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

Doug Smith W9WI wrote:
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.

I can't find the reference I thought I kept, but some top-secret business
plan is in action to use a giant ultracapacitor instead of batteries to
power an electric automobile.

--
One meter, to within 0.0125% accuracy (off by just under .005 inches):
Three feet
Three inches
Three eights of an inch

clifto wrote:
SNIP

I can't find the reference I thought I kept, but some top-secret business
plan is in action to use a giant ultracapacitor instead of batteries to
power an electric automobile.


Here's one:

http://www.worldcarfans.com/2060724....-electric-mini


Charlie.

--
M0WYM
www.radiowymsey.org

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

On Sat, 03 Nov 2007 12:03:44 -0500, clifto wrote:
Doug Smith W9WI wrote:
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.

I can't find the reference I thought I kept, but some top-secret business
plan is in action to use a giant ultracapacitor instead of batteries to
power an electric automobile.

And, they'll use Cold Fusion to charge it.

On Sat, 03 Nov 2007 10:41:44 -0700, K7ITM wrote:
(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.

Actually a slip of the brain, but same idea. Oops...

Now I'm confused...doesn't Tau (Time Constant) = 1.1RC, hence 1 farad
and 1 ohm would give a time constant of 1.1? Yes, it's only 6AM, but am
I missing something?

Scott
N0EDV

Doug Smith W9WI wrote:

On Sat, 03 Nov 2007 10:41:44 -0700, K7ITM wrote:

(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.


Actually a slip of the brain, but same idea. Oops...



--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

On Nov 4, 4:02 am, Scott wrote:
Now I'm confused...doesn't Tau (Time Constant) = 1.1RC, hence 1 farad
and 1 ohm would give a time constant of 1.1? Yes, it's only 6AM, but am
I missing something?

No, the time constant of a resistance and capacitance is exactly R*C.
The time constant of a resistance and inductance is exactly L/R.

If you want to know the time at which something special happens in an
RC exponential decay, you may have to use the formula: v(t) =
v(0)*(e^(-t/RC)) (or a variation of that which accounts for the RC
changing from one voltage to a different one). So, for example, in a
555 one-shot timer circuit where the capacitor starts at (essentially)
zero volts and charges to 2/3 of the power supply voltage, we want to
find t such that e^(-t/RC) = 1/3, and that will be when t=1.0986... *
RC -- or sensibly t = 1.1*RC. And that likely is where you're
thinking of the 1.1 factor from. That's not the RC time constant;
it's the time period of a 555 one-shot timer.

Cheers,
Tom

I knew a fellow who had a bank of 1700 amp hour batteries (2 volt cells in
series). he dropped a crescent wrench across the terminals and it started
spttiong out molten BB sized pellets. He dove for cover and when he finally
recovered, he discovered the crescent wrench had disintegrated, and he had
pellet sized holes melted into his jacket.

This will be great delight when we can do the same thing with capacitors.

Colin K7FM

Hey, I think you're right! Sucks to get old! The good news is that
I've never had to be SO precise that using RC or 1.1RC would make any
difference for me Thanks for reminding me (The bad news is that
I'm an electronics tech by day and should have remembered that! I guess
I'm waiting for the day when I get in an argument and can say I've
forgotten more than "the other person in the argument" will ever
know!!)...Cheers!!

Scott
N0EDV

K7ITM wrote:

On Nov 4, 4:02 am, Scott wrote:

Now I'm confused...doesn't Tau (Time Constant) = 1.1RC, hence 1 farad
and 1 ohm would give a time constant of 1.1? Yes, it's only 6AM, but am
I missing something?


No, the time constant of a resistance and capacitance is exactly R*C.
The time constant of a resistance and inductance is exactly L/R.

If you want to know the time at which something special happens in an
RC exponential decay, you may have to use the formula: v(t) =
v(0)*(e^(-t/RC)) (or a variation of that which accounts for the RC
changing from one voltage to a different one). So, for example, in a
555 one-shot timer circuit where the capacitor starts at (essentially)
zero volts and charges to 2/3 of the power supply voltage, we want to
find t such that e^(-t/RC) = 1/3, and that will be when t=1.0986... *
RC -- or sensibly t = 1.1*RC. And that likely is where you're
thinking of the 1.1 factor from. That's not the RC time constant;
it's the time period of a 555 one-shot timer.

Cheers,
Tom


--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)