# The Phase in Tapped Inductors

Discussion in 'General Electronics Chat' started by sjgallagher2, Jan 19, 2016.

1. ### sjgallagher2 Thread Starter Member

Feb 6, 2013
106
7
I was working through a circuit called a T-coil, and the reference is one of Jim Williams' books on Analog Design. The circuit is simple looking, it is just ac input to a center-tapped inductor with a resonating capacitor and an output cap. That's all good and dandy, I thought. Then I wondered- if the inductor is center-tapped, does that mean there's a 180 degree phase shift from one end to the other, leaving 90 degree phase shift in the middle? It reminded me of center-tapped transformers, and Hartley oscillators, and I was really baffled coming up with a reasonable answer. The inductance of a coil, I then thought, is defined by the way it reacts to changes in current. Of course the 90 degree phase shift each makes sense mathematically - when there's a current peak, instantaneous change in voltage is 0. So if I just put two inductors in a row, will I get 90 degrees overall or 180, and 90 at the center tap? Plenty of thoughts have gone through my mind, and even after putting it into SPICE, and considering how in a hartley osc the inductors are tapped to ground, I'm still not convinced at any given answer. The spice simulation, by the way, showed with a given ac input the inductors act as simple voltage dividers, and they don't contribute phase shift to each other. Sure, inductances' reactance adds up like that nicely so you get jX_L, total. But what if the inductors share a common iron core and have mutual inductance? Will the back EMF affect the voltage in the second coil?

With all these thoughts in mind, would someone explain to me how a center-tapped inductor works? First, with an air-core, then with iron-core? Let's talk in general ways, no one in specific. Thanks so much in advance!
Sam Gallagher

2. ### sjgallagher2 Thread Starter Member

Feb 6, 2013
106
7
An update. I took the time to analyze the phasors in a simple inductor voltage divider with one series resistor and one parallel resistor. The result seemed random and unintuitive, as it had a voltage phase shift that was negative, as if it were a capacitive circuit. Where is the intuition in using such a circuit as a tapped inductor? I have also realized that a tapped inductor with a common core is an autotransformer, so you can nix that! Thanks.
Sam Gallagher