power transformer inductance

I'm wondering if anyone knows the approximate inductance of the primary and secondary on one of those little 1 watt 120-12.6 power transformers, at _audio frequencies_ (~15kHz)?

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Reply to
bitrex
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No idea, Will it be mostly resistive (or maybe capacitive) at those frequencies? If you have one drive it with a sig gen. and look at the current.

George H.

Reply to
George Herold

Watt is the wrong unit.

I've been told that the toriodial power transformers work better than E-I in the audio range. (I think due to the tape being thinner than the laminations, less gap, and better magnetic coupling.

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Reply to
Jasen Betts

You can get a rough idea of the inductance by figuring the magnetizing current at 120 VAC to be something like 10% of the fully loaded current. So a 1 watt transformer would draw about 1 mA at 60 Hz which is a reactance of

120 kOhms or an inductance of 318H. The impedance at audio frequencies will likely be lower than that of a pure inductance because of parallel capacitance.

I measured a 12V 400mA (4.8W) wall wart and it has a resistance of 275 ohms at 100 Hz and 76 kohms at 10 kHz. Its impedance is 1.75k at 100Hz and 102k at 10kHz. It is 2.75H at 100Hz, 2.37H at 1kHz, and 1.08H at 10kHz.

These measurements may be significantly off because of the rectifiers and capacitors in the wall-wart, and of course it is 5W and not 1W. So a 1W unit would probably measure about 14H. I would expect the 5W wall wart might draw about 120/1000 or 120 mA at 120 VAC, so my initial estimates above are probably off by a factor of 5 to 20 or more.

As a further point of reference I measured a Signal 241-6-16 transformer rated 30 VA. The secondary is 160 mH at 100Hz, 48 mH at 1kHz, and 8.1 mH at

10kHz. Resistance 32 ohms at 100Hz, 90 ohms at 1kHz, and 1.97 k at 10kHz. Impedance 104 ohms at 100Hz, 305 ohms at 1kHz, and 2.04k at 10kHz.

My LCR meter was acting strangely so I don't guarantee those readings. The secondary reads about 0.5 ohms using a DMM.

Paul

Reply to
P E Schoen

Why do you care? Probably best to figger that out and make a measurement that's actually relevant to what you're doing. It's surely not a pure inductance and parasitics may be more relevant. Perhaps resonate it at 15kHz. and calculate the inductance from that? Depending on why you care...

Reply to
mike

That's so far off 50/60 Hz spec that you'll have to try it. I'd bet that it looks more like a resistor than a transformer above 10 KHz.

For a 10:1 ratio at higher frequencies, check out old speaker impedance matching transformers that were popular when copper was cheap and pocket radios ran off 9V batteries.

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Reply to
Kevin McMurtrie

** That is not the case with such small transformers, often Imag barely changes when the rated load is applied because the core is in saturation all the time.

Measuring the primary inductance is treacherous, since it is non-linear with voltage level and frequency.

Iron core inductors have a deliberate air gap that stabilises the L value, transformers do not.

... Phil

Reply to
Phil Allison

** Very large and very non-linear.

IOW near useless.

... Phil

Reply to
Phil Allison

What type of transformer is it ? You might want to check up Lyman's classic text at:

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Lots of detailed, useful information and formulas.

Reply to
dakupoto

nice reference, thanks

Mark

Reply to
makolber

Actually, more like 1kHz. 3kHz for very thin laminations or tape.

The Z(f) function goes up linearly for low frequencies (like you'd expect from a proper inductor), then flattens out about as sqrt(f) above the eddy current frequency (roughly, where thickness of the laminations is a skin depth or three). Eventually, it peaks, then falls over (due partly to inter-lamination capacitance, but mostly winding capacitance) and becomes capacitive (more or less Z ~ 1/f), until still higher resonances do complicated things (dips and peaks due to equivalent transmission line modes in the windings and layers).

If this model is actually literally representative, then we can actually make an estimate...

Supposing Imag ~= Imax/10 (a reasonable guess, see also Schoen's reply) at

60Hz, then at 1kHz we have Imag ~= Imax / (10 * 1000/60) = Imax / 167. Or for 1W at 120V, 50uA. Then from 1 to 10kHz, it goes as sqrt, or 3.16 times less, or 15.8uA. The impedance will be fairly resistive, if it's still below the peak (or fully resistive, at the "resonance" peak).

So the inductive reactance will be roughly sqrt(2)/2 of that impedance, and resistance the other (vector) half. Or about 85H + 4.3Mohm. (Compare to the figure at line frequency, 382H and ESR not much more than DCR.)

Tim

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Reply to
Tim Williams

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