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adjustable AC power supply

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  1. #1
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    Default adjustable AC power supply

    Fellas,
    I need to make a power supply for a hot-wire foam cutter. There are many websites detailing how to do this. Most recommend a standard light dimmer connected to the primary of a suitably rated 12 or 24V transformer (for my purposes around 60VA). The result is a power supply with a variable AC output (to be suitably fused of course).

    Iíve built many variable DC power supplies over the years, but have never built a variable AC power supply. Technically this is a piece of cake to build. What Iím not sure about is whether itís a sensible way of going about it. I know this isnít really sparky territory, but I would appreciate any comments on the pros and cons of this method.

    Regards,
    Chris

  2. #2
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    G'day Chris

    Here is the hot wire cutter power supply I built for cutting the polystyrene blocks for my ICF house.



    It is a Variac (Variable auto transformer) supplying a step down isolating transformer. The isolating transformer is 240V to 20V so the output is always extralow voltage and so reasonably safe. The Variac allows the output to be varied from 0 - 20 V ac at a current of up to 30A. I use it with a hot wire block cutter or the wall chasing gadget seen in the photo.

    In your case a light dimmer supplying a transformer would do a similar job and I have also heard of it being done this way but not tried it as I had the Variac.
    There are a couple of things I think you need to watch.
    The dimmer should be rated for an inductive (transformer) type load, a dimmer suitable for SELV halogen light transformers springs to mind.
    The transformer MUST be an isolating transformer and with an output voltage of say not more than 24V for added safety but definitely not more that 50V.
    The supply wiring and the output wiring must be separated and secured so there is no chance of the isolation of the transformer being bridged.

    If you have a fixed load (eg. a hot wire that you don't change the length of) you don't really need to vary the output. A simple isolating transformer will do the trick and be the simplest solution.
    A toroidal transformer would be a good one to use. You could wind a few of turns of heavy gauge wire through the hole and try it. If the cutter wire is too hot drop a turn or two and try again, likewise if it is not hot enough add a couple of turns. Once you have the number of turns worked out, put the thing in a box with a fuse on the supply side and you are done.

    Caution: if you have any doubts about what you are doing or are not familiar with AS3000, AS3100 and electrical safety, I suggest you involve the help of someone who is.

    Hope this is of some help

  3. #3
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    Hi Chris
    It sounds as though you just need to heat up a length of resistance wire to cut the foam. Your 60VA suggests something like 5A at 12V. For intermittent use, you might find a car battery will do. Anyhow, there is no need for a DC supply, AC will heat just the same, so a simple transformer as suggested above will be fine.

    If you don't need to variably control the heat, rather than controlling the input you might also consider using a tapped secondary unit (say tapped at 6V, 12V etc) and tailoring your heating element size to this. In any case, the heat generated is a function of the supply voltage and current drawn by the heating element, which in turn is limited by the resistance of the element or wire.

    Forgive me if I am telling you something that you already know, but the fundamental arithmetic that you need to do is based on Ohm's Law which in its various forms says Amps =Volts/Ohms, Volts=AmpsxOhms, Ohms=Volts/Amps, Watts=VoltsxAmps, Watts=VoltsxVolts/Ohms, etc, etc. (i.e. substitution as per Algebra 101 if you can remember back that far).

    So, for example a 60W heated wire (say 12V @5A) of a particular length will need to be 2.4 ohms, if you only want 30W then you need 4.8 ohms etc (or double the length). Of course, the total power required will also depend on the length of the cutting wire since if you double the length you will also need to double the total heat or watts consumed for the same unit heat per unit length of wire.

    The only trap I can think of is trying to draw more power than the transformer is rated at, but I am sure you are across that.

    Good luck!

    P.S. Remember, don't put your fingers on the hot bit!!!!

  4. #4
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    Thanks for the feedback.

    A Variac is one of those things I've always wanted but could never justify the expense of. I was just going to buy a 12-0-12V 80VA toroidal tranny for Altronics. I'll parallel the secondaries for 12VAC at up to 6.7A.

    As the length of hot wire will depend on the job at hand, I'll need the ability to vary the voltage, hence the dimmer. The wire might be quite short to cut through 50mm foam up to a metre or more to cut foam blanks for model aircraft wings. The cutting wire will be much thinner than the wire in the picture above.

    Regards

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    Sounds like fun! If you go that way, check that the outputs are "parallelable" (if there is such a word). The 12-0-12 might not be two 12V windings but one 24V winding with a tap in the middle. Joining the 12V ends together could result in a 24V secondary short circuit... whoops!... it would certainly get hot!

  6. #6
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    Yeh sorry, bad terminology on my part. It has two separate windings which can be wired either in series or parallel.

  7. #7
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    Quote Originally Posted by jack620 View Post
    A Variac is one of those things I've always wanted but could never justify the expense of.
    I find Variacs very useful and have a good collection of them, ranging from the little 2.5A type used in the hotwire cutter right up to a 28A three phase one.

    Quote Originally Posted by jack620 View Post
    I was just going to buy a 12-0-12V 80VA toroidal tranny for Altronics. I'll parallel the secondaries for 12VAC at up to 6.7A.
    Sound good to me. The way to check you have paralleled the winding correctly is to connect one end of each pair together and measure the voltage across the other ends. If it is zero you have it right, if it is 24 V you have them in series and need to swap one.

    Quote Originally Posted by jack620 View Post
    As the length of hot wire will depend on the job at hand, I'll need the ability to vary the voltage, hence the dimmer. The wire might be quite short to cut through 50mm foam up to a metre or more to cut foam blanks for model aircraft wings. The cutting wire will be much thinner than the wire in the picture above.
    This is exactly why I made a variable supply and with a decent current capacity. I use a thin taut wire for cutting the foam blocks to length (and for model aircraft wings) but use the heavy wire (old coat hangers) when I need a rigid shape to cut a chase or use as a hot knife.

    Chris, how goes the progress with your cutter? I would be interested in hearing how successful the dimmer design is.

  8. #8
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    No further progress BB. Christmas and New Year got in the way. Will report back when I have a prototype up and running.

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