Transformers - where to start!!

[Bushbaby]

Hi All,
Firstly happy Easter to one and all.

I must admit I haven't posted for a while due to workload!! (I always thought that as I got older things would get easier....??)

I decided that its time I used my Avo Douglas for making some transformers. I thought that I would start be endeavouring to rewind an old one from a 'junked' radio (ones that are truly beyond economical restoration and are only useful as doner items). However, all the posts that I have read appear to show this a complex task for a novice. Questions that spring to mind include:

• copper wire gauge selection
  
• insulation material thickness (laminate sheets)
  
• laminate design etc
  

Maybe I'm expecting too much here but I'd really like to give it a try so any help will be most gratefully recieved.

Many thanks

Rob

[pwdrive]

If Ed Dinning frequents here then he would be the chap to speak to.

Lawrence.

[Alan Douglas]

I've rewound lots of transformers; mostly interstage with typically 4000 turns primary, 12000 turns secondary, wire sizes #38-#40 AWG. I built a high-speed winder to do it.

[ppppenguin]

Best intro to the subject that i've seen:
http://www.ludens.cl/Electron/Magnet.html

[Andy M0FYA]

I recommend you look out for the book 'Practical Transformer Design Handbook' by Eric Lowden. This succesfully covers both the theory (without getting bogged down in mathematics for its own sake) and the practical aspects of making transformers.

Andy

[Radio Fixer]

If you are a BVWS member (and everyone should be IMHO) then look in 2007 Summer and 2007 Autumn for two articles "Making a coil winder" Part 1 and 2. You wont need the coil winder but two examples are given of winding coils with little maths but very practical on how to get the right wire gauge, make or alter bobbin and formers and how to terminate the wires and more.

Gary

[Ed_Dinning]

Hi Rob, somewhere in the depths of the forum are a set of notes that I put together with Retired (Colin) that dealt with the empirical methods that can be used to wind transformers.
This was possibly 2 or 3 years ago.
Feel free to PM me if you get stuck and I'll see what can be done.

Ed

[Alan Douglas]

If you're rewinding, most of the design decisions have already been made, and you copy what's there.  Audio interstage transformers can be considerably simplified by winding on bobbins rather than in layers with paper between (modern magnet-wire insulation makes this possible).

I've done a couple of power transformers but now that I'm retired I don't have the use of the large winder at work.

During summers while in college I worked at a motor repair shop and rewound a couple hundred motors. 

[Bushbaby]

Hi Alan,
Many thanks for your posts.
I have looked at many articles and books which are great regarding theory but not much by means fo practical application. I've an old coil winder (Avo Douglas No 3) but yet to use it. I'm seeking fundamental answers such as - selecting the correct size copper wire and how to insulate windings (correct selection of insulation material).

Regards

Rob

[Alan Douglas]

I don't know how British wire gauges go, but there are accepted standards for how much current can be carried in a certain cross-sectional area, depending on temperature rise (modern wire enamels can stand more than old materials). Here it is measured in Amperes per circular mil (in reality you just look it up in the wire tables).

Older practice for insulation was to wind in layers with paper between to keep the layers straight. The finished winding was impregnated with wax or varnish to keep the wire immobile. Nowadays transformers are wound on bobbins with no paper.