making a transformer

Discussion in 'The Projects Forum' started by ddickey, Nov 29, 2011.

  1. ddickey

    Thread Starter Member

    Sep 28, 2011
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  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    I don't think it matters too much for the purpose of experimentation. Try something like a 1/4" thick by an inch wide or whatever you can find in that area.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    As thin as possible, and interleave paper between the sections.

    This is to stop eddy currents wasting most of the input energy heating the iron core.
     
  4. ddickey

    Thread Starter Member

    Sep 28, 2011
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    Ok, I made the transformer and it doesn't work. 100 windings on the primary and 350 on the secondary. As soon as i hook the transformer to my low volatge power supply the voltage goes down to 2 instead of the 12.6 volts coming out of my power supply. I'm getting 1.5 volts out of my secondary. Any suggestions on why? I'm going to try more primary windings next time.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    The primary is too big a load on your power supply - it looks much like a short. Try putting a resistor in series. This will maintain voltage at the supply but won't lead to any more power from the secondary.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    I assume you are applying 12.6ACV @ 50/60Hz to the primary(?).

    It's drawing too much current because the inductance of the primary is not sufficient to provide a low magnetizing current. The core is being saturated thus you need more turns on the primary. If the voltage drops to 2V, then as a rough estimate I would guess you need at least 6 times the turns or 600 turns.

    As a test try applying the 12.6VAC to the secondary and see what happens.

    Edit: I forgot that inductance is proportion to the square of the turns, so you may need to increase the number of turns by a only a factor of √6 = 2.5 times to 250 turns.
     
    Last edited: Dec 14, 2011
  7. ddickey

    Thread Starter Member

    Sep 28, 2011
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    yes. 12.6 AC. Ok thanks. I'll try.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Another way to think about this is the length of your primary; if you're using 28 gauge copper, it's only 6.445Ω per 100 feet. If your power supply can supply 1 amp at 12v, you need a load of at least 12Ω to stay below the 1A limit, or 200 feet of windings. Tedious. ;)
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Real transformers are made with enough primary turns, so that the inductance is the main thing that sets the magnetizing current. The resistance of the primary has little to do with limiting the current in good practical transformers
     
  10. crutschow

    Expert

    Mar 14, 2008
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    True. Ideally you want the winding resistance to be zero so there is no IR power loss in the windings.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    The idea is to get counter EMF to work for you. It is what limits current going through the transformer. Resistance in windings is usually a bad thing.

    I've never built a transformer, but I have wondered how to figure the ideal number of wirings. I don't think it is a simple math problem in the slightest.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    At the low frequency (50-60 Hz) and low inductance of the OP's 100-turn coil, DC resistance is the primary contributor to the coil's impedance. Do the math. I stand by my statement that such a coil is essentially a short to the power supply. It's better than one loop but it's still too short.

    Inductance of a coil increases roughly with the square of the number of turns. At ~500 turns or more, at this frequency, the impedance due to coil inductance will begin to exceed the contribution of DC resistance. Together, the DC resistance and the inductance will keep the current low enough.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The laminated iron core makes every winding "worth" quite a bit more inductance, though. I forgot the permeability offhand, but I'll look for it, or at least a range. At 120V, I don't think inductance will fully cancel out resistance, otherwise, idle transformers shouldn't get warm, as the inductance is a reactive load.
     
  14. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The primary inductance dominates the impedance, but as the inductance can't be made infinite, there is some heating due to current flowing in the primary resistance, even with the transformer idle. That said, most of the idle losses actually come from the core: these typically approximate to 50% of the full-load losses. In a well-designed transformer, copper losses are relatively small in the unloaded condition.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    In a normal transformer for sure, but absolutely not in the OP's first attempt.
     
  16. ddickey

    Thread Starter Member

    Sep 28, 2011
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    I increased the primary windings to 350 turns. Now when i take a measurement between the transformer i made and the power supply i have 12.5 volts, pretty much exactly what i want. Howerver, I can only now get 1 watt out of the secondary. I did run out of Magnet wire but still got just over 150 turns.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Wattage delivered to the load will depend on the load itself. Maximum power will be transferred when the impedance of the load matches the impedance of the primary. Like being in the right gear on a bicycle. You can map out your transformer by applying a series of increasing loads (decreasing ohms) across the secondary, and measuring the voltage across them (and thus current). You'll find a fairly sharp peak, probably around 30Ω or so. That's just a guess on my part, and the actual peak may be a ways off from that.

    But as you near the peak, the resistors will have to be large enough to handle the output. Since you're already at 1 watt, they might get kinda large. Use 'em if you got 'em.
     
  18. ifixit

    Distinguished Member

    Nov 20, 2008
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    To get better help...
    1. Can you post a picture of your tranformer?
    2. What is the primary AC voltage and frequency?
    3. What is the maximum current you need from the secondary at 12.6VAC?
    4. What wire gauge are you using for the primary?
    5. What wire gauge are you using for the secondary?
    Regards,
    Ifixit
     
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