What happened to toroid power transformers?

Spehro Pefhany wrote:

You hope the PSU filter caps hold up !
Graham
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On Mar 12, 5:58 pm, John Larkin

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.
Paul Mathews
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Paul Mathews wrote:

The omly reason I've never done that is the issue of dissipation in the triac.

A certain designer uses that technique to modulate the output voltage.
Graham
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wrote:

In one of our products, we use a primary-side resistor-triac as both a surge limiter and a crude bang-bang regulator, to reduce the stress on downstream stuff as line voltage changes.
John
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Diagram, please (c:
--
John English


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John Larkin wrote:

It also has a small distributed air gap.

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 btw.
Graham
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Eeyore wrote:

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 :(
Cheers Terry
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John Larkin wrote:

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.
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Winfield Hill wrote:

In which case you'd probably like R-cores and O-cores even more.
Graham
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wrote:

This was for an nmr gradient amp, in a rack with lots of stuff that doesn't like 60 Hz fields. We finally talked them into letting us use switchers - it took about 10 years - and they work great.
John
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John Larkin wrote:

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.
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Winfield Hill wrote:

R-cores are vastly better.
Graham
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Winfield Hill wrote:

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.
--
Regards, Joerg

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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.
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Lostgallifreyan wrote:

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 them yet: http://www.electroassemblies.com/r-core.htm
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.
--
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Joerg wrote:

Expensive ! You also can't bump up the copper as you can with a toroid.

??????
Not the same thing at all.

An R-core is made from continuous strip like a toroid. No clamping is invoved.
Graham
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Eeyore wrote:

Hmm, so how does that make winding easier then?
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Joerg wrote:

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 with the winding machine.
Graham
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Eeyore wrote:

Ah, thanks. To be honest I have never seen one. But it sure looks like a cool trick.
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It does if you can wind the bobbin round the former it's clamped round.
Eeyore wrote:

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 wind?
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