How much number of turns in a transformer needed?

Discussion in 'The Projects Forum' started by apolo02, Apr 30, 2011.

  1. apolo02

    Thread Starter New Member

    Apr 30, 2011
    9
    0
    SORRY,I HAVE A CENTRE TAPED TRANSFORMER FOR MY SUB WOOFER MODEL SP-7700F (SEA PIANO 5.1) THE TRANSFORMER GET SHOCKED ,THEREFORE I WANT TO REPAIR IT

    I WANT TO KNOW THE NUMBER OF TURNS TO PUT IN THE PRIMARY WINDING AREA SO AS TO PRODUCE 14.2-0-14.2VAC BUT WITH OUTPUT VOLTAGE OF 220-240V

    HOW MUCH NUMBER OF TURNS CAN I PUT?
    HOW TO KNOW AND CALCULATE THOSE NUMBER OF TURNS?

    PLIS HELP ME;u can use even my email;apolinary-mahilane@yahoo.co.uk
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    950
    What gauge wire is on the transformer now?

    What output did it have before it was 'shocked'?

    How large is the transformer?

    Is the primary or secondary the outside winding?

    Is the winding center tapped, or does it have two independent windings?

    Is the winding on a removable spool or is it wound onto the core directly?

    Is the core welded or simply epoxied together?

    Have you taken resistance readings on the primary and secondary to the core to see if there are shorts in the windings?

    Do you have access to form winding machinery?

    Do you have thousands of feet of copper magnet wire ready at hand which you can use for the rewinding?

    Have you taken micrometer measurements of the wires to verify the gauge sizes used?

    Do you have any Hi-pot testing equipment which can be used to verify isolation of the windings from the core after the job is completed?

    Finally- have you ever done any electronics repair work that is at all similar in nature before this?
     
  3. designnut

    Member

    Apr 21, 2011
    33
    1
    I knew a transformer manufacturer, he took the old onew apart and counted the layers turns and wire size.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    In theory, the voltage ratio is the same as the turns ratio. But when it comes to practical reality, all those questions above come into play. IMHO, unless your time is worth zero or unless you really enjoy winding a transformer, you'd be MUCH better off to just buy one. The copper wire alone will cost you a lot, maybe more than a new transformer.
     
  5. apolo02

    Thread Starter New Member

    Apr 30, 2011
    9
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    I repaired it with the same wire gauge bt the new one then i put 58 turns,bt the output of the centre taped wire become 6.3v-0-6.3v which means there is 12.6total output voltage bt i want to make 14.2-0-14.2v whch make 28.4total output voltage so how much number of turns can produce 28.4vac?(14.2-0-14.2) ,input voltage is 220v to 240vac
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    Again, voltage is proportional to the number of turns. If you can add more turns, theory says you'll need 14.2/6.3•58 = 131 turns. Note that this will increase the resistance and impedance, and the length of wire required for each turn will increase as the diameter gets larger.
     
  7. apolo02

    Thread Starter New Member

    Apr 30, 2011
    9
    0
    How about the relationship between current,voltage and turns in a transformer? Is there any relation between them for example if u add or reduce number of turns will it affect the current?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    Of course it will. For crude estimations, current through the primary will be roughly the supply voltage divided by the DC resistance, which is proportional to wire length for a given gauge. (It's a lot more complicated, though, due to impedance which also increases with turn number. An offsetting magnetic field sets up that reduces the unloaded current.) Current in both the primary and the secondary depends on the loading. The secondary coil's EMF is also proportional to the turn number, but that EMF must overcome the impedance of the coil which is again proportional to wire length and turns.

    In short, it can be complicated to model a transformer with certainty. But for rough approximation, assume the transformer is maybe 85% efficient. That means that power consumed in the primary is conserved in the secondary. So voltage depends on turns ratio and current is roughly inverse, so that power is not created.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    No doubt, just buy a 240V:14V transformer of the proper amperage although you may find that 240V:12.6V will be easer to find and less expensive yet should still work just as well since it's probably rectified, filtered then regulated down in voltage.

    Hand winding a transformer is extremely difficult, it's nearly impossible to pack the turns in with the precision and tightness of an original.

    Several of the big wholesalers such as Farnell will ship worldwide.
     
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