Convert a Rheostat into a Variac?

Discussion in 'General Electronics Chat' started by Gdrumm, Jan 13, 2015.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Is it possible to make a Variac out of an Ohmlite Rheostat?
    The one I have is a Model H, and looks like the one in this picture.

    In many ways, it looks identical to my Variac, only it is much smaller (about the diameter of a Silver Dollar).

    Just curious.

    Thanks,
    Gary
     
    Last edited: Jan 13, 2015
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Variac relies on magnetic flux linkage between turns(is a transformer). Rheostat relies on resistance between turns .
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
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    A rheostat is a variable resistor and a variac is an adjustable auto-transformer. Not likely to make one from the other though they both can vary a voltage but using very different principles.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well yes but the whole point of a transformer is to change the voltage without much power loss, which a rheostat will not do.
     
  5. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Okay,
    Thanks for the education.
    Gary
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Though no one specifically mentioned it, the core of the rheostat is a ceramic insulator, while the core of a Variac is ferromagnetic metal.

    Ken
     
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  7. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    If the core could be removed and replaced with a suitable magnetic core, there is a chance that it may perform like a variac but there would be significant drawbacks.
    One drawback which is very obvious is that the wire used for the resistor is made from a special alloy that has high resistance relative to copper. This would mean the variac would be eating up lots of power while delivering current.
    Another drawback is that the size is so small already that only a small core would be able to fit inside, and since the core size and number of turns affects inductance and inductance affects the maximum useable voltage and there is already a very very limited number of turns on the resistor, the useable voltage would probably come in around 5vac if your lucky, but i would not even bet on that.

    The price of variacs has gone up quite a bit in the past, like a lot of transformers.
     
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