autotransformer help!

Thread Starter


Joined Jan 8, 2013
Hello! I have a doubt, i hope someone can help me...


it is written that:

“ When the primary current IP is flowing through the single winding in the direction of the arrow as shown, the secondary current, IS, flows in the opposite direction.”

Question 1:

Does it mean that the primary winding “ IP “ induces a voltage to the secondary winding ” IS“ ? creating a 180degree phase shift in the secondary winding?

Question 2

What happen if i connect the center tap to ground? like in the hartley oscillator

Thank you.


Joined Jul 18, 2013
There may be a couple of reasons for the Earth ground symbol in the link, One, they intended to indicate a general circuit common and used the E.G. symbol, or, Two, that the earth ground conductor is the supply neutral and therefore is connected to ground potential.
Either way it is a misuse of the symbol in this case.
It is unfortunate that articles like this intend to inform, but can end up not only encouraging bad practices but can be dangerous.

Last edited:
The voltage isn't out of phase at the tap. That bottom current flowing up from ground will raise the voltage at the tap and that's in response to the increasing voltage at the top; they're both moving in the same direction. If you ground the tap and disconnect the bottom (to prevent a short across the winding) the bottom of the winding will have an inverted voltage relative to the top. Think of each turn of a single winding as being a voltage source or battery with X "volts per turn." If you ground the center of such a string of batteries, the bottom will be negative and the top will be positive. Obviously, I'm talking about an "ac battery." A variac is an autotransformer with a wiper that can be moved from the bottom to the top, adding so many volts per turn until reaching the top where the full voltage is realized.


Joined Mar 14, 2008
Question 1: The secondary output voltage is in phase with the primary input voltage, as noted, but that means that the secondary winding current flows opposite to the primary current.
That's right. The ideal transformer has "infinite" inductance so all the "amp-turns" have to add up to zero. Otherwise, the residual amp-turns would produce an infinite magnetic field in the core. If there's no secondary or tap load, there's no current flow. If the autotransformer is tapped in the middle, equal but opposite-directed currents flow in the top and bottom windings into the load, resulting in no net magnetization in the core. Move the tap down to 1/3 up from ground and twice as much current will flow in the bottom winding as the top resulting in zero net amp-turns. (Twice as many turns on top mean twice as much current in the bottom winding is needed to produce the same, but opposite polarity, magnetic field.)

Thread Starter


Joined Jan 8, 2013
thank you very much for the help! i will try to find some books or good page where i can study more about electronics.