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Old 11th August 2012, 12:57 PM   #541
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Originally Posted by Terry Given View Post
this transformer might turn out to be a TERRIBLE choice for modelling power supply interactions
Terry, my experience with transformers is just sufficient to get me by with simulations, and the last time I looked at the matter to any degree was a couple of years ago. However, I had a quick rummage around my downloads of the time, and also just had a quick look around now via Google; and it appears that a standard power supply toroid of that rating has magnetising current in the ball park of the figure that you query.

So, for better or worse, those are the order of transformer characteristics that the modelling needs to use, it appears ...

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Old 11th August 2012, 12:59 PM   #542
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Terry, thank you very much for your time and explanation, I would not say I am now qualified in the matter, but I have a far better grasp of where you coming from with your thinking of the power supply modeling.
A typical answer for whom dont know an answer.
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Old 11th August 2012, 01:07 PM   #543
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Liching is "resting" for a few days.
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Old 11th August 2012, 01:12 PM   #544
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Not to take anything away from Terry's excellent and comprehensive explanations ... for the tiny number of those not familiar with this material, there's also Inrush Current ...

Frank
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Old 11th August 2012, 01:14 PM   #545
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Liching1952,

lets stop fooling with everyone, I don't think you are stupid, but I also do not think you totally understand what we are trying to argue here.

The question is to try to establish a simple set of rules that will mostly satisfy the power supply requirement of a typical DIY power amplifier, given rudimentary power requirements.

We are not trying to establish who is the smartest kid on the block but trying to learn from each other, especially from those in the knowing.

So far some members were really forthcoming by putting a lot of effort into the matter and I thank you all without singling out anyone. It has made this thread genuinely worth following and I again thank you all for your contributions.
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Old 11th August 2012, 01:21 PM   #546
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What are your comments volks?
While you're chilling, the harmonics produced you do not want to hear or have them cause the amp go boom. Hence the NFB loopfrequency transient response must be capable of erasing those harmonics and otherwise intrusive transients to be suppressed down to below the noise level. You need a regulatory frequency at least 10 times the desired signal-to-be-reproduced-by-negative-feedback bandwidth to reproduce accurately. To do that, the logical system that is the closed negative feedback loop is going to need current when it wants to. Preferrably 'now' and not in a few units of time.
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Old 11th August 2012, 01:23 PM   #547
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Andrew,

if we ignore core saturation, it pops straight out of the solution to V = L*dI/dt if we take the initial conditions into account. this is pretty easy to test in spice (where its kinda hard to model actual cores, which is amusing).

the thing is, unless you study network analysis (in say an EE degree) you will hardly ever come across initial conditions. they complicate everything, so are almost always ignored. and in EE its mostly covered in network analysis, so all the other fun courses casually ignore it too. hell, I ignored it right up until it bit me in the ****. now I specifically check for initial conditions I have to care about before starting an analysis.

hand-wavey explanation:
(I'm using electrical angles cos its less confusing).
in an inductor the current and voltage are 90 degrees out of phase. with zero-crossing turn-on (assuming no stored flux in the core), V(0) = Vpk*sin(0) = 0 and I(0) = 0. whereas in steady state at the zero crossings V(n*pi) = Vpeak*sin(n*pi) = 0 but I(n*pi) = Ipeak*cos(n*pi) = +/-Ipeak

so turning an inductor on at zero voltage effectively applies a transient step to the inductor current of Ipeak, which will decay with an L/R time constant. so the peak inrush current can be twice the magnetising current.

Any residual magnetism in the core will either add to this, making it worse, or subtract from it, making it better.

when you include xfmr saturation this gets a whole lot worse, as L drops a lot and I skyrockets - my 400VA variac has an isolating transformer, and the bloody thing occasionally pops a 10A breaker (yeah, I know, but its easier to flick the breaker than pull the variac out and put in a soft-start. one day.....)

if the xfmr fully saturates then Lmag = Lp_leakage = bloody small, and that along with Rp are all that limit the primary current.5% - 10% is typical for a mains transformer, so 10x - 20x inrush currents result.

its for this reason that many xfmrs have a small gap. this reduces Lmag, increasing Imag, but ensuring the transformer doesnt saturate reduces the inrush current from 10..20xImag down to 2xImag.

google high voltage inrush limiting resistors. these are AWESOME and are in series with humongous power transformers in substations etc.

here's a better analysis: http://xa.yimg.com/kq/groups/2334943...ransformer.pdf

Last edited by Terry Given; 11th August 2012 at 01:36 PM.
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Old 11th August 2012, 01:25 PM   #548
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"resting"

What a shame, for the occasional sTroll-in, he/she was refreshingly amusing.
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Old 11th August 2012, 01:32 PM   #549
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Frank,
yeah, Rods site is great, and he's pretty good with the technical details, although he tends not to be all mathematical. thats the cool thing about electronics - the practical is at least as important as the theoretical, if not more so.

re. initial conditions - I got really good at laplace transforms years ago. if we ignore initial conditions its actually just algebra, right up until we have to do inverse transforms. which we hardly ever do - the frequency response is usually the desired goal. Other than in maths classes at Uni, I dont think I have ever done an inverse laplace transform analytically - I use an optical-neural pattern-matching heuristic for inverse transforms (I look through a big book of laplace transforms until I see an s-domain function that matches what I have, then write down the time-domain column).

that served me well for 15 years as a power electronics engineer. until one day initial conditions became really important, and I suddenly realised I had to go back and learn how to do them.
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Old 11th August 2012, 01:38 PM   #550
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Originally Posted by MagicBox View Post
Hence the NFB loopfrequency transient response must be capable of erasing those harmonics and otherwise intrusive transients to be suppressed down to below the noise level. You need a regulatory frequency at least 10 times the desired signal-to-be-reproduced-by-negative-feedback bandwidth to reproduce accurately. To do that, the logical system that is the closed negative feedback loop is going to need current when it wants to. Preferrably 'now' and not in a few units of time.
And herein may lie the great answer as to why negative feedback in many implementations seems to fail to live up to expectations, why it has such a bad rap ...

Frank
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