How many steps in a variac

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

John

Gene wrote in news:6564dc10-3bb0-47d4-b2fb- snipped-for-privacy@z1g2000yqn.googlegroups.com:

On most of the variacs I've looked at closely, the brush contacts several windings and it's hard to tell if there's a "step" in voltage. I'd recommend using it with a low voltage transformer; I think you'll get a finer range of voltage control.

--Damon

"Gene"

** Google Groper Alert !

** Connect an AC volt meter to the output and find out for yourself - d*****ad.

BTW

Only an utter moron use a variac to derive a low voltage AC supply.

Cost it is stupidly dangerous.

..... Phil

As a sideline issue, the comment about the wiper arm shorting one winding intrigued me.

That > ** =A0Connect an AC volt meter to the output

Cost is dangerous? Please explain.

If you've got a variac that outputs the range

0-line voltage and you output to the primary on a 12v transformer you get very fine control of the range 0-12 v ac.

With a 6v transformer instead you get very fine control for the range 0-6 v ac.

How is this so dangerous, aside from cost?

"Greegor"

As a sideline issue, the comment about the wiper arm shorting one winding intrigued me.

** There is no hard short.

See the other replies.

That one winding would produce low voltage and high current,

** There is no hard short.

See the other replies.

does that have a history of causing failure?

** There is no hard short.

See the other replies.

Cost is dangerous? Please explain.

** Good safety practice is to *never connect* a load to a variac that CANNOT handle the FULL output voltage - connecting a load that can only handle a tiny fraction of that voltage ( like 1%) is plain NUTS !!!

PLUS a variac provides NO isolation from the AC supply - so a missing or reverse connected neutral conductor means that both connections to the variac are at a dangerous ( ie full line ) voltage.

If you've got a variac that outputs the range

0-line voltage and you output to the primary on a 12v transformer.....

** That was never the OP's Q nor relevant to my comment.

Please learn to read before posting here again and wasting people's time.

Dick wad.

...... Phil

Now that's something I would really like to know!!! I would give my let nut for that answer!

Shorting one (or two, or whatever) resistors in a string is NOT the same as shorting one (or two or whatever) turns in a transformer because the resistors are passive dividers of the applied voltage across the entire chain; while each turn of the autotransformer independently dissipates the applied power, at the same time, each turn also *generates* power as a result of being wound around the powered magnetic core.

huh? A turn generates power and dissipates power? As far as I know a winding doesn't generate any power.

The electric field is converted into a magnetic field which is then converted back into an electric field at a different intensity.

So you disagree with wiki? Maybe you should rewrite the page?

Look at the first figure... do you see that big squiggly line? Do you know that is called an inductor? Do you see the taps?

Do you realize you can draw it as a series of inductors where connected by wires andt he taps taken at the wires?

i.e.

----/\\/\\/\\---tap1---/\\/\\/\\/\\---tap2---\\/\\/\\/\\/\\---

??? Surely you do get that much?

Do you realiez that one can conceptualize it as resistors? Maybe this is the leap of faith you have? Luckily good basic run of the mill impedence comes to the rescue:

OMG!!!!

"just as impedance extends Ohm's law to cover AC circuits, other results from DC circuit analysis such as voltage division, current division, Thevenin's theorem, and Norton's theorem, can also be extended to AC circuits by replacing resistance with impedance."

Do you understand that? The autotranformer is just one big voltage divider. abstractly at least... it does this a bit different than a resistive network but it works out to be the same. After all, we were talking about the current instead of the voltage reduction. For resistors the voltage is reduced by the drop in potential across a resistive element. For the inductors the voltage is converted into a magnetic field but then converted back into an electric field. Different principles at work but same abstract analysis. (This is because the current through the windings is exactly that... regardless of what the inductor is doing to limit the current.

In fact since it seems to escape you guys:

V---L1--A--L2---gnd

What is the voltage at point A? Is it L1*L2/(L1+L2)?? Assuming the inductors have the same construction so there inductance per turn is the same then it works!!! Again, wiki proves this!

A = V*L1*L2/(L1 + L2)

If L1 = L2 then we have

A = V/2*L!!!!

Which is exactly which one expects!

What is that? Voltage divider law? Same principle holds for resistors too, and capacitors? right? Get it now? (Note L really is X_L = jwL but in this case the magnitudes are identical except for the factor of w!!!!!! Or just think of the symbol L as a shorthand for X_L.. nifty huh?)

I know it's hard but think!!! Or you can just jump on the band wagon... I hear Phil "FUCKWIT" Allison is taking applications. (You just have to go around calling people Fuckwits and saying they are wrong and prove it by changing the hypothesis. Benji is doing a good job of that and I think larkin is taking notes.)

Waiting on Benji to reply and call me an asshole again. This time he probably won't even read a tenth of the post.

...which has interesting implications when someone gets the bright idea of using a Variac as a really big high-current inductor.

--
Guy Macon

You're treating all the windings as separate inductors, but ignoring the mutual inductance. All those turns are wound on the same core.

John

[snip]

Been many years since I even thought of a transformer. I seem to remember something tricky about AUTO-transformers??

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

There actually are isolated VARIACs available (I have one), although in general you wouldn't want to that without testing it... Probably less than 5 VARIACs in 100 sold is isolated?

ing

w y

the

s

r.

ork

ed

ct

ors

it

,

his

t

Slaughter murders yet another analysis!

Here's a puzzle for you:

Apply your inductive voltage-divider "theory" to an autotransformer set anywhere else except at the halfway point - say, at the 1/3 point. Then the inductance of the short side is 1/4 of the long side (recall that L is proportional to the number of turns SQUARED). Therefore the output voltage should be 1/5 not 1/3. Where did you go wrong?

-- Joe

On Feb 10, 12:37=A0am, John Larkin wrote

Thanks.

When was the last time you set a variac on your bench and actually MEASURED the output?

Thanks, Rich

It's just a regular transformer with the windings connected in series.

John

Let me count the ways....

John

Do you mean left nut, or lug nut?

John

The brush resistance will kill the Q a little, but the losses are obviously not serious, since Variacs work.

John