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

The use of a variac in series with a SINGLE diode and filter capacitor is no
more dangerous than the light bulb connected in your ceiling. If both are
wired correctly (the shell of the socket should be connected to the neutral,
and the 'common line' of the variac should be connected to neutral, you'll
have one side of your circuit connected to neutral, and the other connected
to the 'hot' wire.

Connect the input wires backwards, and both circuits will work, but you'll
no longer have a 'safe' condition. The shell of the light bulb will be hot
and will be a potential shock hazard when you change bulbs. Similarly,
you'll have a situation where neither end of your power supply will be able
to be connected to ground. That's a definite shock hazard with the machine
since one line of the power supply is usually connected to the machine
chassis.

So, with a SINGLE diode (also called half wave rectification), you can have
a 'safe' system, but you also have an easy potential for error.

But, most unregulated CNC power supplies use a rectifier bridge to produce
full wave rectification because you can use a smaller capacitor and still
have relatively low ripple. You can wire a bridge rectifier circuit with
either a transformer with two windings or a variac as the input. But, with a
bridge rectifier, even if the variac is wired with the neutral wire
correctly connected, you cannot connect either end of the bridge's output to
a ground without shorting out a diode. This is the configuration that is
inherently unsafe, no matter how you wire it.

Now, with a transformer that has two windings, you'll note that there's no
connection (other than the magnetic flux circuit of the core) between the
input and output coils. This means that the power input can have a neutral
line at ground potential. If you don't connect either of the output coil's
wires to the ground, you can connect the negative end of the diode bridge to
the ground, and can also connect your machine to ground, making for a safe
system.

I hope this explains it a little clearer. However, when all is said and
done, my adivce follows the other folks - steer clear of the variac as the
ONLY transformer for an unregulated power supply.

-- Jerry

|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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|OFF Topic POSTS: General Machining
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|aol://5863:126/rec.crafts.metalworking or go thru Google.com to
|reach it if you have trouble.
|http://www.metalworking.com/news_servers.html
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|http://groups.yahoo.com/group/jobshophomeshop I consider this to
|be a sister site to the CCED group, as many of the same members
|are there, for OT subjects, that are not allowed on the CCED list.
|
|NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
|THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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