Re: 577D1 itching issue - Haven't touched since, currently in storage

Chuck Harris


It is a very bad idea to short out a turn on a transformer.

But it is a very good idea to load a turn or two on a transformer...
after all, that is the transformer's job.

Here is what your variac's brush really is:

Turn1 <--Resistor---+
Turn2 <--Resistor---+-----Brush terminal
Turn3 <--Resistor---+

The voltage between turns is around a volt on most variacs.
The resistance of the brush material is chosen so that its
the power dissipation will be 1 or 2 watts, for a typical

If it is essential that a variac go all the way to zero, there
will be a metallic landing pad for the brush to land on after
it falls off of the last turn.

Absent the landing pad, the lowest voltage will be on the
order of 1/2 turn's voltage.

The variac was originally designed to be a lamp dimmer for
theater use... it replaced the very wasteful carbon pile
rheostats originally used.

This curve tracer application is forcing an oval peg into
a round hole. It sort of fits... if you force it.

-Chuck Harris

Dennis Tillman W7PF wrote:

Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac works. They appear to be pretty simple and a very clever. This has accounted for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a transformer with 120 (more or less) turns and 120V across it. So tomorrow I will test this by intentionally hooking a short piece of wire from one turn to its neighbor while I am holding the wire in my hand. If you are right the wire won't get hot and I will learn something new. If I am right I will get burned and never trust the three of you again. Either way you have nothing to lose. :)

I will report the results either way.

Dennis Tillman W7PF

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