Transformer winding help?

Discussion in 'The Projects Forum' started by flightrisk, Sep 8, 2014.

  1. flightrisk

    Thread Starter New Member

    Sep 8, 2014
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    I wanted to build a foam cutting tool and thought of using one of my microwave oven transformers to make a 6v/12v power supply. I am microelectronics guy, not a power guy so it has been 30 years since my college EE transformer course ;)

    There are many options, a tapped transformer, multiple multi-volt windings, multi same volt windings series or paralleled, etc. I was thinking of using 2 windings. The transformer is a standard MOT with the primary wound horizonally and the secondary parallel to and above the primary. I wanted to cut out the magnet wire secondary and imagining that it is 1 turn per volt in stepdown, I would make two, 6 turn secondaries. using a switch to put them in series I would get 14V with maybe 2 to 4 amps and connecting them in parallel I would get 6V at double the current. I would control the output voltage with a light dimmer on the primary. My questions:

    1. What guage for the secondary? AWG 18 stranded? Has to handle the amps but be bendable and fit in the space.

    2. Assuming the primary looking down from above is wound CCW, do I have to wind both the secondaries CCW as well?

    3. Does it matter how I wind the secondaries? Put the wires next to each other and wind both at the same time? Wind one on the botom of the secondary space and the other above it? Wind one around the center of the core and then wind the second one around the outside of the first one? Maybe since I am using only one winding at a time, it is irrelevant?

    4. Should I be aware of anything else or re-think my design?

    5. Any tricks or things to limit core saturation and heating?

    6. I'll need a hefty DPDT Switch. Do I need to worry about transients if I switch voltages with the dimmer on the primary and the power still applied? I want to be sure I don't blow the dimmer or arc the switch.

    7. Should I be concerned if the hot wire breaks and I am running the transformer under no load (secondary open)?

    Thank you.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    1/ You need the know the length & gauge of the nicrohm wire you intend using in order to calculate the secondary current based on what the secondary voltage is, there are plenty of 12vac secondary transformers that should fit the bill.
    2/ doesn't matter if you wind them bifilar you just parallel each end up. If winding separate, they need to be phased before connecting.
    3/ Primary is usually wound first.
    4/ If using a dimmer type switch on the primary, as many builders do, they normally come with an on/off switch, I opted to control the secondary just because it seemed a little more professional.
    7/ Nothing will happen if the wire breaks.
    Max.
     
  3. pilko

    Active Member

    Dec 8, 2008
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  4. flightrisk

    Thread Starter New Member

    Sep 8, 2014
    3
    0
    Thanks to both of you. So I have a roll of zip cord/speaker wire. 18ga, red and black. It is obviously joined together. I should then be able to wind it so that the wire is flat against the center of the core and wind if from bottom to top. Now I will have something like this when I run them in parallel. I am using 12ov and not 240. (image from Ibiblio)
    [​IMG]
    Winding bifilar as I will, am I correct that both positive ends will be together? In other words, if the primary is wound starting in the lower right, going CCW and ending in the upper left, and I do the same thing by dual winding the secondary CCW, all of the "positive" or dotted wires will be together on let's say the right. All 3 coils are phased together. I can then just join the two right wires and join the two left wires and the secondary coils are in phase and parallel. Just double checking to be sure. Seems logical because the same magnetic field is inducing a current at the same time and in the same direction since the wires are wound connected together.

    Just as a curiosity, what is the implication of winding up and down like a sewing machine spool does? I don't have enough windings for that, but if you were winding on layer one from bottom to top, then wind from top to bottom in layer two over top of layer one, then back from bottom to top again until you filled the space. My understanding is that a low frequencies it is irrelevant. It that also correct?
     
    Last edited: Sep 10, 2014
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    The dots indicate the start (or end) of both windings and the dotted ends will be connected together for parallel operation as will the other two ends.
    If you do not need dual windings on the primary, a single 120v will suffice at a larger gauge.
    There does not need to be any correlation between primary and secondary as to the direction of wind.
    Normally you use a coil former and the winding is wound progressively forward and back across the former.
    Max.
     
  6. flightrisk

    Thread Starter New Member

    Sep 8, 2014
    3
    0
    I am wondering where I went wrong. I have a transformer from an old stereo. I mapped out the wiring and took a 10vac wall wart to use as an input voltage to measure the output voltage with no load other than the meter. I got 1.2 vac. The black wire seemed to be the center tap. I measured to a brown wire and got 1.2 volts. I assumed I could get 12v out of it with 120vac input and enough current to power a 12" nicrome were (26guage). I was expecting to get 2-5 amps out of it. But when I hook up my dimmer circuit and crank it all the way up, which should be 120vac, and connect my 12" of nichrome wire, I get only 3.9 volts showing on my cheap non-rms meter across the wire. Did I miss something?
     
  7. paulktreg

    Distinguished Member

    Jun 2, 2008
    609
    120
    Your first part sounds about right. If you have a 120VAC to 12VAC transformer then you would expect approximately 1VAC out if your input is 10VAC.

    Are you sure the VA rating of your transformer is sufficient?
     
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