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Confusing behavior in LCR/stepup transformer circuit - Help?
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me_engineer:

--- Quote from: radiolistener on May 30, 2019, 11:11:48 pm ---your transformer has 1:100 impedance ratio transformation (1:10 turns ratio).
What do you expect from this circuit on the output?
R3 is your output?

Did you check whats going on for entire 0...300 kHz range?

--- End quote ---

My expectation was that the model would match the physical circuit.  Those two features (lack of current dip at resonance and current trending upward, rather than downward, with frequency) mean my model isn't correct.  I'm wanting to find out why so I can model this kind of thing correctly.

The combination of R3 and C2 is the load, but really I want to know why my model is not at all matching my measurements, that's the important part.
me_engineer:

--- Quote from: T3sl4co1l on May 30, 2019, 11:34:15 pm ---Output impedance should be factored out due to measuring the source voltage and calculating gain from that point.

What is this component?  Is there any way for us to check that your LCR measurements are correct?

Tim

--- End quote ---

The stepup gain seems to be correct.  It's the other mismatch between measurements and model that I'm wondering about.

The transformer is something we've had around for a similar project years back, and was intended for this frequency range. I measured it with a decent (if older) HP LCR meter, at the frequency of interest, as:

Primary inductance (and R) - measured L+R series value across primary with secondary open, provides L4 and R6 values directly
Secondary inductance (and R)- measured L+R series value across secondary with primary open, provides L5 and R7 directly
Primary leakage - shorted secondary, measured inductance across primary, provides L8 directly
Secondary leakage - shorted primary, measured inductance across secondary, provides L7 directly

This seem the right way to characterize the transformer?  The numbers match the manufacturer's figures pretty well (though they weren't explicit about leakage, just stated "maximum").

If so, the discrepancy is probably from some substantial output impedance (reactive) in the amplifier that's tough to measure, since the output resistance is very small.  Maybe a huge inductance?  I'm not sure what else to check.
The Electrician:
Can you post the transformer's spec sheet (or a link to it)?  Is it spec'd as an audio transformer?

Measure and plot the impedance at the primary with the load connected over the frequency range 10 kHz to several hundred kHz.
T3sl4co1l:

--- Quote from: me_engineer on May 31, 2019, 01:16:50 pm ---Primary inductance (and R) - measured L+R series value across primary with secondary open, provides L4 and R6 values directly
Secondary inductance (and R)- measured L+R series value across secondary with primary open, provides L5 and R7 directly
Primary leakage - shorted secondary, measured inductance across primary, provides L8 directly
Secondary leakage - shorted primary, measured inductance across secondary, provides L7 directly

This seem the right way to characterize the transformer?  The numbers match the manufacturer's figures pretty well (though they weren't explicit about leakage, just stated "maximum").

--- End quote ---

Ah. Hm. Well the leakage is wrong -- you can't measure these independently, best you can do is hand-wave and split it in half.

The numbers are also inconsistent with the inductance ratio, which I also wonder if that's actually the turns ratio or not.

Tim
MagicSmoker:
Hmmm... what you have drawn is a series resonant LCR network, hence you should see a maximum in current at the resonant frequency.

Also, the preferred way to represent leakage inductance in LTSpice is by reducing the coupling coefficient, K, from 1 (leakage fraction is then equal to 1-K^2). The simulation will sometimes run faster if you model leakage as an external inductor, as you have done, but then you should only put it on one winding as it will be correctly reflected to the other windings by K=1.

Thirdly, inductance scales with the square of the turns ratio, so the secondary inductance should be 250uH, not 168uH.

Finally, yes, the practical way to measure leakage is to short out the primary (secondary) and measure the inductance across the secondary (primary), assuming there are just two windings; if there are more than two then technically you need to short out all of them except for one to measure the leakage, but unless the leakage is really high then just shorting any winding with a high VA rating (e.g. - the primary or the main secondary) will suffice.

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