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Old 16th July 2013, 07:53 AM   #2061
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whoopee!

thanks Frank.

Symmetry's a good sign. Ooh, hyperbola. Air gap is a useful addition. First impression of hysteresis is not especially encouraging because of the need to specify a constant operating frequency. OK for approximations to mains transformers, but no good for audio transformers where frequency varies. Also, because it's a curve-fitting exercise, it's hard to estimate the error compared to the real thing. Worth ripping though. I'll look at how LTSPICE implements it and incorporate it if it's not too hard. It'll take some time though.
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Old 16th July 2013, 10:46 AM   #2062
gootee is offline gootee  United States
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Quote:
Originally Posted by PlasticIsGood View Post
It would be very nice if I could get a copy of Chan's paper without joining Yahoo. Otherwise I'll have to wait until I go to the library to download it. If it is just a reworking of Hymowitz, it may not be worth using.

This paper:

http://www.onsemi.com/pub_link/Collateral/AN1679-D.PDF

was intended for modelling transformers in switch-mode power supplies. Further, its main interest is leakage inductance. An SMPS transformer typically has windings of very low resistance. Leakage is much more important. They work at high frequencies, and core characteristics are very different from mains transformers.

Early in the ramble, the author puts the primary resistance in series with the primary, but later switches it to parallel with no explanation of why, AFAICS. He gets the behaviour near enough, perhaps, because the regulation due to resistance is negligible, and the relationship between Cp, Ll, and Rp is maintained, so filter characteristics may be correct or near enough.

As it stands, it is not appropriate for modelling a mains transformer, where for nearly all purposes leakage is insignificant and resistance is important. It has very limited value as a design tool even for SMPS, because flux is crucial there and it isn't included.

I have to admit I regret binning my model. It's much better and works for everything because it simply arranges the transformer equations into a computational model. If other SPICE engines can do recursion like mine can (I was surprised and amazed, just trying it for amusement) then I recommend it. It allows flux to be graphed easily, so you can see if it's too great even if you can't see the consequences because the BH curve is linear.

The tan function used in other models for the BH curve can be easily adopted, and I dare say Chan's work could be built in too. I doubt either approximation would be useful, however, outside of narrow limits. We are interested in the details of distortion, and a generalised approximation is of no value IMHO.

I binned it because simulation has very little value in designing a linear power supply. I prefer to copy best practice and tweak to suit.
I remember discussions in this thread where it was easily determined that the leakage inductance effects were much more important than the resistance, for power transformers, particularly in terms of regulation.
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Old 11th March 2015, 07:55 PM   #2063
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Default Too much capacitance.

We've talked about how much capacitance is too little and how much is just right.

However, the first posts of this thread were all about the audio quality problems of too much capacitance. If that problem is taken to an extreme, it is kind of like trying to hear through a thick beef stew or other muck. Oh, I like beef stew plenty well, but Not in my ear.

I find out that estimating maximum capacitance much for a given transformer is literally as easy as 123.
VA*123=uF (max)
It is a coarse estimate, per each rail of a split rail supply.
That much is nice.
Much more probably isn't.

If the capacitance is larger than the estimate, then at least one of the following applies:
1). There is too much capacitance, or
2). That transformer is too small (recuperates the charge too slowly)



notes and exclusions:
I suppose that most people won't need this estimate; however, if your power board is a lot more bulky than your transformer then this estimate may help.

The estimate doesn't work conveniently when unregulated supplies have transformers less than 100va.

This estimate doesn't apply to regulated supplies, because when a regulator is used, audible effects begin at the regulator, not the reservoir before it.
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Old 11th March 2015, 11:46 PM   #2064
AJT is offline AJT  Philippines
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Old 12th March 2015, 03:26 AM   #2065
buby is offline buby  Slovenia
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Default Power suplly capacitors....

Sangamd capacitors, 34000mF
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Old 12th March 2015, 01:14 PM   #2066
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Hi and sorry to trivialize
But a Voltmeter across the voltage rails could visualize if the uFs are too few.

Click the image to open in full size.

If the needle stays still even during most powerful passages the capacitance is enough. If it bounces the capacitance is not enough to keep the voltage rails steady.
Am i wrong ?
Thanks and regards, gino

P.S. i am sure that this simple test would provide big surprises on most of the commercial amps, hopelessly underpowered. I am sure of this.
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Old 12th March 2015, 01:26 PM   #2067
DF96 is offline DF96  England
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If the needle stays still then either you have a Class A amp or the caps are far far too large. Voltage rails for Class B will sag. The issue is 'by how much?'.
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Old 12th March 2015, 01:55 PM   #2068
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Hi and thanks a lot for the valuable reply.
Quote:
Originally Posted by DF96 View Post
If the needle stays still then either you have a Class A amp
and i understand that class A amps have many estimators. Maybe also this is a reason ? a steady voltage rail ?
Quote:
or the caps are far far too large
maybe
Quote:
Voltage rails for Class B will sag.
i think that we can agree that this not positive because changes the working point of the output devices. Can we agree on this ?
Quote:
The issue is 'by how much?'
this is a good question. Not easy to answer i think.
Thanks again, gino
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Old 12th March 2015, 02:09 PM   #2069
SonyFan is offline SonyFan  United States
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Quote:
Originally Posted by AJT View Post
i understand what you are saying but.....
isn't the % regulation spec of a transformer supposed to cover this?
do we really need to be this precise or meticulous?
can things be as simple as possible?
after all readers who are also diy'ers are not going to be interested in the
pure theory of things at the mathematical levels,
imho they will want to build practical amps that work without
being saddled with maths....

I'm a nerd. Show me the sine waves!
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Old 12th March 2015, 03:46 PM   #2070
DF96 is offline DF96  England
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Quote:
Originally Posted by ginetto61
and i understand that class A amps have many estimators. Maybe also this is a reason ? a steady voltage rail ?
Class A may require a steady voltage, so it is fortunate that its steady current draw makes this easier to provide.

Quote:
i think that we can agree that this not positive because changes the working point of the output devices. Can we agree on this ?
No. The main problem with sagging is the clipping point reduces. Nothing to do with output stage working point.

Quote:
this is a good question. Not easy to answer i think.
Fairly easy to get an approximate answer, which is good enough for most purposes.
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