Tuesday, February 1, 2011

New Variac

So for along time one of the essential tools I have been missing in my line of work was a variac. A variac is just a coined name for a variable autotransformer. It allows you to control the wall voltage directly. Unlike a rheostat this device simply steps down or up the mains voltage to a desired level. They range in power from 100VA (essentially 100W) to several thousand VA and so much larger industrial ones regulate grid voltage with a motor driven tap changer that can regulate line voltages at thousands of volts and millions of VA.

These things are easy to use! Just plug them into the wall or any source of AC! power, bring the knob down to the 0V position, plug in a load and then flip the switch on. Now by carefully varying the knob from 0 to 100% or even above 100% you control the mains voltage. This is handy for compensation due to voltage drops on long lengths of extension cord or running appliances that need specific voltages. Mostly what they are used for is controlling the voltage while troubleshooting or repairing appliances. That is we keep the voltage low and look for any smoke before powering the appliance at full lines voltage. It ensures you dont blow stuff up right away after fixing if you made a small mistake!

Ok now back to me bragging about my shiny new Variac... : ) I wanted a new one but I decided to look on ebay first for any deals before shopping at my local electrical supply store. Oh look whats this...

Figure 1 - TDGC-3k no-name Variac with compact fluorescent bulb for scale.

I obtained this unit on ebay from user drills12 for 100$ Canadian! and 35$ shipping in 5 days I had it on my door-step! Best deal if there ever was one!

Now I had stared at the unit on ebay for about a week or so and researched the model number because I could not believe a brand new unit rated at 3kVA at this price was even possible. For reasons of frustration in finding any information about this autotransformer I have decided to write down what I have found to help future buyers of the remaining units on ebay or for that mater any other online store selling similar units.

The unit is made in China outsourced by a russian company who's name I cant type in english and by SILICON inc. which I cant seem to find on the web?

The toriodial variable core is rated for 3000VA , 12A RMS, 250VAC and weighs 11.3Kg (11.6 with case around it). Painted red it is sold with a north american - hybrid european/asian plug on its output rather than the original manufacture's bannajack / screw terminal connections. The unit has a built in 0 - 150VAC analog voltmeter on its output so you are limited to running the unit off of 120VAC. It also comes with KIM CHAY 20A 400V circuit breaker/switch combination. The manufacturer shows the unit originally came with a 12A fuse and switch but because the voltage rating went down to 120VAC as its "rated input".

Since the core rating was originally 250V at 12A which is 3000VA im guessing when they derated it to run off north american 120V they had to increase the maximum allowable current to compensate? Since now 120V at 20A, really its a 2400VA variac.. (I^2 R losses are higher for the Rw of the autotransformer!) But given its original rating still stands true, it should work off of 240VAC. However to make it run off of 240VAC safely the 0 - 150V voltmeter must be removed and the breaker replaced with a 12A breaker. Not to mention a replacement power cord for the required outlet type and a new dial indicator for 0-280VAC (original dial is 0-140VAC).

But I am not complaining this is a well made unit. It appears to have once been sold at 3,800$ USD but that was the original, not the knock off version made in china. Considering it cost only a fraction of that and seems to be equal quality! The wiper and brush on the core looks well made and so does the core itself, very neat well contrusted. It feels heavy and sturdy - the handles are secure! The only thing that feels cheap on this thing is the giant control knob. But seriously its a giant knob theres not much you can fuss over when things get huge and become mass produced.

I couldn't resist myself, had to use a label maker and put on the power rating and my name on top. Conclusion, you cant go wrong for only 100$ consider that its made with 20$ in copper wire and transformer iron core you could always scrap if you let the smoke out or use it as a paper weight.


  1. Thank you for writing this blog page! I just bought one from "drills12" based on your observations and research. I will use it to power old tube equipment progressively.

    I printed this page as PDF for future reference.

    1. Oh nice to know this was a useful review to someone. I over estimated the copper wire price for scrapping but included the scrap metal cost of the iron core its more like 20$ in scrap metals. Still after over a year of using it and working it in with some over current on some projects it has turned out to be well worth the price. The only damage I could do to it so far was sinking one of the windings in when I melted the resin around the core after using it at 200% its rating for an hour. It still works fine after some cleaning of the wiper surface.

    2. I'm a little confused, how can you run it at 200% capacity without blowing the breaker?
      Maybe a smaller breaker is required?

  2. Hi, I'm not sure if you're still active on this blog, just a quick question, is the output balanced, thanks.