# autotransfomer vs tapped coil in a hartley oscillator

Discussion in 'General Electronics Chat' started by leonheard, Dec 3, 2014.

Jan 8, 2013
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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I guess the difference is the basis for using a tapped inductor in the Hartley oscillator.
The goal is to achieve the required phase shift in the oscillator feedback path so that oscillation can actually take place.

The Hartley has a dual topology in the Colpitts oscillator where the phase shift is achieved using a pair of capacitors ("tapped" at a common reference point) in the main tuning network.

This perhaps begs the question...

Would the Hartley work just as well using two magnetically uncoupled inductors, rather than the single element "auto transformer" / tapped inductor topology?

Give it some thought ....

Last edited: Dec 4, 2014
3. ### Charles Wenzel New Member

Dec 2, 2014
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In the Hartley oscillator the center-tap is held at AC ground (typically) so one end swings in the opposite direction from the other, like a seesaw, providing the phase inversion needed for oscillation. The autotransformer typically has one end grounded and the tap swings in the same direction (same polarity) as the voltage across the entire coil (more like a pry bar).

4. ### ErnieM AAC Fanatic!

Apr 24, 2011
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They are very similar and completely different.

Similar in that they are both taped inductors.

Different in one is for very small signals and the other for AC power.

They do both work on the same set of equations.

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5. ### leonheard Thread Starter Member

Jan 8, 2013
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wow! amazing how complex a coil and capacitor are, they can behave in many different ways .

I would like to study and have a DEEP understanding in this subject. So I would know in the future for example why the tapped coil in a Hartley oscillator is sometimes connected to ground and in other circuit they are connected to power supply. What book/web page you recommend? mathematics is not a big problem for me.

hartley tapped coil connected to ground:

http://www.electronics-tutorials.ws/oscillator/osc6.gif

hartley tapped coil connected to power supply

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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One of the most important concepts in signal analysis is that the effectively decoupled DC power supply ac potential is equivalent to signal ground.
However, it is not a fundamental requirement that the tap point be ac or signal grounded for the Hartley oscillator to function. Other topologies are possible.

Last edited: Dec 4, 2014
7. ### leonheard Thread Starter Member

Jan 8, 2013
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Superposition theorem?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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That's too cryptic for me. What do you mean?

9. ### leonheard Thread Starter Member

Jan 8, 2013
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I mean, in ac analysis you use the superposition theorem, is that correct?