Re: [CAD_CAM_EDM_DRO] Re: variable transformer for dc power supply

I think I get the danger but I am not sure. Every light in my house is simply 110 conected to the light the only difference is that the ground is seperate from the neutral. I am not quite there yet on how the variac wiring would be anymore dangerous then the wiring in my house, I think the reason is:
In my drives the dc ground and the negetive side are hooked up together so if the ground fails the chasis becomes hot compared to ground.
Or is the reason due to the construction of the variable transformer that the two legs of the output will read the new voltage accross them but if you measure the voltage of one of the legs to ground you get the full 120. I will put a meter on the thing in the morining and test this theory.
So one way to use the variac would be to ad a line isolation transformer between the variac and my rectifier. maybee something like this http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1790265643 or just buy a fixed voltage transformer and not use the variac altogether.
thanks for watching out for my safety.
Troy




n4onl <umrk@...> wrote:--- In CAD_CAM_EDM_DRO@y..., mmiami johnson wrote:

>
> I am not sure of what isolation of a transformer exactly means.

An isolation transformer has two seperate (insulated from each other)
windings.

ILLISTRATION ONLY, DO NOT DO THIS!!!

Think of it this way, take a piece of insulated wire and wrap it
around a large bolt. Take a second piece of insulated wire and wrap
it over the first winding (wrapped wire). You just made a crude
isolation transformer.

ILLISTRATION ONLY, DO NOT DO THIS!!!

If you put a AC voltage on one winding and were to grab either side
of the other winding and ground, you'd feel nothing, your isolated
from the first (primary) winding and from the AC line.

ILLISTRATION ONLY, DO NOT DO THIS!!!

A variac is like taking one end of each of the above wires and
hooking them togeather and putting AC on that junction, with the
other AC connection on either of the free ends of a winding. Now if
you grab the last free end and ground you'll get shocked, hopefully
you won't end up in a hospital.

ILLISTRATION ONLY, DO NOT DO THIS!!!

Would you willingly cut an extention cord in two, strip the
insulation off the two wires, plug it in, then grab one of the wires?
Thats basically what you do when you use a variac to power a DC
supply and ground the negative side.

ILLISTRATION ONLY, DO NOT DO THIS!!!

I hope this clears things up.

mike


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