# The wiper in a variac

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Robert Roland
Guest

Wed May 09, 2018 6:45 pm

I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo

Phil Hobbs
Guest

Wed May 09, 2018 8:45 pm

On 05/09/18 13:37, Robert Roland wrote:
Quote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?

I'm not a big variac guy, but in the couple I've seen the wiper is made
of graphite, and spans a couple of turns. The series resistance is
small enough not to matter much for the main output, but large enough
not to cause serious loss due to the shorted turn due to the low voltage
drop between turns.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

John Larkin
Guest

Thu May 10, 2018 3:45 am

On Wed, 9 May 2018 14:47:53 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
On 05/09/18 13:37, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?

I'm not a big variac guy, but in the couple I've seen the wiper is made
of graphite, and spans a couple of turns. The series resistance is
small enough not to matter much for the main output, but large enough
not to cause serious loss due to the shorted turn due to the low voltage
drop between turns.

Cheers

Phil Hobbs

I think there's some anisotropic conduction thing going on too.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

George Herold
Guest

Thu May 10, 2018 2:45 pm

On Wednesday, May 9, 2018 at 1:37:19 PM UTC-4, Robert Roland wrote:
Quote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo

The small one I have here, (1.75 A) has a wiper that spans at least
three turns at a time, but there's ~1000 (?) turns on the thing.
(Staco type 171) I couldn't find a good image.

George H.

default
Guest

Fri May 11, 2018 1:45 am

On Thu, 10 May 2018 05:58:46 -0700 (PDT), George Herold
<gherold_at_teachspin.com> wrote:

Quote:
On Wednesday, May 9, 2018 at 1:37:19 PM UTC-4, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo

The small one I have here, (1.75 A) has a wiper that spans at least
three turns at a time, but there's ~1000 (?) turns on the thing.
(Staco type 171) I couldn't find a good image.

George H.

That's been my experience too. Spans three or four turns with a
graphite wiper.

default
Guest

Fri May 11, 2018 10:45 am

On Wed, 09 May 2018 19:37:14 +0200, Robert Roland <fake_at_ddress.no>
wrote:

Quote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?

Check out:

This is a description of a tap-changing high power distribution
regulator. Basically a Variac with a servo motor doing the switching.
They cover shorted turns when the device is between taps.

whit3rd
Guest

Thu May 17, 2018 7:45 pm

On Wednesday, May 9, 2018 at 10:37:19 AM UTC-7, Robert Roland wrote:
Quote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

Yep, the wiper gets hot, too. Sometimes that kills the
variac... usually not, though. It's just not energy
efficient because of the shorted turn, it might be amusing
to look at a FLIR image of a variac in operation.

Robert Roland
Guest

Tue May 22, 2018 7:45 pm

On Fri, 11 May 2018 04:46:44 -0400, default <default_at_defaulter.net>
wrote:

Quote:

That is interesting. Thanks.
--
RoRo

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