Offline transformer winding...why not always "spiral" the final turn back across the former?

[treez]

Hi,
A contractor has given us a transformer manufacture spec for a 120W Offline Flyback.
For the secondary, he has 11 turns of 3 strands of TEX-FS 0.7mm triple insulated wire.

This 11 turn secondary has a total width of 29.7mm, when the bobbin width is only 27.7mm.

Would you presume that this means he has wound ten turns and then the eleventh turn is spiral wound evenly back round and across the bobbin to be terminated?

This method of spiralling the final turn back and then terminating it, sounds like a great idea....why is it not done as a standard technique? After all, with offline SMPS transformers, one usually terminates all primary side windings to the same side of the bobbin (for isolation reasons)...which means having to bring the coil_end back across the bobbin.....if you've gotta do that, then why not spiral the final turn back as described. Why is this technique not standard?
(The bobbin is a 12 pin E42/21/15)

E42/21/15 former

http://en.aet.com.pl/Portals/en/Kat...EK 12PIN HY4214.pdf?ver=2015-06-11-125237-890

Tex FS 0.7mm triple insulated wire:

Home Page

www.pacecomponents.co.uk

 

[betwixt]

If it wasn't standard you would either have to use wire that was an exact fit or a bobbin size to suite the wire you chose. There is no alternative if you want the connections to terminate at one side of the transformer. If the turns don't exactly fit they have to 'spiral' to the terminating end or else do a 90 degree bend above the termination and that would be difficult to manufacture.

Brian.

 

[treez]

Thanks, i must admit it does beg the question, with a high voltage offline flyback primary.....if you would rather put tape over the winding before spiralling the last turn back across the winding layer?....or just spiral it straight back and rely on the enamelled copper wire covering? I've never specified to use tape like that but i bet most of the winders do it anyway?

 

[betwixt]

The problem with tape is its width would have to match the length of the spiral. Imagine the winding was one and a half layers, the spiral would complete the path across the last half so taping under it would need a width of half the winding. If it was 1.75 widths the tape would have to be 0.25 of the winding width. Either that or the final full winding would have a full tape layer to ensure the spiral sat on top of it..

Brian.