[Amps] Transformer material weight

[Manfred Mornhinweg]

Hi John,

> Thanks, for your excellent review of transformer design.  Coming down to 
> a more practical problem that has bugged me in the past when I've tried 
> to rewind the secondary of a transformer is how to deal with the last 
> two laminations.

A good question! I used to have that same problem, until I found the 
solution! ;-)

> It doesn't seem to matter 
> how much you clean up the laminations the penultimate one usually goes 
> in with a bit of a struggle with the aid of a blunt instrument but then 
> the last one ends up bent and useless.

Cleaning any varnish off the laminations is the first step, but you have 
that right. And here comes the trick:

When you assemble a core, you normally put in the E's from alternate 
sides. That's fine, but next time you assemble a core, ocassionally put 
a PAIR of E's in from THE SAME side, that is two neighboring laminations 
going in from the same side. Do this a few times, so that you end up 
with enough such pairs distributed throughout the height of the stack. 
Now, when you are reaching the end, and it starts becoming hard 
inserting the last E's, insert each of the remaining ones BETWEEN two 
E's that point the same way. These two E's will guide the third one, so 
that you can get it in without applying so much force that you bend it! 
I usually push them in using a block of wood between the lamination and 
my hands.
Inserting these last laminations at one end of the stack, where they 
chafe against the bobbin, is hopeless, but between two E's they will 
slide in!

So you end up with a few groups of three neighboring E-s looking to the 
same side, but that's not a big disadvantage. In any case it's better 
than leaving some laminations out.

When you insert the I's after that, of course you need to look where you 
need two or three I's together.

> So far the transformer I've done this with seem to work OK despite 
> missing one lamination 

Yes, usually it's not too terrible to end up with one or two fewer 
laminations. It just eats away a little of the saturation headroom. The 
worst problem actually is that the stack will end up too loose inside 
the bobbin, and might vibrate, causing audible hum. So it's always good 
to get those last laminations in!

Manfred.

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