Toroid second secondary?

[Bloy2004]

Can a second wrapping of wire over the top of the existing secondary successfully create a second secondary? Or do the windings have to be intermingled after the initial primary winding of the transformer?

Thanks,
Bloy
(trying to understand what you CAN or CAN'T do)

[gar]

050818-0750 EST USA

Bloy2004:

That is no problem.

Go back to what I said about volt-amp loads to determine wire size.

Generally you only have so much window area in a transformer for wire. When a transformer is designed and rated the usable window area is fully utilized. Somewhat less well defined in a toroid than an EI type core.

A major consideration in winding a coil is the maximum voltage between any two wires. If the wire insulation has a breakdown of 1000 v, then you might not want to apply more than 10 v between two wires in contact with each other. So the start and end of a winding should not touch. Two different secondaries should not touch.

Why did I pick a 100 to 1 ratio? Because there are external causes that may produce much higher voltages. For example, take a low voltage secondary and pass a dc current into this coil, open the circuit and you might produce thousands of volts from the inductive kick.

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[miljnor]

I don't know jack about electronics but if you wind a secondary on top of a secondary wouldn't that cause some sort of feadback or something or wouldn't the second secondary try to develop voltage based of the other secondary (heating at the least).. like I said don't know anything so this is probably a "duh" stupid question.

[ViperTX]

miljnor...there is probably some mutual coupling taking place....not sure how "commercially wound transformers" deal with that. I would suspect that the secondaries are wound in opposite directions...I actually had never thought about that.....so, definitely not a "duh" question! Darn another word with 2 meanings...his wound is not serious.....he wound up where he began.....gotta check dictionary.com......

[gar]

050818-1230 EST USA

The basic construction of a transformer is such that the primary winding area will occupy approxoimately 1/2 of the available window area for wire. As many secondaries as you want will occupy the other half. The winding direction is of little concern except for the phasing of the wires coming from the transformer.

The sum total of the heat from all windings, and core losses determines the average temperature rise of the transformer. An important parameter that is hard to measure is the maximum hot spot. This will certainly be determined by how the various windings are electrically loaded and how they were designed ( wire size and placement ).

There is what is called leakage flux and this inturn produces leakage inductance. If all the magnetic flux from one coil linked another then there would be no leakage. The induced voltage in a coil is = K * N df/dt where df/dt is the rate of change of the flux linking the coil. K is some constant to make the units come out correctly , and N is the number of turns on the coil.

If all of the flux generated by one coil does not link another coil, then the induced voltage will be less than if there was 100% linkage. In an iron core based transformer because of the magnetic characteristics of the core there is very high coupling between windings on the same core. A toroid of the correct material being better than an EI structure because the toroid has less equivalent air gap and tighter coupling. However, both are very efficient compared to an air core.

(edit) As I have previously mention, somewhere, the primary is that winding that you feed power in to. (end edit)

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