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Re: [TCML] variac question


When you way "switched off", do you mean that there is a toggle switch which you have opened, or just that the tap knob is set to "zero" volts?

If there is a toggle switch, then it may be incorrectly placed in the neutral side of the circuit, instead of on the hot leg of the circuit, or the switch is shorted, or else your outlet is wired backward. A DPST switch on the power which breaks both the hot and neutral lines would be even safer, but probably would not meet the economy standards of an inexpensive import.

Ordinary Variacs are not line isolated, so their outputs are directly connected to the hot lead of your house wiring.

If you look at the schematic of a properly wired Variac, you will see that there is a direct path for the 120 VAC from the hot input line, through the winding, to the tap brush, limited only by the impedance of the winding. If your tap is even one turn above the neutral end of the coil, you can still get a full 120 VAC on the tap. Since the impedance of your hand is probably a lot higher than the impedance of the winding, you will get most of the 120 volt line potential on your fingertip.

I would pop the cover on the unit and see if the toggle switch is properly wired in the hot end of the circuit, and that the switch is not shorted. If it is all OK, then check the wiring of the outlet to make sure the black lead is on the brass screw. For extra credit, you can install a DPST power switch to open both the black and white power leads to the Variac.

In any event, you have to remember that the output of any Variac is still "HOT", at line potential, regardless of the knob setting. You have to run it through some sort of transformer isolation before you want to risk touching it. Even better practice is just to never touch a potentially energized lead. You never know what sort of potential may actually be connected to it.



On 1/25/2015 12:33 AM, John Cooper wrote:
I have a 2000VA chinese variac off ebay looks similar to a staco(.  No schematic.  It works fine when used normally.  Varies my voltage from 0-148V.  But when its switched off and left plugged in I have 0 V between and neutral and 120V between hot and ground and neutral and ground.  Is this normal for a variac?

I was hooking up a electromagnet @5V with the variac off but plugged in and I got a 120V bite even when the dial is all the way to 0.  That’s why I started checking it. Ive been running my coil off it with no problems.

John Cooper WT5Y

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