# Variable transformers and speed control differences.

Discussion in 'Electronic Basics' started by ME, Apr 13, 2005.

1. ### MEGuest

Hello,
Can someone tell me what the differences are for:

120-volt, 15-amp variable transformer - (about 150.00)
120-volt, 15 amp ac motor (router) speed control -( about 20.00)
and are they interchangeable

I will be using it as a temperature control for some nichrome wire to melt
and bend plastic.

or anything else cheaper to use for this.

Thanks

Ron

2. ### ChrisGuest

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).

____ .-.
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.

Good luck
Chris

3. ### John FieldsGuest

Nice description of the VARIAC, but considering that 1800 watts from a
120V source requires 15 amps into the load means that the TRIAC dimmer
with its inherent 0.7 to 1V drop will need to get rid of from about
11 watts to 15 watts worth of heat when it's cranked. Plus, there's
the EMI which will be generated and the harmonics sent down the
mains...

4. ### John GGuest

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).

____ .-.
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.

Good luck
Chris

Whatever else is needed an isolation transformer will be necessary or
the cutting wire will have mains voltage on it and therefore be lethal.

A variac is certainly an autotransformer but a lot of auto transformers
are not variacs and in fact just have a fixed tap somewhere on the
primary winding to give a ratio of voltage between the input and output.
These are not suiable for this operation because they cannot be

The power levels quoted do seem too high for cutting and just a wire
will be difficult to use to warm the plastic to bend it. Of course we do
not know what the actual material to be cut or bent is so I am just

5. ### Rich GriseGuest

For nichrome, get a light dimmer at the Home Club or equivalent. Just
be sure it's good for 15 amps - that's only 150 watts of lights, so
a dimmer that's good for 300 watts would give you a decent "safety"
margin.

In fact, since they go about 20 bucks, it and the motor control are
probably about the same - the motor controller might have power factor
correction or something, so the dimmer would probably be cheaper.

Or get a mechanical thermostat for about five bucks.

Cheers!
Rich

6. ### Rich GriseGuest

OOps - 1800 watts; I guess that's a pretty mongo dimmer - but do
you really nead that kind of heat to cut and bend plastic?

Good Luck!
Rich