Re: Voltage division along NON-equal caps in series? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 11 Sep 2007 14:14:15 -0700 (PDT)
From: G Hunter <dogbrain_39560@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Voltage division along NON-equal caps in series? (fwd)

Even at DC, the voltage won't divide evenly across
unequal caps in series.  The small caps will reach
peak charge before the bigger caps.

There's an easy way to visualize this using a simple
rule:  "current is common in a series circuit".
  
Since all the caps in a series string will see exactly
the same current at the same time, it stands to reason
the smaller caps will "fill up" first.

Once the smallest cap is "full", it feels the highest
voltage, behaves like an open, and all current stops,
leaving the bigger caps only partially charged.

You are correct that Xc is a non-factor at DC.  The
reason is simple.  Xc=1/2pi(fC), where f=frequency. 
Since DC is zero Hertz, and division by zero is
undefined, you can't calculate reactance.

Regardless--DC voltage will not divide equally across
unequal caps in series.

This doesn't mean you can't use unequal caps in your
tank circuit--you can.  You just have to be mindful
that the smaller caps will feel higher voltage.

Cheers,

Greg


--- Tesla list <tesla@xxxxxxxxxx> wrote:

> 
> ---------- Forwarded message ----------
> Date: Mon, 10 Sep 2007 13:51:11 EDT
> From: FIFTYGUY@xxxxxxx
> To: tesla@xxxxxxxxxx
> Subject: Voltage division along NON-equal caps in
> series?
> 
> Folks-
>  
>     I've got a number of caps of varying ratings
> that  I'm considering 
> combining for primary tank use.
>  
>     I know we assume equal sharing of voltage
> between  identical caps in 
> series. 
>  
>     But what about non-identical caps? I know that
> an AC  voltage will be 
> seen across the caps as if it was a resistive
> voltage divider,  with the 
> reactances of the various caps corresponding to the
> voltage seen across  each. In this 
> case, larger caps will have smaller reactances, and
> the voltages  across them 
> will be proportionately smaller. 
>  
>     But what about the DC case? The primary charging
>  frequency is so low, we 
> consider the DC case. And here's where I get
> confused.  It seems 
> (intuitively) that if each cap was a single plate,
> then the voltage  would be distributed 
> evenly between each cap, no matter what the area of 
> each plate may be. I 
> could also see that the spacing of each  plate could
> play a role as well - the 
> thinnest dielectric in a series of plates  would
> have the highest voltage stress 
> (intuitively?).  But what if some  caps have many
> internal plates, and some 
> have only one? Does cap value have any  effect on
> the DC voltage seen across it 
> when it's in series with other  caps?
>  
>     For instance, if I have a 1uF cap, a 10uF cap,
> and  a .1uF cap in series, 
> and I put them in series, and put 1V DC across this
> series  arrangement, how 
> is the 1V DC distributed across them? Are the
> particulars of  their 
> construction important?
>  
>     Does this sort of mismatch cause problems during
> TC  use?
>  
>     Thanks!
>  
> -Phil LaBudde
> Center for the Advanced Study of Ballistic 
> Improbabilities
> 
> 
> 
> ************************************** See what's
> new at http://www.aol.com
> 
> 
> 



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