How do I connect a variac?

[slidincharlie (Carlo P)]

Absolute beginner question:
how do I connect a variac?
I bought a variac a few years ago, just in case. Now I need it to test a piece of gear but I don't know how to hook it up and use it
I just need to supply 230 VAC power to my amp to test it with optimal supply (the wall power supply at home these days is pretty low).
Below I post two pictures of the variac.
How do I connect it to the wall outlet with a three-wire cord and then to the amp? Which lugs should I use and how?
It is sort of open frame unit. May I use it safely just putting it on my wooden bench or do I need any extra safety measure?
Many thanks in advance.
--Carlo

[g1]

Wow, that is a very nice (and expensive) unit.
Here is a link to the manufacturers info for their entire variac line:
www.ietlabs.com/pdf/Datasheets/Variacs_all.pdf
pg.16 shows specs for your model (W10H). It can give 280V out from 240V, so I think even with your 217V in, you should be able to get near 240V out. It is rated for 1200W, so no problem with current capability.
The unit will be as safe as you are, some people are ok with a loaded gun, some aren't . If you have it properly wired and only touch the dial, you are safe. But you may want it in some kind of enclosure (even wooden box) so there is no chance of touching any exposed parts aside from the dial.

I'm hoping someone more familiar with european voltage will explain how to wire it up. I usually only deal with 120V, so I don't want to overlook anything.

[eschertron]

The conversion from Euro Volts (EUV) to US Volts (USV) is about 1.8:1 so that's why the voltage is higher in the European Union

[but seriously] If you use terminal 2 as your common (per the nameplate suggestion) then hooking the line hot to terminal 5 will give you the design 240vac in 280vac out (max) that G-one mentioned. It's all printed on the terminal nameplate, with the design voltage at all the different taps. With 217vac as your hot voltage (connected to terminal 5), then you'll get a max of about 250-ish on the wiper.

So the neutral, or common, output of the variac is on terminal 2 ( or wherever you're tapping into for common), and the hot output from the variac is terminal 3, the wiper. Be sure to set the wiper voltage (checked via multimeter) to what you want for the amp before powering up the amp.

I'll affirm that you don't want to handle the variac, or touch any part other than the bakelite knob, once it is all wired up. Whil the thing is heavy, and probably won't go sliding around your workbench, a nonconductive wooden frame is easy enough to slap together, and can keep fingers away from the sides or back of the variac.

[slidincharlie (Carlo P)]

Thank you g-one for the link to the pdf. I had searched for Technipower (the label on the nameplate) but the only thing I got was a link to a staffing solutions firm
eschertron let me see if I got it right:
1) neutral from the wall socket to terminal 2
2) hot from the wall socket to terminal 5
3) neutral from the amp power cord to terminal 2 again
4) hot from the amp power cord to terminal 3.
Correct?
Is there any difference between the two wires going to the amp, which I called neutral and hot in points 3 and 4 above?
Is there not any use for the ground wire that is in the wall socket as well as in the amp power cord?
Please forgive me if I said nonsense

[g1]

Is there any difference between the two wires going to the amp, which I called neutral and hot in points 3 and 4 above?

No, there is no difference. You are calling one neutral for convenience but you actually have 2 hots. Properly we should probably call them H1 and H2 but calling one neutral will work. Your understanding of which number tabs to use is correct.

Is there not any use for the ground wire that is in the wall socket as well as in the amp power cord?

The ground (middle pin) wire should be connected for safety. Ideally from the outlet to somewhere on the casing of the variac, then on to the ground wire of the cord going to the amp (which should be connected to amp chassis).

[slidincharlie (Carlo P)]

The ground (middle pin) wire should be connected for safety. Ideally from the outlet to somewhere on the casing of the variac, then on to the ground wire of the cord going to the amp (which should be connected to amp chassis).

May I use either screw connecting the nameplate to the front metal housing (red arrows in the photo below) as a grounding point? Each screw does have continuity with the front housing.

[g1]

Yes. The screws also go into that baseplate, which is where I thought you should ground to.

[slidincharlie (Carlo P)]

I did the hookup job. I tested the variac without any load and it works.
Only odd thing is that the AC voltage increases when I turn the knob counterclockwise. Shouldn't it be the other way round?

[eschertron]

I did the hookup job. I tested the variac without any load and it works.
Only odd thing is that the AC voltage increases when I turn the knob counterclockwise. Shouldn't it be the other way round?

If you look at the way the wiper moves across the coil, and the parts of the coil that lead to the fixed taps, you should be able to see how the variac is constructed (electrically as well as physically). Now that you've reported the unit's behavior, I can imagine - while looking at the faceplate diagram - that as the wiper moves clockwise it travels from terminal 4 to terminal 2, not the other way around. And, yes, it surprises me also.

Swap terminals for their mirror image (2 for 4, 5 for 1) if you want the mirror (CW versus CCW) behavior. Good luck and be safe!

[slidincharlie (Carlo P)]

Swap terminals for their mirror image (2 for 4, 5 for 1) if you want the mirror (CW versus CCW) behavior. Good luck and be safe!

Since terminal 2 (that I am using for commons) is on the same side as terminal 1 on the wiring diagram on the nameplate, maybe I should only swap 5 for 1, right?

[eschertron]

Since terminal 2 (that I am using for commons) is on the same side as terminal 1 on the wiring diagram on the nameplate, maybe I should only swap 5 for 1, right?

No, Non, Nyet! To change rotational direction, if the wall voltage (217, 230, whatever) is moved from terminal 5 to terminal 1, then we need to move the commons (all of them!) to reference ~240vac away, which is terminal 4. Look at the faceplate with the voltages (actually voltage differentials) printed. One of the two endpoints (2 or 4) MUST be the common reference, and then the terminal closest to 217 volts difference from that will be the line hot wire. So if 2 (neutral) and 5 (hot) worked but CCW, then 4 (neutral) and 1 (hot) will work for CW. Remember to make both circuits use the same common wire (also called neutral). The only wire not moved is the wiper. Oh, and the ground can stay where it is.

[Jazz P Bass]

It's just like a potentiometer (err, big rheostat)

[Leo_Gnardo]

It's just like a potentiometer (err, big rheostat)

Indeed, an electromagnetically enhanced potentiometer. Whooeee! (put mad scientist smiley thing here.)

[Jazz P Bass]

[Leo_Gnardo]

[ATTACH=CONFIG]32225[/ATTACH]

Yessindeedy, just like THAT! Nice find - thank you Jazz! Note a family resemblance, maybe Vector is one of my distant cousins.