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How hot might this 3 amp transformer get?

  1. Dec 23, 2014 #1
    20 gauge copper wire is rated for 5 amps.
    Suppose someone use 18 gauge copper wire and wraps it 60 times around one side of a 1 inch iron ring, with the two ends then connected to a plug.
    A separate piece of copper wire wraps around the other side 20 times, and goes to a 13.3 Ohm resister.
    So now this transformer has reduced voltage from 120 volts to 40 volts, and the 13.3 Ohm resister regulates the current to 3 amps.
    Suppose the resister has a good heat sink on it and thus won't melt.

    How hot will will the iron right and copper windings get?
    Would using electrical tape to guarantee electrical insulation result in burnt electrical tape?
    What do fully enclosed (no fan) 9 volt or 12 volt AC DC converters use to dissipate their heat? I'm guessing they use less than 3 amps, which dodges my real question. I notice computer power supplies, which use more current, have fans.
  2. jcsd
  3. Dec 23, 2014 #2
    Magnet (enameled) wire should withstand at least 100°C with no problem. Generally, it depends on the class of insulation of the device:

  4. Dec 23, 2014 #3


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    This is not possible to answer since we know nothing of the properties of the iron ring. I could take a single strand of 18 AWG iron wire and wrap it around to form a 'ring'. It is not much of a transformer core and will cause the primary in your transformer to appear as a short circuit.
  5. Dec 23, 2014 #4
    On the input size, where there is 120V ac, there is a 10amp fuze in series with the 18 gauge copper wire and 60 windings.
    The ring is solid iron, 5mm tall, square cross section, 30mm outer diameter, 20mm inner diameter, wrapped in one layer of electrical tape to prevent conduction between wires and ring.
    On the converted side, which is 40v, there is a 13.3 Ohm resister, to produced 3 amps.

    I don't need an exact temperature calculation, just a ball park of where most of the heat will be produced. I already know how to calculate the heat produced at the resister, just not the transformer. I'm guessing the transformer windings would give impedance, which is kind of like resistance.

    1. Does it matter whether the transformer had 60 and 20 windings or 12 and 4 windings?
    2. Without a fan, would the electrical tape likely melt/burn?
    3. Where would more heat be produced? At the 13.3 Ohm resister on the transformed side, or at transformer?
    4. Would the 13.3 Ohm resister on the converted size provide a load for the input side to control current on that side, or would the input side need its own resister?
  6. Dec 23, 2014 #5


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    Without doing the calculations I am going to ball-park not very hot since I am pretty sure the 10 amp fuse will blow. The core is not large enough with the number of turns you have on the primary. If you eliminated the secondary I think it would still blow. Google transformer design. I know in the past I have found some ball park formulas that guide you through some very basic transformer design.
    Basic transformer design:
    The core size and number of primary windings need to be large enough to prevent excessive current with no load.
    The size of the wire in the windings needs to be large enough to prevent resistive losses. Also, the number of turns needs to be kept low enough to help prevent resistive losses. Do you see a tradeoff here?
    Core size also comes into play when considering the load placed on the secondary. If the core is not large enough too much current drawn from the secondary will cause the core to go into magnetic saturation which will cause excessive current to be drawn from the primary.
    These are some things I know about transformer design. I am by no means an expert. I know enough to know there is PLENTY I do not know.
  7. Dec 23, 2014 #6
    Thank you for that second keyword, design; that got me some good Google results.

    OK, so the transformer connects their voltages but not their currents. Loads must be in both loops. I have plenty of reading to do.
  8. Dec 23, 2014 #7
    Considerations regarding magnetizing current.All other things being equal, 5x less turns means aprox. 25x less inductivity, which means 25x smaller primary impedance, which means 25 more primary current, which means aprox. 25x more joule heat for winding and 625x more for the fuse ..
    Last edited: Dec 23, 2014
  9. Dec 23, 2014 #8
    This thread is a reason why engineers often don't hire mathematicians, at least not mathematicians who don't also have a degree in engineering.
    Still, this math guy wants his physics education.

    Power (heat) = (V^2)/R. I just now realized it, and see you kind of posted it here too. Suppose instead of more coils, a 1000 ohm resister is put in series with the primary. That means maybe 1/8 amp in the primary, and 1.5W of heat in that resister, which is manageable.
    1. Would the primary coil still have 120V? Would it still impart 40V to the secondary coil?
    Would the fact that there is only 0.125A in the primary have any effect on the secondary's ability to have 3A?
    My limited understanding of the equations says the secondary would have all the desired properties. My intuition highly questions the transformer's ability to pass any significant energy to secondary.
    My aversion to all the coils is what if I tiny scratch shorts out the whole thing? Fewer coils and a resister means a gap between coils so that a scratch would be a non-issue.
  10. Dec 23, 2014 #9
    Notice in my sentence "All other things being equal". I was refering just to magnetizing current (no secondary load) which is problematic by itself in your design.
    edit: And careful with words becouse you're obviously very ignorant.
    Last edited: Dec 23, 2014
  11. Dec 23, 2014 #10


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    If you use just a lump of Iron as your core, you can expect the core to get hot due to Eddy Currents. All Iron core transformers are made with laminations to prevent this.
    Just go out and buy a transformer or even a really made low voltage power supply. You can get low voltage PSUs that will supply several Amps (AC or DC) off the shelf for not much money - probably cheaper than actually buying a transformer.
    You can also buy transformer kits along with the design parameters for a range of Primary and Secondary voltages. The suppliers of these kits have done all the sums for you and you can sleep easier at night, knowing that it was designed by someone who knows about these things. :)

    The cheap alternative is to find a piece of old equipment that is clearly much higher power than you need and scrounge the transformer out of it.
  12. Dec 23, 2014 #11


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    No. For what you have in mind, most of the voltage will be lost across that resistor.
    For your transformer, no, nothing like 40V.
    There is nothing that replaces practical experience. I suggest that you rip apart an old transformer from some broken electronic gear to see exactly how it is constructed and wound. (If you scavenge from a discarded appliance, first short across the electrolytic capacitors to remove any residual charge using a screwdriver with an insulated handle. You hold the handle.)
    BTW, it's spelled resistor http://thumbnails111.imagebam.com/37333/4d0696373324852.jpg [Broken]
    Last edited by a moderator: May 7, 2017
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