# Coupled Inductors

Discussion in 'General Electronics Chat' started by Adanovinivici, Sep 28, 2014.

1. ### Adanovinivici Thread Starter Member

Sep 5, 2014
58
0
Hello All,

I'm trying to figure out how you know how to decide the dot convention on a set of coupled inductors. Does it depend on how they are wrapped around the cross-sectional area? If a dotted region is at one end, how do you know what the corresponding dotted region is on the other coil (top or bottom)?

Thank You

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,062
322
Excellent question. The dots indicate voltages that are in phase. The tricky part is that with certain types of transformers, such as transmission line transformers, the leads coming out of the same physical end of the device will be of opposite polarity. So, when in doubt, always measure!
Eric

3. ### Adanovinivici Thread Starter Member

Sep 5, 2014
58
0
Thank you for the response. I also have another question: Assuming a rectangular cross sectional area and the dot on each side both reside on the positive node of the voltage source with current moving through the positive node, the flux caused by the magnetic field seems like it should induce a current in the opposite direction so why is the induced term Mdi/dt, positive? The change in direction of magnetic field should be resisted? I know this applies to AC current, but my question is why the convention is still positive?

4. ### crutschow Expert

Mar 14, 2008
16,227
4,339
If you have two windings (assume a 1:1 ratio) wound in the same direction on a core (with a secondary load connected), then the two leads coming out of the same end will have the same polarity. That means current will go into the plus primary winding with an applied positive voltage which then generates a positive voltage at the secondary plus output, causing a current out of the secondary. Thus the current into the plus primary input is matched by the current out of the plus secondary terminal meaning the flux from the primary current is countered (cancelled)by the flux from the secondary current.