Re: elctronic components

[roger haar <haar@soliton.physics.arizona.edu>]

Hi,

	On the question of the discharge of your capacitor:  Did the
meter change scales?  I have seem moderately expensive meters NOT have the
same reading between two scales even though the current was constant.  
The piece wise behavior points to something like this first.  Value 
written on the sides of capacitors are often wrong by quite a lot.

	The very high capacitance, supercaps are a batttery-capacitor
hybird. They are more like batteries with a very thin electrolyte, so
the time for the chemicals to drift from terminal to terminal is short.
These supercaps are the subject of much research because of their high 
energy density.  Hoped for uses, are things like storage as part 
of regenerative braking in electric buses.

Thanks
Roger Haar
U of AZ   



**********************************************************
On Thu, 26 Dec 1996 kowalskil@alpha.montclair.edu wrote:

> Now that the pressure from grading is over I decided to collect some data.
>
> I charged C=1 F to 5 volts and discharged it through an ammeter in 
> series with a resistor of 100 ohms. The current was measured to see if 
> it decreases exponentially, as it always does for a "non-chemical" 
> capacitor. I do remember (reading it somewhere) that chemical reactions  
> take place in electrolytic capacitors during charging and discharging. 
> Speculations apart, I am puzzled. Please help me to interpret the discharge
> data below. (Actually I have many more data points than shown in the table;
> they fit nicely between what is presented.)
>
> t (s)   0   60  120  180  240  310  420  500  660  800  1120  1320  2100
> I (mA) 40   27 17.5 12.0  8.0  5.0  2.5  1.5  0.8  0.4  0.17  0.12  0.05
>
> Here is what I did.
>
> 1. A linear plot of I versus t was used to get the total charge delivered
>    by the capacitor (charge is the area under the curve). To my great
>    surprise Q was found to be 12.9 coulombs. This is 2.6 times as much
>    as Q= V/C = 5 volts / 1 farad = 5 coulombs. WHERE DOES THE EXTRA Q 
>    COME FROM? IS CAPACITOR ACTING AS A BATTERY (CHEMICAL SEPARATIONS)?
>
> 2. A semilogarithmic plot of I versus t was not a straight line over
>    the entire range of current. It was a piece-wise exponential curve.
>    The value of RC was changing from about 150 seconds at the beginning
>    to nearly twice as much at the end. The expected nominal RC was 100 
>    ohms times 1 farad = 100 seconds (or 260 seconds if I assume that 
>    the C of my capacitor is 2.6 F, based on 1 above).
>
> Can somebody verify that R*C is indeed changing during the discharge
> process? Perhaps the capacitor I have is a fluke. (It was made by NEC
> and labeled as 5.5 V and 1 F. Does it matter which end is + and which
> is -? Electrolytic capacitors which I used before were always marked 
> for + and -.)
>
> 3. The initial discharge current (40mA) is smaller than what I expected
>    (5 volts /100 ohms = 50 mA). I am sure of my 100 ohms. Does it mean
>    that the esr (equivalent serial resistance) of my cap is 20 ohms?
>    That is much more than about 0.1 ohms we were talking about.
>
> I wish I had a 500 F (or even 10 F) super-capacitor to see if the 
> "battery-like" effects (such as delivering more charge than expected) 
> are stronger. Please share your experimental data.
>
>            Ludwik Kowalski         kowalskiL@alpha.montclair.edu