Re: How many steps in a variac

On Mon, 9 Feb 2009 22:36:05 -0600, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:v0v1p4tdafeqnkn8t6n99mo9a7t6oag82k@xxxxxxxxxx
On Mon, 9 Feb 2009 19:49:54 -0800 (PST), Gene <genecolgrove@xxxxxxxxx>
wrote:

For a lab bench variac that has a dial connected to a brush that
slides across the winding, does anyone know how many winding/taps
there typically are? Or another way of putting it, what is the typical
voltage step between windings? Say there are 100 windings, meaning
about 1.2 V per winding (ignoring for the moment that it might have
taps for output voltages > input voltage). Would it really go down to
1.2 volts (very first tap/winding)? I'm interested in voltage control
at low voltages.

Gene

The carbon brush "interpolates" between windings, so resolution is
better than one turn of the winding.


Thats simply not true(for the most part). It will take ignore any windings
after the tap. You are essentially shorting out the windings with the brush
so how could any of them after the first matter? (they do matter in that
there is non-zero resistance but 99% of the current will go through the
first winding.

It imagine if you think about it for a second you'll realize your error.

|
|
-----
--/\/\/\/\/\/\/\

(think of it as a variable resistor if you like)


I prefer to think of it as an autotransformer, which is what it is.
Note that BOTH winding ends are connected across the AC line. You only
drew one end, like a rheostat, which it ain't.

Why do people make aggressive, authoritative, and totally idiotic
statements without taking a couple of minutes to google the facts, and
maybe learn something?


http://en.wikipedia.org/wiki/Autotransformer

"By exposing part of the winding coils and making the secondary
connection through a sliding brush, an almost continuously variable
turns ratio can be obtained, allowing for very smooth control of
voltage."


http://www.meinstrument.com/variac-transformer.htm

"SMOOTH AND LINEAR OUTPUT: As voltage between turns is small and as
the brush-arm is in constant contact with more than one turn, harmful
sparking is avoided and output voltage can be set very easily to a
fraction of a volt from zero onwards by rotation of the knob."


http://books.google.com/books?id=rZXg6hL93HEC&pg=PA593&lpg=PA593&dq=variac+autotransformer+brush+smooth&source=web&ots=csenlQB2Xj&sig=BDownR0oCCUTe3jSvIUCkkLlo0Y&hl=en&ei=pxuRSeWJB5nMsAPcu9y3Cw&sa=X&oi=book_result&resnum=3&ct=result

section title "Stepless Control by Variacs"



The bush contacts several windings. Any windings after the closes one
contacted will be ignored(again, for the most part). Simply the past of
least resistance in action. (the windings are not in parallel.


It's a TRANSFORMER. Shorting turns would fry it, except for the
special resistive carbon brush. The brush does interpolate the turn
voltage and give, as they all say, smooth and stepless control.

I read in an old GR manual that the brush is an anisotropic electrical
conductor, which improves the shorted-turn-current versus contact
resistance tradeoff, but I can't find any quick references online.
Probably the old GR patents mention this.



similarly you have something like

|
|
+----+----+
| | |
---R1--R2-+-R3-+-R4-+-R5----


The current will ignore R3,R4, and R5 assumine the wire thickness is much
lower than the resistor/winding resistance.

i.e., it is identical to

|
|
+----+
|
---R1--R2-+-R3-+-R4-+-R5----


I imagine it might be possible for the windings to share the current do some
degree because of the contact resistance(specially if lots of current is
going through it causing it to heat up locally). Obviously the contact(in
my diagram) forms a voltage divider at the points.

If I took lessons from the assholes on here I'd bitch you out for being
wrong...


But I'm not. Look it up.

John


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