Sciencemadness Discussion Board - Powerstat help? + another question

Powerstat help? + another question

Endo - 22-7-2012 at 12:41

I recently purchased this variable transformer for $15 at a techno junk store nearby. I took it home and cleaned up the brush as well as the areas across the top of the windings where the brush contacts. I hooked 120V AC across terminals 1 and 4. For load I hooked up to the center terminal 3 and terminal 4. With a tester initially it showed the supplied voltage AC varying with the setting on the dial. I came back later, after buying a step-down transformer and hooked that up as part of a project my 7yr old and I are working on. I find now that the values on the dial are reversed, I pick up 0 ac volts at the 100% setting and full 120V at the 0% setting.

Am I missing something about how this works? Do I need to use a different pair of terminals? Or is there something I got turned around mechanically?

2nd Question, On the shelf at this place they have a bunch of different high voltage step up transformers. They range with a 110V primary from 5KV up to as much as 22KV. I am thinking about buying one, but I am not sure what I really want. Projects may range from a Jacob's ladder, up to ozone or NO2 generation. If you were buying a high voltage transformer what output range would you find the most useful?

Also does a high voltage transformer behave in a linear fashion when used with a variable transformer?

[Edited on 22-7-2012 by Endo]

watson.fawkes - 23-7-2012 at 07:21

Quote: Originally posted by Endo

I hooked 120V AC across terminals 1 and 4. For load I hooked up to the center terminal 3 and terminal 4. [...] Do I need to use a different pair of terminals?
[...]
Also does a high voltage transformer behave in a linear fashion when used with a variable transformer?

You want to take your output off terminals 1 and 3 to get an output that matches the dial. The legend on the dial shows that when the dial is all the way counter-clockwise, that 1 and 3 should be shorted, or an output of 0 V, which matches the dial. Note that terminals 5, 6, 7, and 2 are fixed-output taps, which could be useful in their own right.

The output of a step-up transformer is linear as long as its input voltage is less within its rating. Go higher and you might saturate the core, which would introduce non-linearity.

Endo - 23-7-2012 at 09:05

Thanks for the help with this Watson. I see what I did now thanks.

So with a step-up transformer I can vary the input voltage to less than the rating without problem. The only problem comes if I exceed the voltage rating for the primary windings.

So choosing my step up transformer based on amperage, and maximum output at the working rating for the primary will be the best path forward.

Swede - 24-7-2012 at 07:45

Quote: Originally posted by Endo

Thanks for the help with this Watson. I see what I did now thanks.

So with a step-up transformer I can vary the input voltage to less than the rating without problem. The only problem comes if I exceed the voltage rating for the primary windings.

So choosing my step up transformer based on amperage, and maximum output at the working rating for the primary will be the best path forward.

Correct. Input voltages below the unit's rated voltage will be divided proportionally. And the limiting factor will be power. Sometimes it's possible to sneak a bit extra through the device with assisted cooling, but keep track of heat generated.

These things will not control the speed of some AC motors, which is a bit of a bummer for some people. They are happier doing resistive loads, like nichrome heater elements, cartridge heaters, etc. They are great for mantle heaters so long as the current is not excessive.