Help - Transformer Voltage drop

[Ian Finch]

.... I've heard of winding turns to increase voltage but not decrease......

You wouldn't likely find textbook discussion on reversed sense windings because it is usually an inefficient use of power but it follows simple inductor theory quite "sensibly" - just as when you goof up and connect the dual secondary windings of a power transformer in reverse sense (or winding direction) and the output then cancels or subtracts the voltages to 0V instead of resulting in the sum both windings.

 

[wg_ski]

Put more turns, and add them in series with the primary. That will lower the secondary voltage, and be better than adding bucking windings. #14 THHN (insulated) solid core wire works well and is safe. Really easy to do with toroids, harder with EI's. You don't even have to take it apart or re-wrap tape, which can degrade safety. Always test the result with a dim bulb tester to be sure polarity is right before hooking to your circuit - if you get it backwards voltage will rise and the trafo can saturate.

 

[MarianB]

I will try the Voltage divider using two resistors to drop the Voltage a little from my 28VAC transformer.

That's the worst possible solution you could use, because the divider does not only waste power, but it's voltage drop is directly proportional to the load current, this means that the output of the divider will change very much with the load. That is by far the one thing you should NOT use. You should either use a DC-DC step down regulator with that 28Vac transformer ( DO NOT forget to use a bridge rectifier and filter caps ), or you should buy another power transformer, 15 of 18Vac max.

PS: I am sorry to say this ( and i'll apologise if it offend's you ) but you seam to wanna run before you could learn to walk. Reading your postings here i see you lack in basic electronics theory, you should try to correct that before wasting time and money on building stuff.

 

[scottjoplin]

Put more turns, and add them in series with the primary. That will lower the secondary voltage, and be better than adding bucking windings. #14 THHN (insulated) solid core wire works well and is safe. Really easy to do with toroids, harder with EI's. You don't even have to take it apart or re-wrap tape, which can degrade safety. Always test the result with a dim bulb tester to be sure polarity is right before hooking to your circuit - if you get it backwards voltage will rise and the trafo can saturate.

Thanks PRR, Ian and wg_ski for the info.
Your suggestion above seems the most logical to me if I ever decide to do such a thing 🙂 Thanks

 

[MarianB]

Put more turns, and add them in series with the primary. That will lower the secondary voltage, and be better than adding bucking windings. #14 THHN (insulated) solid core wire works well and is safe.

That's good for the core, because it increases the turn/V number, thus reducing the core losses, but it does tend to increase the primary's dc resistance, so the copper losses could increase as well, making the overall power rating slightly less than normal. Also, if doing so, the wire used should be the same gauge as the primary winding is.

 

[scottjoplin]

^ Good points, thanks