Extra turns on transformer secondary

[alibear]

Hi all, I have some new Novutem transformers with 25-0-25 secondaries, I really require 28-0-28 is it possible to add some extra turns to both secondaries to achieve this without unwinding the secondaries first. I know very little about toroid constuction but I assume that the secondaries are wound one on top of the other. Is there an issue if I wind some extra turns on each secondary, eg if I wind some extra turns on secondary No. 1 ( the first secondary to be wound when the transformer was made) these extra turns will then lie on top of the original No2 secondary, is this a problem, and then the same thing applies when the turns on No. 2 secondary are increased.
I do not want to attemp to unwind the original secondaries as I think this will end in dissaster.
Any help or advice greatly appreciated

 

[KatieandDad]

YES.

As an experiment wind 10 turns of wire around the core and power it up to measure what voltage you get from the 10T. Divide this voltage by 10 which will give you the V/T figure for the transformer.

Then divde 3 by this figure to give you the number of extra turns that you need.

Ideally you want to space the turns around the toroid equally so they are evenly spread.

After you have wound the extra turns you need to ascertain the correct phase of the new windings. This is easilly accomplished by winding each new secondary in SERIES with the existing secondary. If the voltage goes UP you are IN PHASE which is what you are looking for, If you are OUT OF PHASE the voltage will go DOWN by the calculated amount.

Personally I would make the connection between the existing secondary and new secondary away from the transformer, less chance of sharp edges damaging the insulation.

 

[woody]

I would expect in most applications your transformer will work fine as is. That 25-0-25 rating is not cast in stone it depends some on what the transformer load is and the real
line voltage in your location. It might not be wasted effort to try that transformer as is
and then go from there.

 

[AndrewT]

If you have a 230:25+25Vac transformer, then running it from a 240Vac supply gives you ~240:26+26Vac. Add on a few percent of unused regulation and you are not far away from 28+28Vac.

 

[KatieandDad]

As everyone is saying 25-0-25 is generally close enough to 28-0-28.

Going the other way and using an anti-phase secondary to reduce the voltage is sometimes more critical.

I can already hear the God Squad saying where does the extra power go - Please read Kirchoffs laws.

 

[DUG]

Yes

e.g.

25V @ 10A = 28V @ 8.9A

maintain total VA

 

[alibear]

Thanks for the replies
Andrew T the makers specs for the transformer are:
230v in 26.85 out with no load
I have been monitoring my incoming mains supply throughout the day and it seems to stay relatively constant between 239 and 240, I have not seen it go above 240v. So calculating from the makers figures I get 28V exactly with no load. I require a DC voltage of 37V at 1.9 Amps. I think with the 25-0-25 I will end up with 35V under load ( transformer is 300VA ). If I increase the number of turns to give me the extra volts I realise that the VA will be slightly reduced.
KatieandDad, If I decide to try increasing the number of turns I was going to make the connection at the end of the secondary wire( leaving it 150mm from the core) and then wind the required number of turns and terminate again 150mm from the core. this would then leave me with a secondary of 28-25-0 obviously the same with the other secondary. I was going to try putting the extra windings on top of the existing insulation tape, so as not to cause any damage to the transformer. Does this sound reasonable.
Thanks everone for your help
Alan