Analyzing circuits with tapped inductors

If you know how to analyze transformer behavior then you can just treat a tapped inductor as a transformer whose two coils happen to be connected together.

If not, then you need a 2nd-year circuits analysis book from a EE course.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

"M. Hamed"

I am currently looking at the Hartley oscillator (yes I'm getting serious about that stuff), and I currently find it hard to analyze circuits with tapped inductors.

** The Hartley oscillator will operate with independent inductors.

I thought about treating them as two separate inductors, but then there is mutual inductance which complicates matters.

** With independent inductors in series, the impedance ratio determines the voltage ratio at a given frequency.

With a tapped inductor, it is the turns ratio that does it.

... Phil

Phil, to analyze as series inductors, doesn't that assume the current going into the tap is negligible? Is it always a safe assumption?

For example in this circuit,

I couldn't really figure out why I can assume there is small current going from ground into the tap. Thanks.

** Not possible if the inductor **IS* in fact TAPPED. ** Because I posted a reply to your question does NOT mean you can turn around and ask me anything you like.

ALL questions MUST be posted to the group, unless you only want clarification of what some poster has said.

Usenet 101 - pal.

.... Phil

into the tap is negligible? Is it always a safe assumption?

I couldn't really figure out why I can assume there is small current going from ground into the tap. Thanks.

I don't know what spice program you're using, if you are using one at all, but with LTspice the L value also reflects the turn ratio.

There is another L model in Ltspice you can use which will allow you to better specify the coil properties. Just look at the help file for that.

If there is a way to do this with a directive using the default L model to specify turn ratio, I don't know what it is. Maybe a LtSpice or spice in general expert can answer that question. I had to use the other model one day to do what I needed. (John Chan) model, that allows you to specify the turns with out changing my L value.

Jamie

Two inductors, which are magnetically coupled go as:

Ltotal = L1+L2+2*M, where M is the mutual inductance, one to the other.

Where coupling is unity (like two inductors wound on a common core which has very low magnetic leakage, toroid or pot core for example), M=sqrt(L1*L2).

Hence, for tightly coupled inductors, Ltotal = L1+L2+2*sqrt(L1*L2).

This brings us to the concept of coupling coefficient, K, which is commonly used in SPICE analyses. It is simply M/sqrt(L1*L2), which is always

More accurately the turns ratio squared. L1/L2=(N1/N2)^2

The Chan model requires a knowledge of the core hysteretic properties:

The parameters Bs, Br, and Hc define the permeability of the core, with respect to the magnetizing force, H. L is then derived from that, the number of turns, the magnetic path length, the effective area, and the air gap.

For an air cored coil, Bs is infinite, Br=0, Hc=0, Lg=Lm. Hence the Chan model is not suitable.

The inductance of an air cored solenoidal coil depends not only on turns squared, but on its diameter, length, and winding pitch.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

"Fred Abse"

** Put far more simply, if you measure the overall L figure and you have the turns ratio at each tapping - the L value of each tap goes as the square of the turns ratio.

Egs:

For a centre tap L = 1/4 of the total L.

For a 10% tap, L = 1/100 of the total L.

Too simple.

... Phil

I'm not sure why you're assuming a small current going from ground into the tap -- I don't see it in that schamatic.

I really strongly recommend that before you start simulating circuits you understand how they work mathematically -- simulations are never, ever,

100% accurate; if you don't understand how circuits work you don't really have the basis for sorting out real results from simulation scrud.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Quite so.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Noted. That wasn't my > I'm not sure why you're assuming a small current going from ground into

That was just a misunderstanding of what Phil said about series vs. tapped inductors.

Noted. That wasn't my > I'm not sure why you're assuming a small current going from ground into

That was just a misunderstanding of what Phil said about series vs. tapped inductors.