Re: MMC resister problem

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Crow Leader by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, June 05, 2002 9:12 AM
Subject: RE: MMC resister problem


> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Gary.Lau-at-hp-dot-com>
>
> I fully agree that battery packs can achieve reverse-charges on cells, but
> this is very different than a string of caps.  A NiCd cell is not a
> capacitor and is not at all a linear device.  A "bad" cell had radically
> different V-I properties than the remaining good cells.  Even good cells
> have different V-I characteristics depending upon their state of charge.
> But in MMC strings, we don't have a "bad" cap that is responsible for the
> imbalance.  As far as we understand it, all caps are electrically the
same.

Maybe in textbooks they are all the same. Capacitors in real life are a bit
more than just something to store a charge. They exhibit capacitance,
inductance, resistance and are by no means act the same across all
frequencies. An item being mass produced does assure that they will all
behave the same. Take a look at the screw bins at a hardware store. They all
look about the same, but you will find lots of "junk" as well with botched
up threads, wrongly formed heads or even nuts with off center holes. Below
is a tacky ASCII art scenario of hysteresis in caps and voltage reversal.

we have say two caps in series

-------||--------||-------
       +  -       +  -

They are all happy and charged up. Then we place a load on them, such as a
transformer winding or screwdriver


|------||--------||----------|
|      +  -       +  -           |
|                                   |
|---------------\/\/\/--------|

As long as they are all charged up their voltages all add up and they look
like batteries for the most part. Their total voltage appears across the
load. Now some time has passed and the caps all seem discharged though their
resistor.

Now if one capacitor is smaller in capacitance than the other and they were
charged to close voltages, the one with the lowest capacitance will
"drain"first and be no different than the resistor. If it is not supplying a
flow or current anymore, it is using it up and it will see a reversed
voltage across its terminals. Pretend the second cap ran out of charge
first. Once that happense is will see a negative charge on its formerly
positive terminal.

I even made a test circuit to demonstrate this very simple effect. You can
do it at home with any voltmeter and two capcitors and a power supply and
maybe a resistor. The scope pics show what happens during the discharge
process.

Here is what I did. I took two GE motor run style film capactors. One was
6uF and the other 17.5uF. They were quite different in size, but it will
only make the point of the project clearer. Each one was charged to about 24
volts and they were connected in series. I attached a scope to each cap.
Ground leads for the scop were connect between the caps. I had the scope
plot the two channels. One showed about -24 volts (the 17.5uF cap) and about
+17 volts for the other. It should have been 24, but it discharded faster
than I could play with the test leads. Then I removed discharged both caps
in series though a 140k resistor that was laying around. The scope as you
can see was set to sweep at 1sec/div to catch everything nicely. The dashed
line in the center if the screen is the ground voltage reference.

http://users.symmetric-dot-net/kkanno/images/Krodak-series-RC-voltage-reversal.j
pg

As you can see, the smaller cap DOES experience voltage reversal. It's
voltage clearly crossed the zero mark. I got the same results with a volt
meter. I measured cap voltages before and after being shorted for a bit. In
ever case the small cap reversed voltage.

>  The observed results of course suggest otherwise, but I've not yet seen a
> model suggested that can explain the effect.

you have one now.

> While iron-core inductors exhibit hysteresis, I wasn't aware that caps had
> this property?

Capacitors do experience this. That is why big caps ship from the factory
shorted out. They will will appear to charge themselves up after even being
dead shorted as the dielectric slowly returns to it's original state. Mylar
filter caps are great for demonstrating this. Charge one, crowbar it then
short it again a minute later. You'll get a spark again even though for a
period of time there was no voltage across it.

KEN

> Gary Lau
> MA, USA
>
>
> >Original poster: "Crow Leader by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <tesla-at-lists.symmetric-dot-net>
> >
> >Voltage reversal is possible in a series of capacitors the same was it is
in
> >"unbalanced" nicad battery packs.
> >
> >If your cap string is building up voltage due to hysteresis and a few
caps
> >don't make any voltage themselves (leakage capacitance or combination of
> >both) and the entire string has a load of some sort like your transformer
> >secondary, they become part of the load and will see a voltage reversal
and
> >possible charge up from this current flow.
> >
> >KEN
>
>
>
>
>