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Old 22nd November 2010, 07:38 AM   #21
akis is offline akis  United Kingdom
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I will use enamelled wire for all additional work. The sellotape I was talking about is not the very long and continuous band of polyester tape that took me an hour to unwind, but basically plenty of ordinary sellotape strips used to hold together bits and pieces and in addition to insulate the soldered connections to external wires. I was expecting to see something better to insulate the exposed soldered wires, eg mica, plastic connectors etc

Questions:

1. Do I need to replace the polyester tape which was wound all around the transformer (I think my wife threw it away)? Can I use, say, electrician's tape to secure the secondary turns firmly onto the core ?

2. The toroidal creates a magnetic field around it. Would enclsosing it inside an earthed aluminium case curtail that field, or would the field go through the case nevertheless?

3. When winding a secondary, does it matter if a few threads cross over each other? Does it matter if in cases they go backwards with respect to the angular direction?

Last edited by akis; 22nd November 2010 at 07:40 AM.
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Old 22nd November 2010, 08:54 AM   #22
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Quote:
Originally Posted by akis View Post
Questions:

1. Do I need to replace the polyester tape which was wound all around the transformer (I think my wife threw it away)? Can I use, say, electrician's tape to secure the secondary turns firmly onto the core ?
Electricians tape goes soft when hot, use packing tape if nothing better is available. Electrical insulating material is readily available, look in the phone book for motor rewinders or electrical insulation.
Quote:
2. The toroidal creates a magnetic field around it. Would enclsosing it inside an earthed aluminium case curtail that field, or would the field go through the case nevertheless?
Very little field escapes a toroidal core due to the grain orientation and no joints to lower permeability. Look up the right hand rule to see the direction of the field induced by a coil Aluminium is poor as a magnetic shield anyway, steel is much better.
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3. When winding a secondary, does it matter if a few threads cross over each other? Does it matter if in cases they go backwards with respect to the angular direction?
No, modern insulation is very tough, direction and angles are unimportant. There should always be a barrier between primary and secondaries and it is good practice to keep the secondaries apart
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Old 22nd November 2010, 10:18 AM   #23
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by akis View Post
I have opened up the transformer and noticed an abundance of ordinary sellotape/scotch tape
it is almost certainly Mylar tape. Much tougher than "Sellotape" and if you measure it, much thicker as well.

Go to Scientific Wire Company.
They will sell you all the copper wire you could ever want.
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Old 22nd November 2010, 10:20 AM   #24
AndrewT is online now AndrewT  Scotland
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Originally Posted by metalsculptor View Post
...... it is good practice to keep the secondaries apart
bi-fillar secondaries by definition cannot be kept apart. They are adjacent to each other. Similarly for tri & quad fillar.
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Old 22nd November 2010, 11:12 AM   #25
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Quote:
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bi-fillar secondaries by definition cannot be kept apart. They are adjacent to each other. Similarly for tri & quad fillar.
I doubt many mains frequency power transformers are wound bifilar though I could be wrong as it would make winding a dual secondary toroid easier I would still prefer separate windings as the user may put hundreds of volts between them depending on the application.
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Old 22nd November 2010, 11:29 AM   #26
AndrewT is online now AndrewT  Scotland
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I have had to open up the outer insulation of quite a number of dual secondary toroids.
ALL have been bi-fillar or tri-fillar wound.

Yes, that means the two layers of enamel is all that separates the voltages between the dual secondaries.
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Old 22nd November 2010, 06:07 PM   #27
akis is offline akis  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
it is almost certainly Mylar tape. Much tougher than "Sellotape" and if you measure it, much thicker as well.

Go to Scientific Wire Company.
They will sell you all the copper wire you could ever want.
I meant real sticky pieces of sellotape holding together the Mylar tape, as well as insulating the solder joins between winding wire and "output" leads.

The secondaries (2 x 25 VAC) are wound together in parallel. As you said it is the enamel providing the insulation between them. On the outer perimeter they fit precisely next to each other, but towards the centre they climb and cross over each other and in some cases there is much sharper pressure point than when they are simply in parallel next to each other.

So I presume the enamel is quite tough and robust.
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Old 22nd November 2010, 07:14 PM   #28
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Was the "sticky tape" yellow or clear?
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Old 22nd November 2010, 08:33 PM   #29
akis is offline akis  United Kingdom
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ordinary sticky clear tape, sellotape
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Old 23rd November 2010, 10:35 AM   #30
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Quote:
Originally Posted by AndrewT View Post
I have had to open up the outer insulation of quite a number of dual secondary toroids.
ALL have been bi-fillar or tri-fillar wound.
Thanks for that info it is something to watch out for, I am assuming that you mean general purpose consumer dual secondary toriods with the same voltage on both windings, most of the dual secondary toroids I get are 110V / 24V with a 415V/ 240V primary this is a very popular industrial control transformer.

The clear sticky sellotape is probably sticky polyester (mylar) tape the stuff we call packing tape in Australia is clear or brown sticky polyester tape. Packing tape has good high temperature resistance though the glue dries out eventually. Upon lookup sellotape brand tape is polypropylene. even their packing tape. Polypropylene has excellent electrical properties, low loss tangent and high dielectric strength etc and good mechanical strength but it has less hot strength than polyester. It should be fine for sticking down the polyester unless you plan on running the transformer at elevated temperatures.
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