[Homeroast] variac basics

msmb at suddenlink.net msmb at suddenlink.net
Sat Nov 13 11:54:23 CST 2010

Thanks!  By the way, are you  Ray Owen, the amiddle school shop teacher?

---- raymanowen at gmail.com wrote: 
> Let me think about this...
> You said:
> "I was amazed at... how evenly it roasted."
> [Sounds like you got this tiger by the tail already. What more do you want?]
> For the formula-challenged, Power = Volts x Amps
> Suppose the nameplate on the PS popper says: 1250 watts  120 volts [1250/120
> = 10.4A]
> Suppose: 1250 watts  12 amps [1250/ 12 = 104 V] Impossible! 104 v does not
> exist.
> "divide 1250 by 84.5, which suggests a 15 AMP variac" [84.5v x 15a = 1267w]
> 1250 watts by itself is meaningless, and from where did you get 84.5? 84.5
> what?
> Variacs are variable auto transformers normally used to lower the input
> voltage to as low as 0 volts, or raise it to as high as 1.4x the input
> voltage.
> With an essentially resistive load like a popper, if you raise its input
> voltage by the 1.4  factor, the current goes up by the same 1.4 factor and
> the power about doubles. 1.4 x 1.4 = 1.96 (2!)
> If you need 15 amps for sure, you can use a Variac rated at 10 amps. It will
> develop heat faster than convection air currents can remove. It will get
> warm. A little fan can be added to completely eliminate heat problems.
> If your device- popper or whatever, normally draws 15 amps at its rated
> voltage, and you jack it up by 1.4, now it's drawing about 20 amps. You can
> still use a 10-amp rated Variac, but it will just get hot faster. Bigger fan
> and don't keep doing it very long.
> Let the Variac cool completely after you run it at 100% overload. No formula
> here, just don't spike your Edsel with hydrazine very often.
> From Wikipedia:
> Operation
>  <http://en.wikipedia.org/wiki/File:Tapped_autotransformer.svg>
>  <http://en.wikipedia.org/wiki/File:Tapped_autotransformer.svg>
> Single-phase tapped autotransformer with output voltage range of 40%–115% of
> input
> As in an *ordinary transformer*, the ratio of secondary to primary voltages
> is equal to the ratio of the number of turns of the winding they connect to.
> For example, connecting the load between the middle and bottom of the
> autotransformer will halve the voltage. Depending on the
> application<http://en.wikipedia.org/wiki/Autotransformer#>,
> that portion of the winding used solely in the higher-voltage (lower
> current) portion may be wound with wire of a smaller gauge, though the
> entire winding is directly connected.
> [edit<http://en.wikipedia.org/w/index.php?title=Autotransformer&action=edit&section=2>
> ] Limitations
> An autotransformer does not provide electrical isolation between its
> windings as an ordinary transfomer[sic] does. A failure of the insulation of
> the windings of an autotransformer can result in full input
> voltage<http://en.wikipedia.org/wiki/Voltage>applied to the output.
> This is an important safety consideration when
> deciding to use an autotransformer in a given application. Furthermore, if
> the "neutral" side of the input is not at ground voltage, the "neutral" side
> of the output will not be either.
> Note- Variac autotransformers normally don't have taps as shown in the
> drawing. Usually it is a single winding on a circular toroid-shaped coil
> form. The "wiper" can contact any turn of the winding.
> Cheers, Mabuhay -RayO, aka Opa!
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