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Old 21st February 2011, 10:48 AM   #31
miklos is offline miklos  Canada
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Quote:
Originally Posted by Richard Ellis View Post
Like this here.




__________________________________________________ ______Rick...
This is not exactly a new idea, I believe in 1885 the Ganz factory in Budapest made a transformer like this.
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Old 22nd February 2011, 06:30 AM   #32
Tyimo is offline Tyimo  Hungary
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Just a single progressive wind layer for each winding across the full core with good insulation between windings.
Why is SmokingAmp suggesting the progressive winding?
Is there any reason except to decrease layer to layer capacitance?
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Old 22nd February 2011, 02:33 PM   #33
ilimzn is offline ilimzn  Croatia
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Of course there is - it comes down to the voltage difference between particular turns in the winding. Although the turn to turn capacitance is the same, the AC voltage between adjecent turns is generally very small, while the AC voltage between turns across multiple layers can get very large, especially in a toroid. A larger AC voltage makes the apparent capacitance between turns that are adjecent physically but spaced in terms of the amount of winding between them, act as a larger capacitance, proportional to the AC voltage at that point in the winding.
With a toroid, the winding is done across a relatively long length of core, several times that of an equivalent EI. This means that when you make a full circle around the core, and start on the next layer, you normally do that without an insulation layer inbetween, the next layer turns have the same turn-to-turn capacitance to the previous layer, as do adjecent turns in that layer, but the voltage difference can easily be several orders of magnitude higher. The layer to layer capacitance will also behave that way.
However, if you wind progressively, putting the whole winding in one core turn, the AC voltage between each physically adjecent turn is kept low, so the whole distributed capacitance appears much lower (the difference can easily be an order of magnitude).
Keep in mind that this capacitance and the leakage inductance form a resonant circuit, the fo of which you want to keep as far from the audio frequency band as possible.
One of the reasons mains toroids can be used as OPTs is that it's very difficult to wind a toroid for very large leakage inductance. So, even though the distributed capacitgance may be very high, the low leakage inductance usually results in acceptably high resonance frequencies, while the low end is still acceptable compared to the price you pay.
However, a winding machine capable of progressive wind takes exactly the same time and labor to make one, as it takes for a normal winding - so there is a LOT to gain in performance with no additional cost.

There are other pit-falls in toroid OPT design, though. For one, to get acceptable tolerance to DC flux, one can wind for a low flux density, however, the OPT becomes very large and the number of turns increases which increases idistributed capacitance (becoming a problem for the primary windings), plus the total length of wire increases, resulting in substantial DCR of the winding (becoming a serious problem for secondary windings). Adjusting wire sizes and winding styles usually results in non-standard core sizes (usually with a larger opening in the core). Most winders also wind cores from tape lamination, so that is usually not a problem.
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Old 22nd February 2011, 02:47 PM   #34
ilimzn is offline ilimzn  Croatia
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Re bifilar windings on a toroid, I will share a recent experience:
The capacitance between two quarters of a PP primary that are wound bifilar, on a 30W OPT (admittedly calculated to go down to 20Hz), is about 65nF. Capacitance between two separately would standrad windings with the same number of turns, separated by the minimum amount of tape insulation is about 2nF. If you want the resosnances of the transformer to be pushed far outside the audio band where you practically do not need to be concerned about them at all, it is obvious what method of winding you have to chose.
Keep in mind that even a half-properly would toroid achieves an order of magnitude less leakage inductance compared to EI cores. This is a very direct manifestation of excellent magnetic coupling between windings, which is generally the reason why bifilary winding is used at all in transformers.
CFB is a special case because the windings can be arranged so that parallel wires only have a higher DC voltage differential, whereas the AC voltage between the wires is very low, in this case bifilar winding is the best you can do, because the capacitance between adjecent wires will not be an issue - dielectric strength of the enamel might be, however. For all other primary windings, bifilar winding on a toroid is not a good idea.
If your aim is to balance the DC resistances of the windings, sectioning is better. It will also reduce parasitic capacitances if the sections are interleaved with parts of the secondary winding, although more than acceptable results can be had just by putting a layer of standard insulating tape between the sections. For PP windings, the number of sections must always be even if DCR balance is the aim - the minimum is 4 sections, one half of each half primary per section. Connect the two outer sections in series to get one half of the primary, and two inner sections to get the other half - the mean turn length is the same this way and the total length of wire in each half-primary will be very close, so the DCR will be balanced.

TIP: grounding the core reduces the parasitic capacitance by a small, but often useful amount.
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Old 22nd February 2011, 03:16 PM   #35
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Please note that progressive winding to lower layer to layer capacitance only tackles one of the capacitive components in a transformer.
The primary by itself also has a capacitive component, the longer the wire the more capacitance.
Therefore choosing a big core with "little" copper is IMHO always better than a small core with "much" copper (apart from nowadays catastrophic copper prices!).
With the big core / little copper combo the lower capacities and DC resistances (copper losses) make better transformers.
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Old 22nd February 2011, 05:20 PM   #36
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Quote:
Originally Posted by ilimzn View Post
quote....One of the reasons mains toroids can be used as OPTs is that it's very difficult to wind a toroid for very large leakage inductance.
Fallacy No1! I've come across some dreadful examples of bunched unequal windings on a toroid with a far worse leakage inductance than an E&I and it's remarkably easy to get it wrong as well as getting it right. The 4 ohm tap is a well known example.

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Old 22nd February 2011, 06:28 PM   #37
ilimzn is offline ilimzn  Croatia
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Originally Posted by richwalters View Post
Fallacy No1! I've come across some dreadful examples of bunched unequal windings on a toroid with a far worse leakage inductance than an E&I and it's remarkably easy to get it wrong as well as getting it right. The 4 ohm tap is a well known example.
richy
True, buit you do not get the luxury of a 4-ohm tap when using a mains toroid as an OPT. You must admit that bad winding techniques really are just that - I was refering to a properly made mains toroid. Having dealt with toroid winding machines one on one just recently, it really takes a rather monumental amount of incompetence to set it up so that it's badly wound - yet I do know of a couple of examples myself. It's bad practice which leads to loss of copper, something most winders today don't ignore with copper prices at current levels.
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Old 22nd February 2011, 06:30 PM   #38
Raj1 is offline Raj1  United Kingdom
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Originally Posted by pieter t View Post
Therefore choosing a big core with "little" copper is IMHO always better than a small core with "much" copper (apart from nowadays catastrophic copper prices!).
With the big core / little copper combo the lower capacities and DC resistances (copper losses) make better transformers.
Having heard some of your transformers i would agree with this
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Old 22nd February 2011, 07:25 PM   #39
Tyimo is offline Tyimo  Hungary
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Thanks Ilimzn!

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However, if you wind progressively, putting the whole winding in one core turn, the AC voltage between each physically adjecent turn is kept low, so the whole distributed capacitance appears much lower (the difference can easily be an order of magnitude).
I don't understand this.
How can wind the whole primary winding (many hundreds of turns) in only one core turn???
I konw that by toroid wind anyway I have to care about to spread the turns around the whole core.
An ideal toroidal coil has its winding spread over 330 degrees of the core. This leaves ca. 30 degrees gap an the ends of the winding.
I thougt that progressive winding is good for sectioning and for perfect splitting the primary.
Like one half of the core is for the half of the primary windings and the another half of the core is for the rest of the primary. 50%-50%.
Could you show me a picture or a drawing to understand it?

Tyimo

Last edited by Tyimo; 22nd February 2011 at 07:34 PM.
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Old 22nd February 2011, 07:56 PM   #40
Tyimo is offline Tyimo  Hungary
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May be you mean something like this:

"Progressive Winding:
Divide the total number of turns in two parts (50% each) like in a common mode choke and start to wind following the sequence indicated in figure 1, starting in ‘a’ and following the turns for each side in layers until obtain the total number required, neither start or finish can be in touch each other and the space between they must be maintained by any physical way according to the specified in the IML for the product."
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