[Rob Flinn]

I've just bee looking at an interstage transformer.    It has centre tapped pri & sec but they are not completely seperate windings.  i.e you don't need to add a link to use them centre tapped.     If one was to use just one half of the secondary, would this affect the performance of the transformer ?  Would I need to load the unused secondary or could it be left flappin' in the breeze ?  It would be a bit like an autoformer I guess.

[EmRR]

You'll have to test.  It may have resonant contributions if floating. 

[CJ]

you will have more leakage inductance,  anything in the core window which is not an active winding represents leakage inductance, that is why they try to fill the window up to the brim with wire. not so for power transformers because all they care about is 50/60 Hz.

so insulation between layers and pri/sec sections represents leakage inductance, as there is less wire to catch the flux set up in the core, but insulation also reduces capacitance, so it is a balancing act between leakage inductance and capacitance, so you can tune a transformer's response by altering insulation thickness and type, 

it may turn out that the un-sused winding will reduce capacitance and actually improve response,  but the transformer will drop level faster when loaded on the secondary side with a resistor etc.

[cyrano]

What happens if you load the unused winding with a resistor? Or even an LC circuit?

[EmRR]

What happens if you load the unused winding with a resistor? Or even an LC circuit?

You can control ringing that way, yes.

[Rob Flinn]

Thanks for the responses guys.

If one were able to separate the 2 halves of the winding would the ringing still be a problem ?   i.e is the problem because the extra bit of winding is hanging off the end of one side of the used winding ?

[CJ]

you would probably have to rip that section out along with the insulation and move the primary close to the secondary,

this is one reason guitar amp output transformers sound better when using the 16 ohm tap instead of 8, you are using the whole winding,   

[EmRR]

Thanks for the responses guys.

If one were able to separate the 2 halves of the winding would the ringing still be a problem ?   i.e is the problem because the extra bit of winding is hanging off the end of one side of the used winding ?

Yes.   It's just like a secondary reflecting something back.  Loading it may be required.  May not.   

Have you seen the other current thread about transformer behavior?   

I have this transformer - how to find out the windings

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