Thanks that sounds like an excellent idea, I'll have to try it next
time. Do you have any tricks on how to achieve neat side by side turns?
On 17/09/2011 00:19, Manfred Mornhinweg wrote:
> 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!
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