Variable autotransformer and earth connection

[xtian]

I got this Adjust-a-Volt variable autotransformer. It's not wired with an earth connection. Should I just add a new 3-conductor mains box, where the earths connection is common and runs thru the box to the mains outlet?

http://www.electronics-tutorials.ws/tra ... ormer.html

[Leo_Gnardo]

I got this Adjust-a-Volt variable autotransformer. It's not wired with an earth connection. Should I just add a new 3-conductor mains box, where the earths connection is common and runs thru the box to the mains outlet?

Yup that's a good plan. fuse holder riding atop the switch. Nice ol' relic.

[JazzGuitarGimp]

Yes, and also ground the shell of the variac.

[martin manning]

Also be aware that the drawing above is incorrect in that there is actually only one winding (as shown in the link), and therefore no isolation of the output from the input.

[xtian]

Right you are, Martin. One coil, variable tap.

[David Root]

My Variac is newer than that and uses a 3-pin connector. But I was also told that you should use an isolating transformer ahead of the Variac/autotransformer anyway.

So I bought the Weber isolating buss box. Was that overkill?

[xtian]

I started testing this thing. Works gud. Voltage markings on dial are very accurate, according to my multimeter.

I noticed that if I have the dial set for 100+ VAC, when I turn it on, current SLAMS into it, tripping my UPS (battery backup) fault state. But if I power it up when the dial is set lower, the surge is not a problem.

Does this make sense?

[Leo_Gnardo]

Does this make sense?

Does seem odd. My 5A bench variac draws some miniscule amount of current less than 50 mA, never any funny business on power-up. While running what current does yours draw?

Possible shorts between windings where the wiper touches? With the typical graphite brush, could load up the gaps between windings there.

[xtian]

It's drawing just under 6 watts, regardless of dial setting.

Repeated the experiment several times, at various dial settings--always a big surge when set high. But the mechanism seems smooth, and my voltmeter shows a steady sweep of voltages as I turn the dial.

[Leo_Gnardo]

You'll have to develop the habit of dialing down before powering up then. Don't forget to remember.

[sluckey]

I noticed that if I have the dial set for 100+ VAC, when I turn it on, current SLAMS into it, tripping my UPS (battery backup) fault state. But if I power it up when the dial is set lower, the surge is not a problem.

I have a 10A Powerstat brand variable auto-transformer. When I turn it on I always notice a flicker in my bench fluorescent lamp, so I know there is some kind of surge going on. It's very quick and has not been a problem. It's not plugged into any kind of UPS or line conditioner though. Maybe you could just plug yours straight into the wall. Is there any advantage of having it plugged into a UPS? I would quickly get frustrated if I had to run it up and down to turn it on and off. Mine doesn't have a built-in volt meter. I mostly use mine to set line voltage to 120 when making baseline voltage measurements to document my schematics. Or when bringing up an old piece of equipment slowly to try to condition filter caps.