In article ,
Richard J Kinch wrote:
DoN. Nichols writes:

Powerstats or Variacs are autotransformers- the terminal connectors
have a wire path to the incoming AC line. Do not use this as a power
supply without a transformer for isolation.

Amen!

A rectified autotransformer is simply a DC power supply that is ground- or
neutral-referenced. Nothing wrong with that. Indeed, it protects against
certain faults. And it would be reckless to permit a floating supply to
operate in the presence of faults.

Not if the hot and neutral are interchanged -- anywhere between
the breaker box and the Variac. Get a three-prong plug which someone
has cut the ground pin from, or an older one without a ground pin, and
the chances are that everything will be hot.

And with a bridge rectifier (which he is proposing), each side
of the load alternates being connected to the low or high side of the
feed with each half-cycle. Better to have an isolation transformer, and
to ground the proper side of the output power.

Being ground-referenced, such power supplies cannot be stacked, or used to
power grounded loads, which must be what leads to the fear of using them.
In some applications they cannot thus be used, but in many applications,
where the load is isolated, this is of *no* consequence, and rectified
autotransformers are economical and appropriate.

Consider my points above.

Enjoy,
DoN.
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