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Re: Home Made Variac

At 08:33 PM 02/11/2000 -0600, you wrote:
>Tesla List wrote:
snip...
>  It looked like copper or at least a metal, and I
>> would think it would short out several turns.  I suspect that anyone who
>> would go to that much trouble would know enough to use the right brush
>> material.
>> 
>> Ed
>
>
>
>Hmmmm...   im lost now ....   why wouldnt a carbon brush "short out" the
>several turns as a metallic "brush" would ???  arent both materials made
>to conduct electricity ???  I understand that carbon has some
>resistivity to it but is it that high that it would matter compared to
>say a metallic brush? 
>
>
>Scot D
>

Hi Scott,

	I too wonder this...

I have a big Staco variac and the brush is thin but it will easily contact
two of the windings at the same time.  This would seem to cause a shorted
turn or have the differing voltage problems paralleling two variacs would have.

I think the trick is that the brush is fairly resistive.  The voltage
between the two winds it contacts is only a volt or two.  If the brush is
only one ohm, the power dissipated would only be a watt or two.
Apparently, the larger current going to the load will make a fairly low
resistance contact while the small current flowing between the two winds
has a fairly high resistance.  This is apparently a subtle quality that
carbon brushes have since the same thing happens in DC motors as too.

Perhaps someone knows more detail.

Cheers,

	Terry
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