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  1. #1
    Join Date
    Nov 2008
    Location
    Rosetta, South Africa
    Posts
    175

    Default Power supply question

    Hi,
    First off - sorry but I asked my question on somebody elses post.
    Delete if needed.

    I built a unit from a PC power supply and it worked fine for a while.
    Now works when it feels like it.
    Need to build another unit.
    Question is:
    Is DC a prerequisite or will an AC power source work?
    I ask because my soldering iron works on a AC power supply.
    Thanks
    Phil
    Last edited by Phil Hansen; 10th Feb 2020 at 09:20 PM. Reason: grammar

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  3. #2
    Join Date
    Nov 2013
    Location
    Caboolture QLD AU
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    778

    Default

    As you are heating a wire, AC is fine, even preferred. Low voltage required, around 3 Volts @ a few amperes.

    The biggest mistake I see people make when trying to build one for a typical wood burning pen, is trying to use a transformer supplying around 12v to 15v AC instead of 2 to 4 volts. As you are trying to use a old PC SMPS then likely the 5V supply or even better a 3.3v output if it has one at a few amperes. It's easier to get fine heat adjustment if you can control the AC mains input side, which is why I like a transformer with an old dimmer controller on the input.

    Usual safety warnings for AC inputs and legal requirements / laws will need to be followed.

    Mike.

  4. #3
    Join Date
    Nov 2008
    Location
    Rosetta, South Africa
    Posts
    175

    Default

    Thanks Mike,
    Just the advice I wanted to hear.
    Have got a transformer (multiple taps 2 to 24 V) and a dimmer unit.
    Will have a crack at it today.
    Phil

  5. #4
    Join Date
    Apr 2011
    Location
    McBride BC Canada
    Posts
    3,397

    Default

    Remember that electricity is like a waterfall =
    The height of the waterfall is the voltage and the amount of water going over is the amperage.
    The amps do the work.

    In fact, I^2xR is the resistance heating. I stands for amps and R is resistance. Keep the voltage low.

  6. #5
    Join Date
    Nov 2008
    Location
    Rosetta, South Africa
    Posts
    175

    Default

    Got it up and running in test mode.
    The transformer is not as I previously thought. It does have multiple taps from 12 to 24 V and rated at 2A.
    Put the dimmer in line and can get good results without the pen getting red hot.
    Measured the voltage at that setting and with no load it was 6V. With the pen attached 1V
    Does this all make sense? Does the load cause such a voltage drop?
    Thanks
    Phil

  7. #6
    Join Date
    Nov 2013
    Location
    Caboolture QLD AU
    Posts
    778

    Default

    This type of input voltage control is not any kind of regulation control. That means that no load to load measurements are meaningless.

    You could check the input current under load and check your not exceeding the VA rating of the transformer.

    If you have a number of secondary taps, you may be able to feed the heating element from between two taps, like between a 6v and 9v tap, potentially giving you a 3V secondary.

    Keeping in mind that a heating element (wire) is almost a glowing fuse and a very low resistance value, hence you would expect a very low voltage and high current measurement across the element. This of course is a simplified answer and there are a number of restive factors in play that determine the need to adjust the input voltage to maintain correct pen temperature under varying wood burning conditions.

    Mike.

  8. #7
    Join Date
    Nov 2008
    Location
    Rosetta, South Africa
    Posts
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    Default

    Thanks Mike and also RV for your replies.
    The explanations of how this all works were great. Made sense and I now understand things a lot better.
    I did not know I could get a feed between different taps.
    Well connected between 12V & 15V and surprise got 3V.
    With that setting and the dimmer on 100% I can get a decent burn with 1.2mm wire and no red hot tip.
    If I use thinner wire will be able to turn it down to avoid melting the wire.
    Tapping between 15V and 20V made things red hot at a very low dimmer setting. Will stick to 3V .
    Thanks again for all your help/explanations.
    Cheers
    Phil

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