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Units of capacitance
1/12/2004 4:53:24 PMPaul Jebb
I am a little confused by the units of capacitance on the old
schematics. I see "mFD" a lot and "mmFD". "m" means milli
in conventional SI units. How are microfarads represented if
the normal symbol is "mu"? What is the convention for millifarads
and microfarads?
Thanks in advance.
1/12/2004 8:01:57 PMTom Singletary
:I am a little confused by the units of capacitance on the old
:schematics. I see "mFD" a lot and "mmFD". "m" means milli
:in conventional SI units. How are microfarads represented if
:the normal symbol is "mu"? What is the convention for millifarads
:and microfarads?
:Thanks in advance.

Paul: It has always been a confusing mess but I think this will help.
1. The FARAD (named after Michael Faraday of British fame) is a MAMMOTH amount of capacitance and was based upon a given number of electrical charges (coulombs). Even 1 Farad (F) is seldom needed except as computer memory backup and a few other isolated instances.
2. Most of the capacitance applications applicable to electronics are in the microfard region (uf or mf) which is 10 -6 Farads. You will see the mf abbreviation for this even though the "m" does indeed coincide with "milli" units in the si system.
3. The old "micromicrofarad" (mmfd) is actually a picofarad (10 -12 Farads) and is seen in older diagrams as mmf or mmfd rather than the "si" correct "pf".
4. I am told by a physics collegue that the reason for not using the "micro" term on older diagrams was that typewriters did not have a "micro key" AND the standard California job case that contained printers type did not have one either. I think this is correct in that if you look in early physics texts the "micros" appear to be "scratched in" rather than a legible sharp type imprint.



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