[Homeroast] variac basics

raymanowen at gmail.com raymanowen at gmail.com
Sat Nov 13 02:53:16 CST 2010

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

"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

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
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:
Single-phase tapped autotransformer with output voltage range of 40%–115% of

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
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.
] 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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