On Nov 2, 10:36 pm, Doug Smith W9WI wrote:
Back when I was studying electronics in college in the 1970s, we were
taught that a 1-Ohm resistor and a 1-Farad capacitor would create a RC
circuit with a time constant of 1.1 seconds -- and that since it was
physically impossible to make a 1-Farad capacitor, we would never actually
see such a circuit in real life.

Of course, you can now order a 1-Farad capacitor for a few bucks from
Digi-Key. But they didn't stop there.

On page 18 of this month's RadComm (RSGB) there is a brief item about
"ultracapcitors". The devices are available in values up to 3,000 Farads.
**3,000 Farads**. Amazing.

http://www.maxwell.com/ultracapacito...l/bcap3000.asp

(Scratching my head in puzzlement...) one farad and one ohm give you
a 1.1 second time constant??

;-) I'll assume that was a slip of the fingers on the keyboard.

But even in the 70's, you could easily find computer-type
electrolytics with capacitances that were a very significant fraction
of a farad. I think I may have some around 0.1 farads with date codes
that old, and I'm just about positive I have some at 0.05 farads, and
not physically all THAT large, even. To me, a more impressive trick
has been to get the series resistance down to a low level. Early 1F
caps, used for clock backup and the like, had terribly series
resistance. They were only good for delivering microamps of current,
and they took a relatively long time to charge. Actually, that's not
necessarily a bad thing with respect to charging, in that you don't
need a series limiting resistor--it's built into the cap--to keep the
charging current pulse under control.

Cheers,
Tom