Power Supply for Soldering Unit

Discussion in 'The Projects Forum' started by dpeterson3, Jun 21, 2011.

  1. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
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    I would like to build a resistive soldering iron to do some brass modeling work. The commercial units are well out of my price range, so I look at building my own. Everywhere I look, this pdf keeps popping up. www.girr.org/girr/tips/tips1/solderer.pdf
    I don't want to go out and buy a battery charger for this. I guess I need some help designing a power supply for such an iron. I was thinking of using a 24V 5A transformer I have from a washing machine lying around (or rewinding it for 12V and higher amperage). Do I need higher amperage? I have seen some schematics using linear regulators, but I don't know how well they will hold up for something like this. Most of my electronics experience has been with motor controllers and micros, so I am a little lost when it comes to power supplies.

    Thanks

    (also, no clue why this is italic)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    What kind of temperatures do you need to have at the tip? Would a pinpoint torch do better?
     
  3. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
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    I talked to a few people in a club I am part of about this, and this is what I was told I need. I trust them as they have been doing this for 20+ years. A torch gets the rest of the workpiece too hot and other joints start melting (I tried). Plus, once the unit is built, power is much cheaper than fuel.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    Do you have some specifications of the soldering iron to be used?
    With this information we can help you to construct a suitable powersupply.
    (eventualy with a power regulation).

    Bertus
     
  5. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
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    Looking at other people's stuff online, most units run between 100W and 200W. I believe that the voltage is around 12V, so assuming 200W, the supply would need to be able to source about 17A at 12V. That may be unrealistic with what I have available, but I would like to be able to source at least 9A at 12V for a ~100W iron. The most I would be doing with it is maybe 0.1" thick brass (and that is an overestimate).
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
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    Once upon a time, a guy came into my cave wanting a transformer to steam wood for guitar bodies. 6 volts, 20 amps. He was soo wrong!

    so I did some measurements for you. Free information follows:

    Weller soldering guns, 100W, 140W, 200W, 260W
    all the voltages at the heating tips are .15 to .18 Vac RMS
    The 100 watt gun measures 150 amps and all the others peg the 300 amp scale.

    I can say that I'd trust my Weller 260 to do a fast, clean, spot job with a massive piece of .1 brass, any day. The 140W might work if the brass didn't go on for a couple of inches in each direction. Brass really sucks heat compared to galvanized steel chassis's...however you spell that...

    Happy estimating. I hope these measurements are useful.
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
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    @#12 - The soldering the OP is doing (I think) is just what your describing, large parts that need soldered at small joints. Think model making, rail road models are done like this.

    The tip of the solder "iron" is held to one part the other is ground and the resistance between the parts only heats very locally.

    My Grandpa did this making models of model T Fords and other old cars when I was young. Instead of a big battery charger he used a car battery and charged it with a small charger when needed.
     
  8. #12

    Expert

    Nov 30, 2010
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    This brings up the idea of cutting a Weller tip in half and touching the ends to the track on each side of the joint. Any piece of copper wire would work. Make it so large that it barely fits in the holders. Solder paste is also a good idea. Takes up about zero room in the joint, provides flux, and brings solder to hold the pieces together.

    The idea that I doubt is being able to sustain 12 volts across a few millimeters of brass. The resistance is just too low. The Weller idea is that massive currents do the job and the voltage is only a couple of tenths of a volt.

    dpeterson3 is free to use this information or not.
     
  9. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
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    #12 is probably right. I am basing my ideas mostly on the PDF I posted in my original post and my knowledge of welders. When looking for specs on soldering irons, I was having trouble finding typical voltages/currents. I was saying 12V because that is what a car battery charger puts out, which is what is recommended for the home-brew versions I have found. That is part of what I was asking, since I am not 100% sure of the requirements. I thought that the carbon tip was to keep the voltage up high enough to push current to heat the piece as well as keeping the tip from touching the workpiece. If I am wrong, please correct me, but I thought that was where the large resistance came from.
     
  10. KJ6EAD

    Senior Member

    Apr 30, 2011
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  11. #12

    Expert

    Nov 30, 2010
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    Now that I've read the link, I wonder if the carbon thing provides helpful resistance. I'd sacrifice a D cell in a heartbeat for this project. Does the whole carbon rod start glowing? Is it the pointy tip that concentrates the heat? Using carbon will surely avoid having the soldering tip stick to the victim. But then, the tweezer method doesn't use carbon.

    My next idea is to get a surplus torroid transformer and make a one turn winding. I bought some 12v/150 watt torroids for $7.50/surplus. (They make dandy car chargers.) I had to hollow the epoxy out of the centers to make my own windings. Maybe you can find torroids that aren't filled up with crud...er...epoxy. You can completely ignore the secondary winding that is on the torroid. Just roll your own with some fat copper wire. One turn? Two turns? I don't know. Watch your winding to see if it's going to overheat. Teflon insulated wire is hard to find. 150 watts into .15 ohms will hit 1000 amps before the core gets irritated! Limiting the primary current with a Variac or a light bulb will calm it down.

    There. Some ideas. If the carbon provides useful resistance, the "roll your own winding" might not work. You might really need 6 or 12 volts to make the carbon hot. I don't know.
     
  12. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
    19
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    Sorry about the link. That may explain why everything looked italic when I was posting. I haven't slept much in the past few days, so checking my posts hasn't been happening.

    KJ6EAD: thanks for fixing that

    Any suggestions on finding an old troid to rewind? I am not sure what would have used one big enough? I have also been considering rewinding a microwave transformer as everyone says those can handle the current (diy welders use them). I know I can find a broken microwave without too much trouble.
     
  13. #12

    Expert

    Nov 30, 2010
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    Judging from your location...

    you haven't posted your location.

    Go with the microwave transformer and roll your own winding. Limit the primary current with a light bulb or you might throw 4000 amps into something. Metal in a gaseous form does not hold solder well.
     
  14. dpeterson3

    Thread Starter New Member

    Oct 9, 2009
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    The biggest problem I am having is not the transformer. I am having trouble figuring out how to regulate the current coming out of the transformer. I was hoping there was a nice analog method for regulating the current going to the tip. I know a big resistor will limit the maximum current, but there has to be a better way so that it is adjustable without a resistor bank or linear regulator.
     
  15. #12

    Expert

    Nov 30, 2010
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    What? Build a regulator for over 100 amps and less than a volt?
    Regulate the input. It's so much easier.
     
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