Other approaches commonly taken include a series power resistor
shunted by relay contacts or a triac turned on after a delay. Another
approach is to use SCRs in 2 legs of the secondary bridge rectifier,
using phase control to ramp up the secondary current. This often
works, since part of the turn-on surge....sometimes a big share of
it....is actually the charging current for the secondary side
capacitors reflected back to the primary side, with very little
leakage inductance in series. ST makes a part designed to switch the
line at zero crossings.
John, you can fix this by running toroids at a *lower* flux or you can fit an
inrush current limiting device / circuit.
The absence of an air gap in toroids is a contributory factor to the problem
yeah, but it has to be 50% of Bsat, or the problem still occurs.
So Np doubles.
And given a full winding window, Rp quadruples.
I've since sold it, but I used to have a little 100kVA transformer I
bought for $200, brand new. It ran at 250mT peak flux density. was
designed for a motor test application, where it was switched on & off
about once per minute, hence the tiny Bpeak. but the customer went broke
and never picked it up, so it sat in the factory for several years,
until I came along.
I ended up selling it for $1000 ;)
damn shame though, I could use it now :(
They have poor line-to-output AC isolation. They usually have low
leakage inductance, that's bad for direct bridge-rectifier storage-
capacitor setups. Plus, it's not so easy to add a grounded primary-
secondary inter-winding shield. But hey, what the hell, I like 'em.
Low ac magnetic fields spreading out into my sensitive electronics.
They have poorer line-to-output AC isolation the conventional
types with separated split windings. They usually have lower
leakage inductance, which is bad for direct bridge-rectifier
storage-capacitor setups as it leads to higher peak currents.
Plus, they don't welcome adding a grounded primary-secondary
inter-winding shield. But hey, what the hell, I like their
low ac magnetic fields spreading into sensitive electronics.
We use toroids almost exclusively for medical. A shield layer is no
problem at all. I also use them here in the office and in the lab for
120V/230V conversion because they are almost completely silent.
While following this thread, I saw a couple of people mention R-cores.
Would they not be better?
I'd once seen one and thought it was some eccentric variant on a toroid
that someone made so they could mount it where their design once called for
a chassis mounted E/I type, or had some other odd space restriction. I was
so wrong. :)
From what I saw via Google once I'd seen the name 'R-core', I see that easy
fitting of split bobbins directly round the straight long sections allows
either a commercial firm OR a hobbyist to not only build their windings
quickly and easily, but to modify them, as an assembled bobbin can rotate
freely if wanted. Electrical isolation between windings can be better than
in a toroidal type, which could be important for use in a medical device.
The efficiency is good, and the flux well-contained, and they'd probably
run as quietly as toroidals. Waste heat can escape from them more
efficiently that either E/I types or toroidals. I'm surprised they aren't
much more widespread than toroidals.
They used to be quite popular in TV sets. I believe I still have a few
cores. Nowadays often just called U-U cores. For those who haven't seen
One challenge with these is proper clamping. You can't inspect how snug
the core halves are joining because it is inside the packets.
BTW your follow-up settings aren't right, was missing three NGs. That
would have broken the thread for those folks.
Only easier with the correct machinery.
The formers are made in 2 pieces that clip together and the winding machine spins
the bobbins on the limbs of the core. The bobbins have 'gear teeth' to engage
the winding machine.
It does if you can wind the bobbin round the former it's clamped round.
What does 'bump up' mean? Re expense, if the bobbin can be rotated round
the straight part of the former it's built onto, it would be a lot less
awkward than winding a toroid, it would not be much more awkward that
winding onto any spool. So why would it be more expensive than a toroid,
given that the former is made the same way, and the windings are easier to
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