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Re: large nonpolar caps

Subject:   Re: large nonpolar caps
  Date:    Wed, 14 May 1997 11:37:58 -0400
  From:   "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: large nonpolar caps
> Date: Tuesday, May 13, 1997 5:58 PM
> 
> Subject:  large nonpolar caps
>   Date:   Tue, 13 May 1997 13:00:33 -0700 (PDT)
>   From:   Jeff Mason <masonj-at-belnet.bellevue.k12.wa.us>
>     To:  tesla-at-pupman-dot-com
> 
> 
> Does anybody have a source for large nonpolar caps?  I.E. 25000 mfd
up
> to 
> about 1 farad.  They need to be nonpolar and have a voltage rating
> larger 
> than about 10 volts.  I teach physics and we use them in our
electricity 
> unit.  (They need to be nonpolar because our physics students have
a 
> habit of accidentally switching the polarity.)  Since these are
deviant 
> high schoolers they need to be relatively indestructable.
> 
> Jeff Mason
> masonj-at-belnet.bellevue.k12.wa.us

Jeff,
As a fellow educator I know what you mean. About that deviant stuff.
They do not make non-polarized capacitors in the sizes you want (at
least not at prices you can afford). There are two things you can do.
One is to make a non-polar cap from two polarized caps and two
diodes. The down-side here is that the thing now has the dumb diode
part adding its characteristics to the totality of the capacitor.
Here's the schematic:

Version 1

 (+/-)       -    +          +     -      (+/-)
------*-------| |------*-------| |-------*------- 
      |                |                 |
      |                |                 |
      -------->|-------*--------|<--------

While this is useful for things like a filter cap, I would not want
to use it for true AC work as THEN the diode characteristic becomes a
real bugger.


Version 2
The absolute CHEAPEST method to *protect* just the capacitor is as
shown in the following schematic:


 (-)        -     +       (+) 
------*-------| |------*------- 
      |                |
      |                |
      -------->|--------

That is the easiest, cheapest, probably does at least 95% of what you
want. This arrangement is GREAT if used for DC filtering (as in power
supply circuits). It fails miserably if the capacitor is being used
for its AC characteristics.

Hope that helps you.

Fr. Tom McGahee