The list of components in my SPICE simulator does not include an autotransformer. Needing to have that function, I modeled it as follows (see Figure 1).

T1 and T2 are idealized transformers whose turns ratio can be selected. Looking at the step-up direction, the step-up ratio of the autotransformer is the turns ratio of the upper transformer T1 plus one. The inductance L1 models 100% coupling between the lower and upper portions of the total tapped winding and sets the magnetization inductance value of the simulation.

Before trying to actually use this simulation, I needed to confirm the simulation’s validity and I did.

Since my SPICE simulator also does not provide an instrument to directly measure inductance, I used parallel resonance and the LC-resonance equation to examine the presented inductance of the simulation for several values of turns ratio (Figure 2).

The one Henry value assigned to L1 was chosen with malice aforethought since an inductance of that magnitude might apply for low-frequency, high-power applications. However, we can extend this model into RF applications as well. Lowering L1 to merely 27 µH yields the following result (Figure 3):

Different inductances, different frequencies, but we get the same results within the numerical accuracy afforded by SPICE determinations of the resonant frequencies.

Voltage step-up and step-down transformations work the same way as well (Figure 4 and Figure 5, respectively). We get the same results for whatever frequency and inductance we choose.

Having gone through all of this thought and effort, an actual autotransformer application will be described in my next post.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

I guess you missed my SuperSpice, its free 🙂

https://www.anasoft.co.uk/

This model of a tapped inductor, which is an auto trasformer, is in the transformer model lib

.SUBCKT InductorTapped top tap bottom L=20u T= 0.2 R=20 K=0.99 * LTop top topx {L * (1 – T)} RTop topx tap {R * (1 – T)} LBottom tap tapx {L * T} RBottom tapx bottom {R * T} K Ltop LBottom {K} * .ends

John, does your Spice simulator not include mutual inductance (K)? Mutual inductance can easily be used to simulate an autotransformer.

This model assumed ideal coupling which is/was a convenient simplification of the real world. A mutual inductance set up is easily provided and will lower the coupling coefficient to less than unity. I will write that up.

Wow – this is an unnecessarily over-complex SPICE model for an Autotransformer! All you need is 2 inductors. You used 5 inductors. Wow.

Andy, please try to pay closer attention. There are three components, not five. Also, as shown in the next essay, this model allows you to analyze the energy transfer taking place via the magnetics as a part of the total energy transfer. Your “two inductors” will not suffice.

Each transformer part is two inductors, coupled. You present an auto-transformer requiring two transformers plus an inductor – or alternatively, five total inductors. All you needed was one transformer (with their windings connected in series), or two inductors (not five).

Again, Andy, please pay attention.

This autotransformer model uses three components arranged to make energy transfer paths readily ascertainable. Two ideal transformers of arbitrarily settable turns ratios and one inductor yield a unity coefficient of coupling autotransformer from which the energy handling demand placed on the magnetic path confirms earlier calculations which I do not care to invest the time or the energy to re-derive.

SPICE’s own transformer models will not aid in assessing magnetics performance requirements. “Two inductors”, however you plan to connect such, will not aid in assessing magnetics performance demands.

When I presented the transformer company’s engineer with what I wanted to do so that a 110V fan would operate properly and safely for both 110V and 220V product usage, I provided my algebraic results which allowed him to choose the proper core size. If I’d had SPICE at that time, my presentation to that fellow would have been much easier to make.

I think is time to start using LTSpice. The best spice simulator and it is free. Easy to add inductor with mutual inductance.

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