# I'm confused about current direction of transformer

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 15, 2016.

1. ### Dong-gyu Jang Thread Starter Member

Jun 26, 2015
100
4
Hello.

For typical battery (nothing but capacitor), current direction is clear as electrons can't not flow across the two plates in battery so they can only choose wire as a path from cathode to anode.

But in secondary side in transformer, basically electron also can go through the "connected" coil so I guess electron can choose this path. Maybe it is more favorable as it is short path from cathode to anode.

The Ohm's law on resistor forces me to think that current path should be the same to case of the battery but It doesn't physically convince me.

How we can determine current direction and why?

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2. ### crutschow Expert

Mar 14, 2008
13,496
3,373
The transformer plus and minus signs do not indicate absolute polarity since the signal is AC, they indicate the polarity (phasing) with respect to the other windings (in your diagram you need a + and - sign on the primary otherwise the secondary polarity signs have no meaning).
This phasing is more commonly indicated by a dot at one end of each winding. The phasing dot indicates that the phase of the signal at those two points are the same with respect to the other side of the windings (i.e if one dot has a positive going signal applied then the other dot will also have a positive signal).

As far as what direction the electrons flow, that's determined by the instantaneous polarity of the AC signal out of the transformer.
The electrons will always flow out of the terminal that's negative at any instant in time.
This reverses at the AC frequency of the signal.

Dong-gyu Jang likes this.
3. ### Dong-gyu Jang Thread Starter Member

Jun 26, 2015
100
4
Thanks to give me practical guide of transformer notation.

I'm still not convinced why electrons always flow "out" of the terminal, rather than "in" (across the coil here). For battery, electrons can not flow vacuum or insulator between two plates so direction "in" (across the plates here) is prohibited. Easy to understand. But for coil, what does force electron not go "in"? Coil is nothing but conductor which is electron flow path.

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The standard on flow of electrons is confused. Folk regularly squabble about it on this site. Understand there are two standards, and with some exceptions that deal with physics it doesn't usually matter.

Phase on transformers matter when you are trying for feedback (as with oscillators) and signal canceling (like on telephones).

5. ### Dong-gyu Jang Thread Starter Member

Jun 26, 2015
100
4
Am...so for transformer, current direction is just following the convention of what most people just accept? Does it mean we really don't know true direction of current? It is so wired for me..

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Only AC passes through a transformer. The AC signal can be inverted or not, depending on how you attach the wires of the secondary. So the current goes both ways.

If you want a power supply then you use diodes to convert the AC to pulsating DC. So you know which way the current is going on an instantaneous level, but the correct answer is it goes both ways.

When phase matters I put a phasing dot on my schematics. Most cases it does not matter.

AC = Alternating Current.

https://en.wikipedia.org/wiki/Alternating_current

7. ### profbuxton Member

Feb 21, 2014
233
68
I always use the convention that current flows from positive to negative. Very very rarely to I have to concern myself about electron flow since I don't do quantum mechanics or design IC etc.
For 99.9% of electrical/electronic work postive to negative works just fine.

8. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
A transformer only works in an AC circuit (though that does not mean that it has to be sinusoidal, but merely that it won't work with a DC input). For the sake of simplicity, let's stipulate that we are talking about sinusoidal AC. As such, the electrons move first in one direction and then in the other, changing direction twice per cycle in response to the reversing magnetic field in the secondary due to the reversing current in the primary.

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,850
965
Getting the electrons to 'move' requires work. It takes an expenditure of energy to make electric current move. Something has to charge a capacitor. The work is expended on driving the generator which makes electrons enter a capacitor. That energy is now stored in the capacitor in the form of an electric field. With transformers and AC. The work is stored in the magnetic field.
Electricity is a car without a motor. To make it move you have to get behind it and push.

10. ### withoutego New Member

Dec 22, 2015
23
4
I have the model in my head of current being two flows. One is positive to negative
and the other is negative to positive. One is electrons the other is holes. For every
electron that moves left to right a hole must move right to left.

or, to be practical, current moves against the arrow of a diode...neg to pos, most
cases this is the "pull-up" from a negative ground to the positive rail. Forward biased
diodes pointing toward the neg ground.

Electricity is not dependent on our models. True understanding may be beyond our
little minds. The model you choose just needs to match reality.

11. ### Dong-gyu Jang Thread Starter Member

Jun 26, 2015
100
4
Hey guys.

Thanks to give me feedback. I was thinking this issue seriously and got clear answer. The attached picture made it briefly clear.

In this case, I was confused by + and - notation in secondary wining. I instantaneously thought electric field direction was + through coil to - like capacitor. It was wrong. The induced field direction is from - through coil to +!

If voltage is developed from space charge separation like capacitor, my original imagination is right. However here, + and - notation really doesn't relate to true field direction! They're just indicating current direction in conventional way.

I think this post is clearly finished now

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