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In Reply to: Power Transformer Model Inductance posted by deafbykhorns on August 11, 2015 at 17:19:39:
"does this inductance ratio apply to OPT's? If so, how would I calculate inductance on a 5000K:8ohm OPT?"There are two general aspects to this. First, you can derive values that will allow your SPICE simulator to work as though the transformer is perfect. That's how I usually handle it. I don't consider the degree of coupling or exact inductance of the transformer useful for most sims. I simply accept the fact that actual performance of an output amplifier in terms of response and efficiency (output power) will be somewhat less than shown in SPICE. That's OK for my purposes. If SPICE says 20W output, but I only see 17W from the prototype, I accept it for what it is.
The second way to derive these values is from actual transformer data provided by the manufacturer. This is much more complicated, but still not likely to yield real-world results in the simulator. For one thing, as with many characteristics of iron core devices, winding inductance isn't a constant. It varies with frequency and power. In addition, it's unlikely a manufacturer will provide data such as leakage inductance, inter- and intra-winding capacitance, distortion vs power and frequency, etc., etc. Even if you had access to all the manufacturer's data for a given transformer, it's my opinion such things are best measured in terms of the resulting performance in a live circuit, and that SPICE is best used as a guideline, not as an end in itself. Also, the level of precision possible by entering all the data relative to every component still won't produce sufficient accuracy to substitute for live testing and measurement. The process will, however, consume massive amounts of time that could be used instead to build and listen to the amplifier. :) Incidentally, the concept of a near-perfect simulation is much more difficult with tubes than with SS. I do this sort of thing almost daily for circuits using opamps and the like, but it just isn't useful in the same way for tubes.
Getting back to your questions, inductance varies as the square of the turns. So does impedance. Therefore, when you tell SPICE that a particular transformer exhibits 1H:4H, it knows the impedance ratio is also 1:4. It also understands that the impedance ratio is the square of the turns ratio, so that in this example, the transformer's turns ratio (voltage ratio) is 1:2.
As for actual values, you're trying to analyze and optimize your circuit, not the transformer itself. So, I simply pick an inductance value that won't "interfere" with the circuit's operation. The general rule of thumb for this is that the winding must have XL not less than 5 times the operating impedance of the circuit it's across (and 10 times is better, nearly perfect from both theoretical and practical standpoints). Of course, XL varies with frequency, and more XL is required as frequency decreases. So, I would typically assign a value for XL equal to 10 times the operating impedance at 20 Hz. For an output transformer driving an 8 ohm speaker, this means a secondary XL of 80 ohms is required, or 0.637H. If the transformer is 5000:8, the required inductance of the primary will be (5,000 / 8) X 0.637, or 398H. These values will allow you to install a transformer in the sim that will produce approximately the same voltage stepdown as the real one. The simulated transformer will also "stay out of the way" while you're attempting to analyze and adjust other components in the amplifier. Easy peasy. :)
Edits: 08/11/15 08/11/15 08/11/15 08/11/15 08/11/15 08/11/15
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Follow Ups
- RE: Power Transformer Model Inductance - Triode_Kingdom 18:46:31 08/11/15 (1)
- RE: Piece of cake when you explain it - deafbykhorns 09:37:25 08/12/15 (0)
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