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Re: variac options

Jorge,
    Any relation to Daisey Fuentes??? If so, please give her my
E-mail address.

    Now back to your question! Bulbs are OK as a VERY crude method
of current limiting. A 100 watt bulb is rated as such when it has
the full 110v accross it ( this also being an estimate). When it
is hooked up in series with a transformer you have changed the 
availble current by adding a relected load resistance from the 
primary. The resistance seen by the primary is inversly proportional
to the current in the secondary. Test this by running the setup with
the secondary open (bulb is dim), then with the secondary shorted
(Near full brightness).

    As you see this can be roughly modeled as a thermister (bulb)
in series with a variable resistor (transformer load). 

    Consider the case of two identical 100 watt bulbs in series.
They both have equal resistance, so each has 50% of the voltage.
The current is now reduced by 50% due to double circuit resistance.
This leaves each bulb with 25Watts for a total input power of 50watts.

Things are not always what they seem!

Jim McVey


>From: Tesla List <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: variac options
>Date: Tue, 16 Mar 1999 18:07:07 -0700
>
>Original Poster: "Jorge Fuentes" <jlfuentes-at-infosel-dot-net.mx> 
>
>Good day all.
>
>I was some time ago ( 1 year ago maybe), told that a cheap option to a
>variac was using 100w lightbulbs. Arrange them in a paralell way (see
>drawing below) and as you turn on the bulbs that's the wattage going to 
the
>system,  turn on the first lightbulb you get 100w in, turn on the 
second
>light bulb on let the first one on and you'll get 200w in etc, when you
>finish turning all the lightbulbs on, turn off the lightbulb bank off 
and
>turn on the main switch on.  
>
>Is this correct, that's the way I understood it. Does anyone have any
>comments or sugestions, drawbacks on using this setup ???
>
>
>
>AC______________________> to neon trans.
>                                         
>                                       
>                  main switch                 
>AC_________/  ___________>to neon trans.
>        I           I   
>            /               I
>            I               |      <---- light bulb bank
>        I--100w--/  -I
>        I--100w--/  -I
>        I--100w--/  -I
>        I-- etc..--/  -I
>
>
>

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