Potentiometer vs. Variacs - what's the diff?

[Patrick Bloofon]

This is probably a *really* stupid question, but what exactly is the
difference between a potentiometer and a variac? Is a variac simply an
industrial strength potentiometer?

Thank you kindly

[Ratch]

A potentiometer is a 3 terminal variable resistor. A variac is a
autotransformer. Ratch

"Patrick Bloofon" <trans...@home.com> wrote in message
news:mkvq7.54705$sM1.14...@news3.rdc1.on.home.com...

[John Popelish]

A potentiometer lowers voltage by voltage divider operation. It
wastes lots of power compared to what it supplies to the load. A
variac is a variable auto transformer. It transforms one product of
voltage and current to a nearly equal product of a different voltage
and different current while wasting only minor core and copper
losses. The mechanical equivalent of a variac would be a variable
ratio pulley arrangement that converts one product of speed and torque
into a nearly equal product of a different speed and torque, while
wasting only a small fraction of the energy that passes through as
heat in the belt, sheaves and bearings.

--
John Popelish

[Peter Bennett]

A potentiometer is a variable resistor - by turning the shaft, you can
slide the tap along the resistor element.

A Variac is a variable transformer (usually an autotransformer (only
one winding)), where you can slide a tap along the windings of the
transformer.

You can use the potentiometer as a voltage divider for both AC and DC,
but the Variac will only work on AC. (and I believe Variac is a
registered trademark)

--
Peter Bennett, VE7CEI
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[David]

A variac is a 'variable transformer' such that you can tap a percentage of
the input. It does not offer isolation from the supply though so care must
be taken. A potentiometer is a fixed value resistance track with a wiper
blade that can tap this resistance along its length.
Entirely different items and ceratinly not transposable!

[Andrew Cooke]

In that case (if it's an autotransformer) could you (at the risk of
confusing everyone) think of it as the inductive equivalent of a
potentiometer - the resistance being purely reactive?

I think really I'm not sure what an autotransfomer is because what I
described above is what I understand from your description, but I don't
see how it will source higher current at lower voltage (as an ordinary
transformer does). I'll go and search the net for autotransfomer and
see what I can find...

Confused,
Andrew

[Ratch]

"Andrew Cooke" <and...@andrewcooke.free-online.co.uk> wrote in message
news:3BAD9E98...@andrewcooke.free-online.co.uk...

>
> In that case (if it's an autotransformer) could you (at the risk of
> confusing everyone) think of it as the inductive equivalent of a
> potentiometer - the resistance being purely reactive?

A Variac is a autotransformer, no doubt about it. Resistance is never
reactive, especially purely reactive. It doesn't do much good to try to
find similiarities between a potientiometer and a transformer.

>
> I think really I'm not sure what an autotransfomer is because what I
> described above is what I understand from your description, but I don't
> see how it will source higher current at lower voltage (as an ordinary
> transformer does). I'll go and search the net for autotransfomer and
> see what I can find...

An autotransformer is a single winding transformer. You need to obtain
and read a good basic textbook that covers this subject. Ratch

[John Popelish]

Andrew Cooke wrote:
>
> In that case (if it's an autotransformer) could you (at the risk of
> confusing everyone) think of it as the inductive equivalent of a
> potentiometer - the resistance being purely reactive?

Not at all. If a voltage divider is made of two separate inductors,
then it is an analog of the resistive divider. But the coils of an
autotransformer all share the same magnetic field, so there is an
additional coupling between the inductance above the tap and that
below the tap that is missing from the two series inductors. This
coupling forces all the turns in the auto transformer to have
approximately the same voltage per turn. This is not the case for the
pair of inductors. In effect, an auto transformer is a transformer
except that some of the turns are part of both the primary and
secondary. But all transformer effects (except isolation), like
voltage ratio approximately equals turn ratio and current ratio
approximately equals inverse of turns ratio, apply, once you figure
out which turns make up the primary, and which make up the secondary.
And you have to take into account that some of the turns carry both
primary and secondary current.

> I think really I'm not sure what an autotransfomer is because what I
> described above is what I understand from your description, but I don't
> see how it will source higher current at lower voltage (as an ordinary
> transformer does). I'll go and search the net for autotransfomer and
> see what I can find...

--
John Popelish

[Andrew Cooke]

John Popelish wrote:
>
> Andrew Cooke wrote:
> >
> > In that case (if it's an autotransformer) could you (at the risk of
> > confusing everyone) think of it as the inductive equivalent of a
> > potentiometer - the resistance being purely reactive?
>
> Not at all. If a voltage divider is made of two separate inductors,
> then it is an analog of the resistive divider. But the coils of an
> autotransformer all share the same magnetic field, so there is an
> additional coupling between the inductance above the tap and that
> below the tap that is missing from the two series inductors. This
> coupling forces all the turns in the auto transformer to have
> approximately the same voltage per turn. This is not the case for the
> pair of inductors.

Thanks - that makes it obvious (I'd forgotten about the magnetic field,
as you guessed).

Andrew