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Old 13th July 2013, 11:15 PM   #2041
gootee is offline gootee  United States
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I think that secondary resistance is just measured with an ohm meter.

Attached is an image of my original spice transformer simulation circuit, which contains all of the transformer measurement instructions.

But the much better model that I created, after that, with the help of Terry Given, is SCALABLE, i.e. you can change the output voltage, the Volt-Amps rating, the mains voltage, and the mains frequency, and the model parameters are all scaled automatically to reflect the changes! An image of that LT-Spice model is also attached. (Note: I am not sure how the accuracy varies, as the model is scaled.)

Besides using the model in LT-Spice, I also implemented the scalable transformer model in MS Excel, as part of the power supply simulation I created. If you download the spreadsheet and open it, click on the Transformer tab at the bottom, to get to the transformer model. It, too, includes the transformer schematic and the measurement instructions.

MS Excel 2007 version:
Power Supply Resevoir Size

MS Excel 97-2003 version:
Power Supply Resevoir Size

Cheers,

Tom
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File Type: jpg transformer_meas_pu.jpg (460.7 KB, 161 views)
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Old 14th July 2013, 12:25 AM   #2042
fas42 is offline fas42  Australia
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Quote:
Originally Posted by PlasticIsGood View Post
One thing I'd like to see modelled is mains transformer distortion.

PSUD is useful.
LTspice will easily demonstrate what happens to the mains power waveform, which does become badly distorted across the secondary, when real behaviours of transformers are included.
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Old 14th July 2013, 03:35 AM   #2043
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Quote:
Originally Posted by gootee View Post
I think that secondary resistance is just measured with an ohm meter.
...
Thanks for links.

I asked because I wondered if Rp is taken into account, but that was the version linked to from another thread, and now I'm disoriented

Anyway, AFAIK the primary resistance should be in series with the primary current, so that it is effectively reflected to the secondary, as is primary capacitance. Measuring the secondary with DC doesn't take this into account, and your model appears wrong in this respect. I might be missing something though.
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Old 14th July 2013, 04:19 AM   #2044
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Originally Posted by fas42 View Post
LTspice will easily demonstrate what happens to the mains power waveform, which does become badly distorted across the secondary, when real behaviours of transformers are included.
I use another version of SPICE 3f4, and developed a model some years ago intended for valve output transformers.

The model allows the transformer to be specified in term of physical parameters, and allows cores and windings to be connected via a bus. This was quite educational but ultimately I ran out of steam when I got to hysteresis.

A static BH curve can be approximated but not very well. With considerable difficulty and cheating, hysteresis can be modelled at one frequency. If there's an LTSPICE model that does all this for all frequencies I'd love to see a listing.

The issue is important but often ignored. For example, a commercial general purpose toroid, to take a worst case, will typically be nudging the top end of the BH curve, close to saturation, when lightly loaded. In such a state it produces its maximum mains harmonics and its inductance is furthest from linear. Those who deliberately oversize transformers in pursuit of lower regulation are not necessarily improving matters. There's no substitute for purpose-built quality.

I don't understand why all DIY amps aren't class A, or shunt regulated. Some people get their kicks from saving energy, I guess.

Maybe it's not as significant as I suspect. That's why it would be nice to see a simulation.

P.S. This is related to my other point above about Rp. Because it is in series with the primary inductance and passes all primary current, the voltage across the primary inductance, and hence the magnetising current, drops as secondary current increases.

Last edited by PlasticIsGood; 14th July 2013 at 04:29 AM.
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Old 14th July 2013, 04:57 AM   #2045
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LTspice handles hysteresis using the John Chan model - there's a thread here that mentions the use: Free Spice Or Cheap Spice Simulator-Where To Start?. In the Yahoo LTspice user group, the Files section has examples in a Magnetics folder, along with a copy of the original paper by John Chan.
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Old 14th July 2013, 05:46 AM   #2046
gootee is offline gootee  United States
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Quote:
Originally Posted by PlasticIsGood View Post
Thanks for links.

I asked because I wondered if Rp is taken into account, but that was the version linked to from another thread, and now I'm disoriented

Anyway, AFAIK the primary resistance should be in series with the primary current, so that it is effectively reflected to the secondary, as is primary capacitance. Measuring the secondary with DC doesn't take this into account, and your model appears wrong in this respect. I might be missing something though.

All models are wrong. It's just a matter of how wrong they are.

Did you also see all of the AC measurements that were made?

Anyway, it's not my model. There is a link to an OnSemi appnote PDF file, right on the schematic. Christophe Basso justifies the model fairly well, in the appnote, in my opinion.
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Old 14th July 2013, 09:47 PM   #2047
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Originally Posted by fas42 View Post
LTspice handles hysteresis using the John Chan model - there's a thread here that mentions the use: www.diyaudio.com/forums/everything-else/96115-free-spice-cheap-spice-simulator-where-start.html. In the Yahoo LTspice user group, the Files section has examples in a Magnetics folder, along with a copy of the original paper by John Chan.
Thanks. I'll read the paper.
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Old 14th July 2013, 10:05 PM   #2048
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Originally Posted by gootee View Post
All models are wrong. It's just a matter of how wrong they are.

Did you also see all of the AC measurements that were made?

Anyway, it's not my model. There is a link to an OnSemi appnote PDF file, right on the schematic. Christophe Basso justifies the model fairly well, in the appnote, in my opinion.
Models can be close enough if used intelligently, for the intended purpose. They can be very wrong otherwise.

Regulation is an important consideration in your use of the model. If it fails to take into account primary resistance, then regulation will be underestimated. It's your job as the model user to check whether the difference is significant.

Just trying to assist your understanding.
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Old 14th July 2013, 10:12 PM   #2049
gootee is offline gootee  United States
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Thanks. I do understand the limitations of engineering modeling and simulation.

The preparation of the model was with the assistance of Terry Given, who is an experienced professional magnetics expert.

The model includes the primary and secondary resistances and leakage inductances, and the primary magnetizing inductance.

I believe that it is sufficiently accurate for simulation of linear power supplies for audio power amplifiers, as long as it isn't scaled too far away from the initially-measured parameters.

But it is always the user's responsibility to validate and verify any model they choose to use.

Last edited by gootee; 14th July 2013 at 10:21 PM.
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Old 15th July 2013, 03:59 AM   #2050
1audio is online now 1audio  United States
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Plastic-
For the denser of us, what exactly is wrong with the model? What is the scale of error the "incorrect" model will create? While we are all content to accept Ohm's law as an absolute most of the simulation work is not totally accurate and in many cases a simplification enables meaningful answers quickly. Most electronic systems can be reduced to problems that can be solved with a slide rule and yield working systems.

In this case small differences in the available supply voltage before clipping from different models is a minor issue. The AC supply can easily vary more than 15% regardless of the load. I don't know if the model adjusts for the heat in the transformer or the change in drop on the diodes with heat. But I suspect the difference in max output from the details in the model will be at or below the threshold of hearing.
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