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Trans former rewind

Discussion in 'Power Electronics' started by tooter, May 12, 2018.

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  1. tooter


    Feb 17, 2018
    Hi I have a transformer that is 240v on the primary and 30v on the secondary. I would like to rewind the secondary to something around 10v but still require around 500ma. How can I achieve this thanks.


    May 20, 2017
    Keeping it simple. Dismantle the transformer by removing the laminations. Take care not to bend or mutilate them as you do as this will affect their performance. Unwind the secondary counting the number of turns as you do.
    Divide the number of turns by 3. Using a slightly heavier gauge of wire, re-wind the secondary and then reassemble your transformer.
  3. duke37


    Jan 9, 2011
    Post a picture of the transformer, a torroid is more difficult to rewind.
    You will not need thicker wire if the original current capability is 500mA. You could take off an estimated half of the turns and measure the resulting voltage, then remove more to get the final value. You then do not need to rewind but will need to replace the laminations to measure the voltage.

    If not far away from me, and you may like a visit to the Crich tramway museum, you could use my winder.
  4. tooter


    Feb 17, 2018
    Hi thanks for the replies. This is the tranny as you can see there is enough room to rewind without dismantling. would removing winding lower the voltage and the amps it can handle.?
  5. tooter


    Feb 17, 2018
  6. ramussons


    Jun 10, 2014
    Wind about 20 turns of 22 gauge enamalled wire through the gap. Measure the voltage. That should give an idea of how many turns are required for 10 volts. Add 2.5% more turns. Hope there's enough space to wind the required turns.
    Take care when pulling the wire through the gaps so it does not rub ahainst the sharp edges and scratch the enamel off.
  7. Minder


    Apr 24, 2015
    I have lost track of the number of transformers I have either rewound or added/removed secondary winding's and to me the Toroidal type is by far easier to add/subtract a secondary.
    It appears to me that the OP just needs to remove 2/3 of the secondary?
    At around a typical 2 to 3 turns /volt, I would remove about 10 turns and check the result to get the turns/volt.
    The Va will remain the same so the current capability will be higher as turns are reduced.
    It looks like the lamination's have a weld bead across them so dismantling is virtually out without some major surgery.
    Last edited: May 12, 2018
    73's de Edd and duke37 like this.
  8. duke37


    Jan 9, 2011
    The primary is the pink winding where the gap is. I would not under any circumstances put a winding on top of this for safety reasons
    You can of course take turns off the secondary as Minder recommends without dismantling

    Edit: The primary should not be modified in any way.
    Last edited: May 12, 2018
  9. kellys_eye


    Jun 25, 2010
    Transformers have VA ratings (basically their wattage) and the volts/amps you can extract always remains the product of the two.

    If you had 30V at 100mA originally then the transformer was 30 x 0.1 = 3W. The secondary can be 'any' combination of volts and amps that don't exceed 3W...... so 10V @ 300mA would be ok, so would 1V at 3A!

    If I read your post correctly then the transformer you have is rated at 30V and 500mA = approximately 15W so you can extract 10V at 1.5A if you wanted (and had the appropriate thickness of wire as the secondary) but using the SAME wire you will always be limited to the original current capacity, in your case 500mA maximum.

    As ever, the current is only drawn to fit the load so if your load DRAWS 300mA then that's all it will take, even if the winding is rated at 500mA.
  10. Minder


    Apr 24, 2015
    I doubt if the original was constructed that tight, to exactly the AWG for 500ma!
    You should be able to calculate the max current based on both the Va and the Actual gauge of the secondary.
  11. dave9


    Mar 5, 2017
    This... or just buy the right transformer as you live in a 1st world country and it can't be very expensive to buy a 10V, 500mA transformer, or if you need DC from it, put 3 diodes in series after a common, switching 12V/1A AC-DC adapter.

    What are you powering? I'm drawing a blank for what would need 10V/500mA... 3 x 3W LEDs in series and you don't want to use a proper current regulating driver? That would put the transformer at full load which is usually not a good design.
  12. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir tooter . . . . . . . .
    WOW !
    and that's being vewy-vewy heavy on the WOW! aspect
    That's a piece of cake . . . after all of the LAYER constructed units that I have either LABORIOUSLY torn down in order to modifiy or built from the "ground" up.

    THAT'S being a split bobbin !

    You just check out the winding at the right . . . . its secondary . . . and find the wire that is its winding END. Then start peeling off turns and use a 20 ohm 5 watt or more watts . . . . as a load resistor . . . . . while progressively checking and measuring the output voltage, until you finally reach that desired 10V output level.
    CONSIDERING that you will need a bit greater than 10V due to different regulator loss, design quirks.

    That photo and its measured reference of sizing, shows that core mass to be adequate for 5 watts + power handling capability.
    That transformer, in its other life, and a ROUGH and unsheltered life it was . . .what with all of those patches of FE O2 deposits, could have actually been a CONTROL transformer of 24VAC out; that would measure as a greater voltage, when being open circuit and unloaded.

    Thasssssit . . .

    73's de Edd
    Last edited: May 13, 2018
    Richard9025 likes this.
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