# Transformer Query

Discussion in 'General Electronics Chat' started by Kimbal, Apr 17, 2014.

1. ### Kimbal Thread Starter New Member

May 7, 2008
6
0
If one takes a transformer with 3 identical windings and they use 2 of those windings as a primaries ( not secondaries ), wired independently of each other and IN PHASE, but also with identical in-phase input signals of the same amplitude and frequency;- would the single winding used as a secondary output be the sum of the two inputs ?
( ie: twice the input signal current and the same voltage.)

Please ignore wire ratings, turns ratio, and insulation specs, etc.
( This is a theoretical question. )

What stops the two primaries fighting each other and one trying to respond as another secondary ? Phasing or something else ?

If the 2 source signals were sensitive to a large output signal back feeding into it - would not one of the primaries back feed into either source and act more like a secondary; hence risking damage to the source ? This I would expect if the real secondary was unloaded.

I'd be interested in further comments.

2. ### MrChips Moderator

Oct 2, 2009
14,504
4,270
The voltage output would be the same.
The current is determined by the load on the secondaries.
It is like having a primary with a thicker wire.

3. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
The physics of transformer action.

The primary or primaries induce a magnetic field in the core.

This field acts on all windings on the core, primary and secondary - it does not distinguish.

Because there is already a voltage in the primary (ies) the result is the vector sum of the induced voltage and the applied voltage.

Since there is only the induced voltage in the secondaries, this is what we see.

Note this is a very simplified version of tranformer action but it should suffice to answer your question.

4. ### daviddeakin Active Member

Aug 6, 2009
207
27
Yes absoluetly.

5. ### MrChips Moderator

Oct 2, 2009
14,504
4,270
No. A second primary has the same voltage induced on it.
It is like connecting two identical batteries in parallel.

Dual voltage transformers are wired this way.
For 240V operation, the two primaries are wired in series.
For 120V operation, the two primaries are wired in parallel.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
I think you will find that the OP means there are two separate sources (which may be different) connected to the two separate primaries.

So there is a definite possibility of (adverse) interaction.

For truly identical signals into truly identical windings there would be no adverse interaction, but we all know there will be differences in reality. The question is (as with the batteries) are these differences significant?

7. ### MrChips Moderator

Oct 2, 2009
14,504
4,270
When two independent sources are connected in parallel, one of the two sources (the higher voltage) will be running flat out until its internal resistance causes its output voltage to fall to match that of the lower voltage source. From there on there will be some load sharing, though not equal.

8. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,647
1,595
Something else, the output impedance of the sources. This is how a telephone hybrid "coil" works. Generally speaking, a transformer doesn't care which way the energy is flowing. If the circuit connected to a winding is a source, or output, it usually has a much lower output impedance than a circuit connected to another winding that is an input. This directs the flow of energy. Also, outputs make outputs and inputs don't (usually). But if, like a telephone line, a connection can be both a source and a load, then the energy it sources goes to all other windings, including other sources, and is dealt with by those circuits. This is a much bigger deal in RF circuits, where reflections from a mismatched load can come back down a coax cable or waveguide and blow up an output stage.

ak

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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I thought this notion had largely been debunked by RF experts. I would refer you to the writings of M. Walter Maxwell, W2DU/W8HKK, for instance.

10. ### alfacliff Well-Known Member

Dec 13, 2013
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a mismatched load on an rf amplifier can damage an output stage. reflected waves cause heating and voltage effects in the output stage unless protected by an swr protection circuit. why would manuacturers waste money on swr protect circuits if it didnt?

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
Of course one generally requires a matching network between the final RF amplifier stage and the antenna feed line. But this is a different issue in relation to the line reflections. One could have a well matched output stage but still have substantial line VSWR without any damage to the final stage occurring.