Gareth Magennis wrote:


OK, so I want to clone this inductor, L9.

http://elektrotanya.com/digipro_1000.../download.html

It is wound on a ETD29 former, I have counted the turns and the wire
gauge.

What I don't know is the core material. There are several types
available.

(The original has zero gap in the core)



** You sure it has no gap ???

L9 is a PFC choke and they use powdered iron cores or gapped ferrite.

Normally, the middle leg is ground short to create a gap of up to 1mm to
allow DC current to pass without saturating the ferrite. Un-gapped ferrite
cores are good for push pull, square wave transformers only.

You have the original core, so can measure its inductance and have a damn
close look for the missing GAP.

Meanwhile, this link should supply some useful clues.

http://www.tme.eu/en/Document/d3a1e8...d_29_16_10.pdf


I'm pretty sure it has no gap.


** There is one - it might be only a few thou of an inch, but it must be there or the whole thing would blow up.


It's almost impossible to take these things apart without breaking the very
fragile core. I've kept all the bits so will go back and glue them together
to recheck this.

I've taken apart other SMPS transformers where the gaps are quite obvious,
even with a core in several pieces, and measurable with a micrometer without
having to reassemble.


** This is NOT a SMPS transformer, it's a ferrite CHOKE that must pass a couple of amps of DC current. It simply HAS to have an air gap !!!


One other thing, Digipro use enamelled stranded wire on this inductor. I
gather this has better HF performance than the much more common solid copper
enamelled wire.
A quick look for stranded enamelled wire of this guage on the Interweb
yesterday showed it is not widely available in small amounts.

Would you need to adjust the number of turns if you used solid copper
instead?


** FFS, make your own stranded wire.

Dare to be great !!!!!!



BTW, this seems to be a common failure mode of these Digipro amps - this is
the second I have seen. The windings short together, and there is no
insulation or tape applied anywhere on the windings except the final outer
wrap.


** Use higher temp wire, 220C stuff is readily available, the usual magnet wire is only rated at 120C at best.

Do not muck about: an active, high frequency, PFC choke is critical component.



..... Phil