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Have Variac, need help
mecompco
Picked this up at a hamfest yesterday for $5.00

i23.photobucket.com/albums/b397/mecompco/Variac.jpg

i23.photobucket.com/albums/b397/mecompco/VariacCU1003.jpg

i23.photobucket.com/albums/b397/mecompco/VariacCU.jpg

I've been unable to find instructions on how to wire it--I'm also unsure what the "20V" next to each of the top terminals means. Are the two bottom terminals the inputs? Is there a facility for a ground?

Any suggestions? Also, any precautions one should take before powering it up, as it appears to be rather old?

Thanks!

Michael
 
hbuchtel
would you mind posting another picture of the front? It's a little hard to make out what is written there.

On my variac (which also had to be wired up) I wired the ground directly to the body, there were a couple of screws which could be used for this. On mine there were only 4 terminals altogether, so wiring it was pretty straightforward. Hmmm . . . 7 terminals . . . strange . . .

To be safe, always turn it to 0 when plugging it in (obvious)

I think some devices can be damaged by too low a voltage. You might want to check into that.

I don't really know much about electricity, I hope someone else can give a better answer,

Henry

[EDIT] after reading Mike's reply I decided I ought to put a heavier gauge wire on my variac, so please don't consider the picture below as a good example.

i25.photobucket.com/albums/c99/hbuchtel/IMG_6105.jpg
Edited by hbuchtel on 02/13/2006 4:17 AM
 
Mike
Yes, I agree with Henry. Would you please post another picture of the terminals with less flash glare. Is it possible to make out the embossing where it looks worn? Two of the screw terminals will be input 120 V from the wall recepticle (use 12 gauge wire minimum - I'd probably go safe with a good flexible 10 gauge (10/3)). Is one of the terminals hooked to chassis (housing) ground? One of the terminals will be hooked to the brush/slip ring. Hold on for a few minutes - I've got an open chassis Power Stat that I can get some pictures and look at the wiring on.

Mike
B)
 
Mike
OK - now let me preface this explanation with a statement of fact that I am not an electrician. I have a ground fault interupter (blended into a short two foot extension cord) that I always power my projects through to afford a little extra protection when playing with a circuit that I have to power for the first time - and most other times as well.

Power Stat is a trademark for a pretty popular brand of variacs (variable transformers). The following pictures are of an open frame Power Stat that I use on my bench. But before looking at the pictures, lets talk about where the terminals are hooked to on a variable transformer (Variac).

Visualize a single sided transformer coil wound around a single core - visualize it in a straight line instead of a circle like the variac. If we hook a power cord (supply side) to both ends of the coil. One end of the coil to the common White wire from your wall receptacle and the other end of the coil to the Black (hot) wire from the wall receptacle and, of course, the green ground wire from the wall receptacle to the housing that is holding our construct together.

If I now hook up another cord (load side) with the white wire to the same terminal as the other white (common) wire and the green wire to the same ground terminal as the other green wire we are left with the black wire from the second cord. Now if we hook that second black wire to a carbon brush that we can rub along the coil and make contact with the individual coils from end to end. We will be able to pick a voltage from '0' if the brush is at the white wire end to the full voltage seen across the original two wires if the brush is at the end of the coil which the initial black wire is hooked to. The voltage on the load side cord will reflect the location of the brush between the two ends. OK - that is how a variable transformer works.

If this visualization works for you - the next step is......to set this system up to allow increasing the voltage above that seen across the initial pair of wires. With everything setup exactly as the previous explanation, lets add another terminal that will allow me to hook a wire into this coil about 20% BEFORE the end that the black supply side wire is hooked to (not the one with the brush ) and lets remove that black wire from that end of the coil and re-attach it to the new terminal.

With this new construct we will find that if the two wires that are hooked to the coil are seeing 120 Volts (supply side) the load wires (second white and black hooked to the brush ) will see 120 Volts when the brush is held against the portion of the coil that the supply side black wire is hooked to. As the brush is moved further down the coil past this point the voltage will increase (proportional to the number of coils between the white and the brush divided by the number of coils between the supply side terminals times the supply side voltage.

If you have been counting you will have come up with five terminals so far, they are:

Common - both white wires (one end of transformer coil)

Ground - both green wires (structural connection)

Brush - load hot wire

End of coil - (other end) to control voltage up to a maximum of supply voltage

Coil tap 20% or so before the end of the coil - to allow increased voltage

Well if for some reason you wanted to control the voltage by moving the dial the opposite direction for voltage increase you could have another voltage tap 20% or so before the other end of the coil. This would be a sixth terminal.

A lot of extra teminals? - yes - gives the variac a lot of flexibility for different applications.

In the following picture you will see five terminals on the black backalite terminal block and the un-hooked green wire should be terminated at the screw hole in the frame that you can see just to the right of the wire end (this is the sixth terminal).

w3.saw.net/~mldhab/Power%20Stat/Power%20Stat%20001.jpg

If you look closely you can see in this composite picture (view from both sides) where the coil ends are brought out to the terminal block (small piece of tape under soldered connection, under the edges of the terminal block). The other two coil taps (near each end of the transformer coil) are more obvious.

w3.saw.net/~mldhab/Power%20Stat/Power%20Stat%20004.jpg

I've obviously gotten way too far into this explanation. But I don't recognize the variac that you are asking about. If the legend is worn bad enough that it can't be deciphered, I would have to take it apart far enough to allow me to trace the wires from the individual terminals (and I have to go through all this logic to figure out what I am doing).

If anyone has a better explanation (or shorter) I'd love hearing it. Or corrections / suggestions would be welcomed - and I'll edit them in.

I think you'll find that one of the terminals will be ground to the structure, and two of the other terminals will be hooked together so you don't have to hook two wires to one teminal as mine has two common (white wires) together.

It is pretty late right now - so if my explanation has done no more than confuse - I could do a quick and dirty schematic and post it if that would help. Let me know.

Mike
B)
Edited by Mike on 02/13/2006 2:30 AM
 
mecompco
Mike, thanks for the detailed and informative reply! (Polish it up and add it to the article archive) Here is a better photo of the wiring terminals:

i23.photobucket.com/albums/b397/mecompco/VariacWiringCU.jpg

The terminals are numbered 1 through 5 across the top, then the lower left one is number 6.

The top five terminals have "20v" next to them, and the bottom two have "37.5v" next to them--this makes no sense to me? From what I've found, this variac is 50's vintage. There must be 20 lbs. of copper wire in it--it is *heavy*.

I've got a GFI outlet I can plug into for the initial test if we come up to a concensus of which terminals I should use.

Thanks again to all who replied!
 
Mike
Ahh, now it becomes clear.

Supply side hookup:

- second from top left terminal, Supply Common (white from wall receptacle - wide blade in plug).

- top right terminal, Supply Hot (black from wall receptacle - thin blade in plug).

Load side hookup:

- second from top left terminal, Load Common (white wire to female receptacle - wide blade).

- top middle terminal (wiper/brush ), Load Hot (black wire to female receptacle - thin blade).

Ground wires from both supply and load hooked together and to housing (probably use one of the lowest non-terminal screw heads).

I've marked the load and supply terminals in the following picture.

w3.saw.net/~mldhab/Power%20Stat/VariacWiringCU.jpg

The way this variac is tapped - it has two extra taps which are the two outboard lower terminals - the numbers are approximate voltage differences between each terminal if wired the way I identified above.

be sure you use the GFI to supply it and have the upper knob turned all the way to the left (down) position when you power it. You could check the voltages between terminals with a meter with it plugged in. Also take a look at the brush, other than overamping and melting the coils or insulation between them - - the brush wearing out is about all that can happen to one of these. If the wiper/brush seems to move a little rough, you may want to take it apart and clean the surface that the brush rides on with a mild non-conductive abrasive (I use a green scrubie pad) and clean it thoroughly afterward.

Slowly - carefully - power through the GFI - stand away when plugging the power into the wall receptacle - first, read load side voltages at different variac settings using a meter without a load plugged in - work your way up to larger loads, use a lamp first, then keep adding 100 Watts or so at a time and excersize the variac with each load before moving to a larger load.

Mike
B)
Edited by Mike on 02/14/2006 12:33 AM
 
mecompco
Thanks! I can follow those simple instructions :-) I will report back on how it works.
 
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