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I want to make a variac

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LAN_403

LuceÅ¡

Sun Oct 02 2011, 08:33PM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
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Posts: 149

I'm getting real sick of how much a blame variac costs! I had the idea... WHY NOT MAKE MY OWN!!?? but what kind of a core does it use? is this even a good idea? looking at my 3.8 amp variac it just looks like a ferrite toroidal core. but I just want to be sure

thanks for you help

Thomas W

Sun Oct 02 2011, 08:41PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

my variac is 1950s era apparently
its iron core and has heavy duty enameled copper wire wrapped around it
(about 1.5mm) the hardest thing about making a variac is probibly getting a iron core and sorting out the rotating dial/ brushes system

Tom Williamson.

Forty

Sun Oct 02 2011, 09:54PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I think you could use a bunch of giant washers (would probably have to make them yourself) stacked on top of each other to form the core. insulate them from one another with either sheets of plastic or some kind of clear coat (the polystyrene dissolved in toluene "q-dope" i've been making comes to mind.)
Once you've got the round core, sand the edges down a bit so that they don't scratch through the enameled wire that you'll wrap over it. Variacs usually use an autotransformer type winding, consisting of a single wire with multiple taps. I think you'll need atleast two layers wrapped onto the core: the first will be fixed and act as the primary, and the secondary will be formed by the center tap and the variable brush contact. for the sliding contact you could just use a carbon brush from a large motor.
Since it will be run at 50 to 60 hz, I don't think you want to use ferrite, but if you could, then the large ferrite rings used on the focusing coils of crt screens would be perfect for a core.

IamSmooth

Sun Oct 02 2011, 10:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

If you are building this because of cost then look at eBay
There are a lot of them for a few dollars

LuceÅ¡

Mon Oct 03 2011, 12:21AM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
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A 25amp is just over 100 dollars... I just spent most of my money on 2 833a's
:D so a variac is last on my list, as far as budget goes :(

dude_500

Mon Oct 03 2011, 12:29AM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

You could make a not-so-nice variac where you have to manually set tap points then power back up, but to actually make a smoothly operating variac that works as well as a retail product will take serious skilled machining and you'll almost certainly dump way more money into the project than you would have just buying one. And that's not even to mention that winding the toroid to sufficient quality will probably make you run away from the project on the first day, anyways.

That being said, if you want to put in the time and effort into making a winding machine and getting all the contacts and mechanical aspects working, it would be a beautiful project. But it would likely take many weeks or months to get right.

LuceÅ¡

Mon Oct 03 2011, 01:31AM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
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Posts: 149

winding it would be a turd, but fortunately I have run a lathe and a milling machine since I was in 2nd grade, so the machining part won't be an issue, I think I will keep thinking about this one for a while, I'm not going to let this idea drop like the rest

radiotech

Mon Oct 03 2011, 07:42AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

If you have a lathe, and are handy at machine work, try and locate an old
electric motor, say 1 HP 1725 RPM. You can push the shaft out of the rotor
in a press. (that's how it was put in) Then mill off one end (aluminum rotor bars
joiner. This will leave you with a iron core made up of circular thin laminations
that can be gotten off fairly easily. If you work carefully you may find enough
wire in the motor to make a variac.

Partial parts list of a 0.75 HP motor (usually only seen by motor winder shops)

Dr. ISOTOP

Mon Oct 03 2011, 09:01AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

A DIY variac would be a great novelty, and would sort of make sense if you had a giant laminated iron toroid sitting around. I wouldn't recommend it to save money if you don't have a core, though. Surplus variacs are not expensive on Ebay (though shipping is slightly painful; then again, shipping a toroid would be equally painful).
With that being said, Finn Hammer made a variac once, so it is certainly doable...though I don't think he laminated his own toroid.

LuceÅ¡

Mon Oct 03 2011, 11:31AM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149

Hmm...I like the motor idea I think I will try to track down a motor like that, I will probably see one at la seur this spring

Would just your every asycronus motor work, because I have an unused one.

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