Transformer/amperage question

[Rysdad]

I understand step up/step down transformers and voltage/amperage changes, but I was wondering about the following scenario:

Lets' say you have a step up transformer with 50 windings on the primary and 100 windings on the secondary. The voltage would double and the amperage would decrease by half, right?

Ok, now let's say you wrap the secondary with another set of windings (on a completely separate circuit)--say 25 windings.

What happens? Would the interference cancel itself out? What would you end up with on the two secondary circuits?

Confused.

[Q.E.D.]

Originally Posted by Rysdad

Lets' say you have a step up transformer with 50 windings on the primary and 100 windings on the secondary. The voltage would double and the amperage would decrease by half, right?

Approximately. Voltage ratio is generally the same as turns ratio, but the exact relationship depends on a number of factors, such as core type and winding geometry. The current ratio may or may not be equal to the inverse of the turns ration; this is heavily dependent upon the various iron and copper losses present. In an ideal transformer, yes, you'd expect the current through the secondary circuit to be about half the current through the primary given the turns ratio above.

Originally Posted by Rysdad

Ok, now let's say you wrap the secondary with another set of windings (on a completely separate circuit)--say 25 windings.

Ok.

Originally Posted by Rysdad

What happens? Would the interference cancel itself out? What would you end up with on the two secondary circuits?

What interference?

If the 25-turn winding was in the same direction as the 100-turn secondary, you'd wind up with 1/2 the primary voltage across it, with the same polarity as the other secondary winding. The current through the primary would depend on the currents through each secondary. Assuming an ideal transformer, if the current through the 100-turn winding was 1 amp and the current through the 25-turn winding was 1/2 amp, the primary current would be 1/2 * 1 A + 4 * 1/2 A = 2.5 A

[Q.E.D.]

Originally Posted by Q.E.D.

...the current through the 25-turn winding was 1/2 amp, the primary current would be 1/2 * 1 A + 4 * 1/2 A = 2.5 A

Grrr. Got the primary and secondaries mixed up. That should read: "the primary current would be 1/2 * 1 A + 2 * 1/2 A = 1.5 A."

[Rysdad]

I mentioned interference because, if the current through the primary was going, say, south, then the current in the 100 winding secondary would head north, right?

If the 25-winding secondary was wound in the opposite direction as the 100-winding, then it would be 'being induced' by both the primary and the 100-wrap but in opposite directions.

I can't wrap my mind around it.

[Q.E.D.]

Originally Posted by Rysdad

If the 25-winding secondary was wound in the opposite direction as the 100-winding, then it would be 'being induced' by both the primary and the 100-wrap but in opposite directions.

Oh, I see what you're thinking. Don't do that, it'll give you a headache.

It's not necessary to think about the effects of the secondary windings on each other, because they effectively cancel out, regardless of winding orientation. You need only concern yourself with the relationship between the primary and each secondary. If the 25-turn secondary was wound opposite in direction to the 100-turn secondary, the only difference would be polarity. The voltage and current relationships would hold.

[Rysdad]

Ok...but just one more question...

How does each secondary winding know which one of the others is the primary?

Argh!