How to connect a variac

[rayma]

This I don't understand, a VARIAC is a variable transformer, so its output
should be isolated from the wall AC right? Even if it is a 1000VA Variac
when set for 10V output, which can easily be confirmed by a DMM, it
should not be harmful right?

A Variac is a variable autotransformer, with one winding. It is NOT isolated.

 

[lgreen]

A Variac is a variable autotransformer, with one winding. It is NOT isolated.

Ahhhh, now that makes sense! In that case I would believe you that it is dangerous. I somehow thought these were built with a primary and variable secondary so it would be isolated. Like the 2nd one in this article-

Where they call non isolated devices "autoformer" not variable transformer.

Winding Configurations | Transformers | Electronics Textbook

but right, "variac" is autotransformer"

And here is something I would think similar to what most DIYers would have- says not equipped with an isolated transformer. Guess I did not know my equipment, thanks for the thread.
Very interesting!

Bench Top 20 Amp Variac Variable Auto-Transformer with LCD Digital Display

So yes, good to know, learned something new.

 

[dinstam]

Hi rayma.
Is there some way to tell one from the other without specs information?
Dimitrios

 

[rayma]

Hi rayma.
Is there some way to tell one from the other without specs information?
Dimitrios

An ohm meter is good enough. Measure from all inputs to all outputs.
You will get a low resistance on most Variacs, so no isolation. If they were
isolated, there would be an open circuit from all inputs to all outputs.

You can be sure that the vast majority of "Variacs" (a company's trade name)
are NOT isolated. By the way, always properly fuse both the input and output
of a Variac.

 

[dinstam]

rayma
What would you expect in auto_ vs isolated?

 

[rayma]

rayma
What would you expect in auto_ vs isolated?

See post #24.

 

[dinstam]

rayma
valuable.
Thank you

 

[audiofan]

Remember that if you wire the variac yourself the fuse must be at the wiper.

 

[john20851]

I use a 500VA isolating transformer with a 500VA variac. All fused and metered. I take it it's ok to use the two together. I've not had any problems, but it's always good to see what other people think.
regards john

 

[Mark Tillotson]

Its pretty standard to run a variac off an isolation transformer, but its probably cheaper to buy an isolated variable transformer rather than two separate units.

 

[john20851]

Hi,
I've not actually seen an isolated variac, although I've not particularly looked. I made this unit some years ago, as I had several 500VA isolation transformers doing nothing. As I almost always used them with a variac, it seemed easier albeit heavier to put them both in one box with meters and mcbs.
Thanks and regards john