Sunday, September 25, 2011

Variac Modification

A while back I purchased a Variac (Auto-Transformer) on eBay; for a rather good low price at the time. I had planned to replace or at-least supliment the cheap inbuilt voltage meter with something more useful and "cool looking". So I decided today was a good opportunity to do just that! and mounted my homemade volt/amp panel meter box to the Variacs output. The project turned out to not be that hard but did take a few hours due to finicky wiring, drilling holes, and hard to get to bolts.

Inside the variac you can see the core/case ground wire (Yellow/Green), Neutral Wire (White), Hot (Brown) and Tap Wire (Red). In the typical variable auto-transformer configuration.

Inside the panel a 20A Breaker / Switch, Analog volt-meter and outlet plug. The 50A shunt was added to accommodate the Amp-meter. The old volt-meter was originally wired to the output of the Variac but is now rewired to the input so I could see the mains voltage which is useful for troubleshooting other issues.

Here you see the panel meter box. It contains the two digital meters on the front (not shown) and 2 isolated switch-mode power supply units in the back (visible). The volt-meter runs off 12VDC and the current-meter runs of 5VDC, both power supplies must be isolated to measure both voltage and current without explosions! I like switch-mode supplies because they can run as low as 50VAC and up to 250VAC so the meters will work under different input supply voltages over a wide range. This variac was originally a 240V input 0-280VAC output so this allows me to still operate it in both European countries and in North-America without any issues (other than input meter being pinned to max).

Here are the results! Nothing high power to test out the output yet but the current meter is rated for continuous 50A I have noticed it can measure several hundred Amps for a short period of time however ;) . The volt-meter is a 700VAC true RMS panel meter both are from Sure Electronics on eBay. (Running a ~23W Compact Fluorescent Light seen below)


  1. Wow, a customized variac meter! I prefer digital meters over meters with needles. how about the accuray? is there a significant difference with the original meter?

    variac transformer

  2. The analog one was pretty poor and didn't have a reflective band so difficult to avoid parallax error so the digital one is much better it is pretty accurate too! Also the digital one lets me use 250VAC but can go up to 600VAC the original can only handle up to 150VAC. as for the current well it has no current meter so that's a bonus all in itself. Not true RMS tho

  3. I have this same variac and wanted to use the house 220 on it and was wondering how I should go about it. Can I use a 220 hook up but how would that be wired because both are in phase...Right? 2 separate breakers 180 out of phase,,,what are your thoughts on this,,,awesome page By the way

    1. I don't visit my blog much anymore so sorry for the late response.

      If it's a split Edison system it should be 180 out of phase then one way you could do this if you had only 1 breaker is to know that the breakers purpose is to protect a load connected to your variac. You could place quick blow fuses on both the hot lines going into the variac and place your breaker off your output tap. This way you are protected from the possibility of shorts to ground or neutral on either of the outputs but still have the single breaker for series load protection that is reusable for most cases. This way the fuses should rarely ever need replacing unless you happen to be running a center tapped neutral load on the output then you are relying on the breaker and the fuses.