schrieb:
>>>> The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
>>>> non-polarized cap.  I don't know that I have any disc or ceramic caps that
>>> IO think we can instantly eliminat 'mf' == millifarad (which is what it
>>> should be!), since that's then 2.2F, which is rediculously large
>> Of course.  I never thought it was millifarad.

I think in this case it was just sloppyness (or even lack of 
understanding, or in best case just a typo) of the circuit designer to 
specify 2200mf.

> Well, it's what it _should_ be. Mind you, millifarad is not a
> commonly-used unit for some unknown reason (I routinely see capacitors
> marked 100000uF rather than 100mF or even 0.1F), although i have seen it
> used in, I think, an HP manual

mF has been uncommon because with the exception of gold caps, you rarely 
find capacitors with such large capacity, and in common circuits, you 
won't need these sizes. For the traditional power supply, I have 
seldomly used larger caps than 10000uF; if one needed more, it is more 
likely the wrong design has been chosen.

> Incidentally, I saw a 2.7kF (no, not a typo!) low-voltage electrolytic in
> a catalogue recently. And to think that when I was at school the physics
> 'teacher' told us that a 1F capacitor would be 'larger than this room'
> and that we'd never seen on. About a week later I dropped a square plasic
> object on his deck -- a 1F capacitor that I'd bought from Farnell or
> somewhere.

Been there, having done and seen that, most teacher with pure physics 
background have no knowledge of practical electronics, so no surprise.

>>> 2200nF (==2.2uF) would be possible, but it's a very odd way to write it.
>> Yes.  This was also written in the 1970s, so I'm trying historical
>> interpretation as well as what's obviously written.
> 
> nF was common in Europe in the 70's, much less common in the States, I think.

Please don't speak for Europe when talking about GB. In Germany, pF, nF, 
and uF are still common nowadays, and everyone uses them where 
appropriate. I had to learn about circuits (mostly from US magazines) 
that contained rather uncommon values like 2200000pF or 0.000001uF which 
would be better understandable when written with less zeros.

> Err, millimicrofarad = nanofarad. The picofarad is the micromicrofarad.
> I've seen  uuF (where the 'u' is actually $\mu$) in old manuals.
> 
> Am I the only person to remember the millimicron as an equivalent for nm
> when talking about optical wavelengths, etc?

Reminds me of describing speeds as angstrom per fortnight. Infact, 
instead of nm, angstrom are frequently still used for wavelengths, but 
SI units are supposed to ever have only one single prefix pico, nano, 
micro, milli etc. and not combining them as in milli-micro-farad.
OTOH, it might be a selling argument for a CPU with 2 MilliPeta-Hz, and 
surely a common distance is more challengingly written as 1000000000000 
femtokilometers.

-- 
Holger



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