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)