A circuit demonstrating exponential decay....

I'd like to hook up a flashlight lamp, 1.5 v battery, and capacitor in
series, close the circuit, and watch the lamp come on and slowly go off
as the capacitor charges up.

I tried it with a double electric layer capacitor (1 farad), but I now
think this type of cap has a big internal resistance, so, nothing
(observable) happened..

Now I'm thinking an electrolytic cap (0.1 farad) and maybe a 10 ohm
resistor in the circuit to get the time constant up to > 1 sec (R*C).

Is this going to work?

Ideas?

Thanks,
Will

 

Or, better yet:


+-----> |
| |
| O
|+ |
[BAT] +------+
| |+ |
| [1F] [LAMP]
| | |
+--------+------+


I like it, but I think the (assumed) high cap resistance of the 1 f
double layer electric cap will prevent it from working.


If I use a 0.1 f electrolytic and the hot bulb has 5 ohms resistance I
can get a time constant of 0.5 seconds, which isn't what I was hoping
for but isn't bad. (If the electrolytic has negligible internal
resistance).

 

I like the LED idea, what is its resistance?

 

.-------------o---o----------.
| | | |
no switch |/ | |
o------o----| ' |
| | |> |/ ---
| | |-| - 3V minimum
| | |> |
'+ .-. | |
=== | | | ---
/-\ | | .-. -
| '-' ( X ) |
| | '-' |
| | | |
'------o----------o----------'

Nice work. But.... I'm trying to put this thing together for a
calculus book for junior high students to illustrate an exponential
function (starting from the DE characerizing the circuit) in nature, so
the circuit has to easy for anyone to wire up. Hence, no
transistors... etc.

If there is even 1 ohm of resistance in the cap, it'll suck up 0.5V, probably preventing your bulb from lighting at all.

I did take a few EE course in college, and as I recall caps always had
0 resistance. What are the facts? How about a 0.1 farad electrolytic?

 

Here is a tutorial containing more that you ever wanted to know about
capacitors:

Nothing on double electric layer caps that I can find .

 

And I've done this very thing, except with a fairly large aluminum
electrolytic cap, maybe 1000 - 10000 uF (I don't remember the exact
value), ~24 VDC, and an LED + 1K resistor. The voltage across the
resistor was 1V per mA, of course.


Excellent. I see that 100000 uF electorlytics are available cheap, so
that's good.... and I suppose I could go with a 9 volt battery.... (not
having a power supply)

I'm not really concerned that the LED brightness doesn't accurately
represent an exponential, I just want the thing to slowly turn
off.....that will be enough.

I'll give this one a go.

Thanks,
Will

 

Take a look at this:

http://www.elna-america.com/PD­F/DZ-N.PDF


Excellent. I didn't find the internal resistance spec for my Panasonic
double layer 1 farad ... but according to the spec for the ELNA the
internal resistance is 10% the usual, and is what .12 ohms which means
the Panasonic should be about an ohm which means my original circuit
should have worked.....

In any case..... I'd like to get one of these caps if they're cheap.

I did hook mine up the wrong way .......once.... maybe I cooked it.

Hmmmmm, in any case, we're getting there.

Thanks,
Will

 

Thanks to all for the assistance. I finally got something to work
using an LED. I found the spec for the Panasonic cap, it has an
internal resistance of 30 ohm at 1000 Hz...this doomed my original
circuit.... I do have the Elna cap on order and will hopefully get
something going with a bulb.....

Will

 

[mike]

wdflannery@aol.com wrote:

I'd like to hook up a flashlight lamp, 1.5 v battery, and capacitor in
series, close the circuit, and watch the lamp come on and slowly go off
as the capacitor charges up.

I tried it with a double electric layer capacitor (1 farad), but I now
think this type of cap has a big internal resistance, so, nothing
(observable) happened..

Now I'm thinking an electrolytic cap (0.1 farad) and maybe a 10 ohm
resistor in the circuit to get the time constant up to > 1 sec (R*C).

Is this going to work?

Ideas?

Thanks,
Will

If it's just a demo of exponential decay, why not make it easier on
yourself.
Use a LED, higher voltage and a MUCH smaller cap.
If you must have an incandescent, use an emitter follower to drive
it from a much smaller cap. Ain't nobody gonna be able to tell by
looking whether it's perfectly exponential anyway.

The key phrase is, "much smaller cap" ;-)

mike

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