Date: Mon, 8 Feb 2010 02:15:12 +0000
From: Missouri Guy <n0tt1@juno.com>
Subject: Re: [Amps] winding xformer
I wound one once....used the formulas from the
ARRL handbook and from an old QST article.
Low tech, but it worked just fine.
The real "trick" is to get the primary
right so it doesn't draw too much current
when there's no load on the secondary. So, I'd
wind the primary first per your calculations, see if
it heats up too much; if not, then go for the secondary.
Charlie N0TT
## agreed. There is always current being drawn on
the primary, called "magnetizing current". If you don't
put enough turns on the pri, you will suck too much mag
current. Of course, the more turns you put on the pri,
the more u have to put on the sec. [ turns ratio]. The
more turns on the pri/sec... the higher the DC resistance
will be on both windings. OK, so we go to bigger ga wire
on the pri/and or sec..[esp the sec].. to get the dc resistance
back down. Now, all that wire won't fir the core ! Now
we need a bigger core. See where I'm going with this?
It's a fine balancing act. Microwave oven xfmr's
suck way too much mag current, and run hot for
that reason.
## If u want to go cheap on a xfmr, you can go cheap
on either the pri /sec , or both windings.
## some xfmrs, esp fil xfmr's... are one off things.
my dahl fil xfmr runs hot, with no load on it. Runs
barely warm with a full bore CCS load on it. It receives
no cooling air from the tube blower either.
## for big stuff, like vault xfmr's... you don't want
them sized too big. 100-2500 kva step down variety.
Rule of thumb is, you want them loaded down to at
least 40% of their rating... other wise, it's costing
huge bux.
## dahl wound the same primary setup on his xfmr's
for a given core. The only diff between any 2 x xfmr's
both using the same core, was the sec winding. Diff
voltages for example. The primarys were wound the
same on both of em.. [ if the same core XXX used]
later... Jim VE7RF
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