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Variable transformers and speed control differences.

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

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  1. ME

    ME Guest

    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. Chris

    Chris Guest

    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.

    Good luck
    Chris
     
  3. John Fields

    John Fields Guest

    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 G

    John G Guest

    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.

    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
    adjusted.

    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
    guessing about that.
     
  5. Rich Grise

    Rich Grise Guest

    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 Grise

    Rich Grise Guest

    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
     
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