Shopping for a variac

[cdev]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

[Cubdriver]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

My guess as to highest frequency would be 'not very' - remember, these are meant to be efficient at line voltages and frequencies - I bet they get lossy fast if you start going higher.  I just grabbed some I had and am taking some photos - one is a French-made GenRad that came out of an old Balzers e-gun supply; it's rated for 2 A @ 220 Vac, 50-400 Hz.  I doubt any of them are meant to operate much above that, and that even going to 400 Hz would be uncommon unless it's something geared towards military and/or aircraft use.I suspect that even the 3 kHz bottom end of the VLF band would be too high.

-Pat

[Electro Detective]

Sounds like a plan guys 

I might one day break out the bits and pump some audio frequencies in mine with an amp and sig gen starting at 50hz and go from there

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fwiw guys: one major design flaw with most if not all variacs IMVHO, and I've seen a few made in the various countries that produce goodies that work beyond 8 hours   
is if the carbon brushes snap or wear down to nothing,
the metal housing comes crashing down and destroying the copper windings

The variac may still work at certain ranges due to direct brass or steel to copper contact grinding along   
till the operator opens it up to investigate why the unit drops out sometimes, or running warm 

Some plastic or isolating material placed just before the 'end of life' point on the carbon brush assembly should stop that from happening,
and alert the operator it's new carbon brush time when the variac starts skipping and arcing/sizzling, or not working at all.

Its a better bet than trashed copper windings which is a pain to fix/maybe,
and I will be rigging up something simple and functional to my humble threesome of variable 240 volt AC units, on the next open and inspect/clean session

Anything is possible with a junk box of 'not ready for landfill yet' scraps, bits and bobs, glue, cable/zip ties, sharp knife,  rotary tool
and bottomless cups of tea etc 

Have a look next time you're in there guys, you'll know what I mean about it,
it may bite you one day or not,
so why not step in and avoid that, with an easy knockup ugly prevention fix no one will see or care,

rather than forced to replace a decent trashed unit with a clickey clickey online shiny red one,
that's apparently only good for 7 hours and 59 minutes..   

 

[drussell]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

Certainly won't go anywhere close to up into the RF range, given the physical dimensions, but might be useful for something-or-other in some sort of project.  The '58 POWERSTAT catalog has the following curve, indicating that those would at least be fine for the 400 Hz frequency used for things like aircraft power:

[cdev]

What's inside the toroid coil, what kind of magnetics do they use in Variacs??

[ArthurDent]

What's inside the toroid coil, what kind of magnetics do they use in Variacs??

It's basically the same type of steel used in a conventional transformer wound up into a coil like a roll of paper towels then coated with something like epoxy to insulate it from the wire wound on it.

https://3.imimg.com/data3/KX/GP/MY-2618182/variac-toroidal-core-500x500.jpg

[cdev]

Ahhh.. so basically impossible to take out and replace with something else.. LOL, oh well..