Hi, Ron. A variable transformer (which can also be called an
autotransformer, or by one of their trade names such as "Variac" or
"V.T."), is basically a transformer with no secondary. There is a
wiper built in which allows you to dial in the tap to the voltage you
want on the primary. The output is a sine wave like the input.
120VAC ____
o--|_--_|---.
FU1 |
C
C
C
C V(out)
C<--o
C 0-120VAC
C
C
C
C
C
C
C
0VAC |
o-----------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Lamp dimmers or AC phase controls are based on triacs. The main
characteristics of a triac are that they latch on, and will only turn
off when the voltage across them is zero (which happens 120 times a
second for 60 Hz AC). In a simple lamp dimmer circuit, a small cap
charges up through a series resistor and the speed control
potentiometer. When it reaches a certain voltage, a triac trigger IC
called a DIAC discharges the cap into the gate of the triac, turning it
on. It remains on until the zero crossing of the AC line. By choosing
components carefully, you can make it so the cap never quite charges up
to the trigger voltage when the speed control pot is turned all the way
down (maximum resistance), and so that the cap charges up almost
instantaneously when the pot is all the way up (minimum resistance).
FU1 LOAD
____ .-.
L1 o-|_--_|--------( X )----o-------o----.
'-' | | |
3AG | | |
___ | | .-.
-|___|-o | | |330 1/2W
| A | | | |
.-. | | | '-'
| | '---' | |
| | | |
'-' _|_ ---
| V_A ---
o--. |<| / | | 0.068uF 250VAC
| '----|\|--| | |
--- |>| | |
--- DIAC | |
| | |
L2 o----------------o---------------o----'
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
An autotransformer is a lot more expensive because of its laminated
metal core, copper windings, labor-intensive lapping of the wiper
mating surface, special composite wiper that's much more conductive
vertically than laterally (to prevent winding shorts), and all the
mechanical doodads and manual labor involved in getting together a
reliable working wiper mechanism. The dimmer has only a triac, a pot,
and a couple of other inexpensive components. Actually, a 15 amp
autotransformer for $150 is a great deal -- most of them are more
expensive.
Good news for you -- the AC speed control should be OK for your
project. As a general rule, nichrome won't mind that it's not being
fed with a sine wave.
By the way, I'm wondering if you haven't over-spec'd your AC control.
Even at the 24VAC setting, you'll be cranking 360 watts. At full
voltage, 1800 watts. That sounds like a lot of power for melting or
cutting some plastic.