Instructables
Picture of Bare Variac Build

A Variac or autotransformer is something I've always wanted. They're an item I just don't see every day though in my travels. One day I did happen to come across one while I was shopping at a flea market. The fellow selling it wanted $5 for it which I thought was fair, so I bought it. Only thing was all I got for $5 was the bare Variac and a panel plate for it too.

In this project I had a real well I never moment that I hope I can share with everyone that reads this. Enjoy!
 
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Step 1: Well Now I Have a Variac

Picture of Well Now I Have a Variac
VariacCCW_R1.png

Sure I'd heard about Variacs, or autotransformers as they are generically called I even had a fair idea what one did, but not exactly how it does it. Just what makes this gadget tick, and more importantly how do I hook it up?

It can't be all that hard can it? After all a Variac is just an adjustable transformer and I've hooked up plenty of transformers in my time. All any transformer is is just a coil of wire right? The five studs that stick out of the thing are obviously the connection lugs for it. Why I'll just get out my trusty multimeter and ring the thing out.

Well those were my thoughts at the outset of this project but once I began things quickly descended into what can only be described as chaos. those 5 studs yield 20 possible combinations of pairs of connections and I rung some sort of a reading out of every one of them!

The graphics on this page are digital reproductions of the tables I made on a piece of paper while I was in the initial discovery phase of my Variac. I made two of these, because of course there is a control on a Variac and these readings change as the control is manipulated. The numbers in the tables are Ohm readings I recorded measuring pairs of terminals to each other.

Measurements that are supposed to match and don't is because my trusty multimeter isn't the best multimeter I guess.
iceng made it!9 months ago

STD wiring lets you use the top to transform for a useful additional voltage above the mains voltage for testing designs for safety withstanding abilities.

A

Variac.gif
laserjocky2 years ago
I was ridiculously lucky when I got my variac. I work in a university lab and the adjacent lab was cleaning out ancient equipment. They left it out as ewaste to be recycled and told me I could have it. It had the old clothbound cord and has a tag indicating it once belonged to a nuclear research group (added fun). I fixed it up with a modern grounded cord and a nice wooden enclosure, and fixed what was actually wrong with it: the contact had filled its track with graphite. Wiped it down with alcohol and it was good as new. I had similar issues demystifying what was actually going on inside the thing when connected to power. Eventually, I just had to take a leap of faith (buffered by a 10A fuse, of course) that my electronics professors were telling the truth about inductors. They were.
Oh, I so envy you! Finding a variac (even one with-out a case) for 5$?!
I would love to have a variac as personally I consider them great for AC bench supplies... and as you pointed out they are great for testing unknown transformers, without receiving any nasty surprises!
Oh well, I will just have to put up with working with DC only for now...
All I have got are a selection of AC-DC power converters, (incl. a nice one variable one meant for laptops - goes from 12- 24 volts and has a 5v USB on it too....) and two ATX computer power supplies,that I am too lazy to actually set up nicely.
pfred2 (author)  BrefelanDesigns2 years ago
I've seen folks get cased variacs for pretty cheap. You just have to look around. Mine isn't very high amps so it's usefulness is limited.

I recently watched this about setting up an ATX supply:

http://www.youtube.com/watch?v=3LBsFrbPzb8

I consider most ATX bench PSU mods pretty worthless, as in the ones with some banana jacks sticking out of the case. The linked video I think has some merit though.
Thank you for the link to the video, I need to keep an eye out for variacs, might try the flea market in Felton,DE... though for me that's a bit of a trip.... Anyways (I seem to use that word a lot it seems) I also found your youtube channel (I think) though your comments on that video....
Dr.Bill3 years ago
You scored on your Variac too for $5. Not bad.
Got any idea how many Amps it will handle?
Mine are rated at 2 Amps output.

I have heard these things crackel when too many Amps are pulled through them.
You don't want to do that.
Your Variac looks like it might be able to handle more.
You might want to Google Staco Energy Products and have a look at all the different models they offer. You might find yours there with all the spec sheets.
pfred2 (author)  Dr.Bill3 years ago
The plate on mine says 3 amps. It took me a while, and trying just about every possible combination of hooking it up, but I have mine doing exactly what I want it to now. The data sheet would have been helpful when I first got it.