Re: Transformers

["Paul O. Johnson" <pojhome@SWBELL.NET>]

My Variac is a five-terminal device, Leigh. It has two terminals at the
extreme ends of the single coil and two more terminals about 10% or 15% in
from either end. The fifth terminal is the sliding contact which is free to
move all the way to the ends

If the input (120 VAC) is connected between the end terminals and the output
to one of those end terminals and the sliding terminal, the output voltage
will vary between 0 and 120 VAC as the knob is turned. If, however, the
input is connected between the two interior terminals, the output can exceed
120 VAC.

I use one of these in a homemade demo that I described on this list a year
or so ago. I discovered that most of my students don't know what an
electrical shock feels like. With this demo, they can put a variable AC
voltage between two fingers of one hand. As they slowly increase the voltage
by turning the Variac knob with their other hand, they can read an AC
voltmeter to determine the voltage level at which they just begin to feel a
tingling in their fingers, and the level at which it begins to be damn
uncomfortable. For my dry fingers, these two levels are about 70 v and 100
v. For wet fingers, the two levels are less than half those values.

Of course, I put a limiting resistor in series with the output which keeps
the current below 1 mA.

Paul O. Johnson
Collin County College

----- Original Message -----
From: "Leigh Palmer" <palmer@SFU.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, November 30, 2000 6:20 PM
Subject: Re: Transformers


> Since no one seems to have complicated this further by mentioning
> another kind of transformer, let me do so. I may save someone a
> nasty shock in the process.
>
> There is a kind of variable transformer called a "Variac". The
> Variac also delivers a stepped down or stepped up AC voltage, but
> it has only one inductor which is tapped at an intermediate point
> and equipped with an adjustable tap attached to a big knob. This
> object is a variant of a beast more properly called an
> "autotransformer". Unlike a real transformer, an autotransformer
> shares a common connection between primary and secondary. It is a
> three terminal device, but it is dressed up to conceal that fact
> in the case of the Variac. A nasty shock can result if it is used
> carelessly, expecting that primary and secondary are insulated
> from one another.
>
> If you have one of these around you should learn exactly how it
> works before you use it, and you should probably not let high
> school students use one at all in any application in which it is
> not prewired into the circuit in which it is being used.
>
> You may guess that I learned my lesson with Variacs the hard way.
> You would be right. I didn't get a shock (I barely notice 110 VAC
> shocks anyway), but I did arcweld a nice hole in the grounding
> lug of a signal generator in the process. The Variac is a useful
> device, but it should be used with intelligent caution.
>
> Leigh