Building one's own transformer

[uncle_leon]

Assume I can buy a good quality transformer core, say grain-oriented steel toroidal, but I want to wind it myself. Can someone point me where to look for information please?
I know the theory of their operation, but I don't really understand which part of the design makes what difference.

For example, if we want a transformer that converts 230VAC into 23VAC - why not just wind ONE winding at the primary side, and ten at secondary?...

Another one, possibly related to the above - does it make any difference at all whether the entire core is covered by wires, or only a part of it?

Why does no one make transformers wound with flat wire (ie. copper tape or foil)? Flat inductors are considered superior, then so should be transformers?...

I know it's a lot of questions, some of them possibly stupid, but I will greatly appreciate any information.

[Minion]

This isn"t my area of expertise (but I am an expert of sitting on my *** and drinking beer) but this PDF might help you .....

http://www.quadesl.com/pdf/trans_inst.pdf


Cheers

[uncle_leon]

This did shed some light on the subject - although, sadly, it hasn't answered any of my questions above... Thanks for your help Minion, much appreciated.

[RJM1]

“For example, if we want a transformer that converts 230VAC into 23VAC - why not just wind ONE winding at the primary side, and ten at secondary?...”

If your secondary has more turns than your primary that would be a step up transformer and your turns ratio of of one to 10 would make your secondary 2300VAC.
A primary winding of one turn or 23 turns would result in the primary inductance to be very low and the impedance at the mains frequency ( 50-60 HZ ) would be so low as to burn up the primary winding.

“Does it make any difference at all whether the entire core is covered by wires, or only a part of it?”

Yes, especially in the case of a toroid. You want to distribute the flux evenly in the core so you want the wires to be evenly spaced over the surface of the core.

“Why does no one make transformers wound with flat wire?”

Some do use square wire instead of round to maximize the amount of wire that they can wind on a core. This is more expensive and it is usually more economical to use a bigger core.

Here is a link to a Mains Power Transformer calculator. For up to 460VA
Download Trafo 0.9 Free - A mains transformer calulator - Softpedia

[seanvn]

I'm kinda interested in making a steel tin can transformer but I haven't gotten around to it yet. I was thinking about winding the coils around the circumference, but I guess toroid would be possible if you put pvc electrical tape over the sharp edges. How cheap am I? Yeh I'm very cheap.

[CBS240]

You need grain oriented silicon steel. There are transformer kits available from different companies. One here in the US is Toroid of Maryland. I had to re-build a 1400VA kit due to vibration issues from the core. I had to epoxy the steel core coil because it was not fixed together. The primay is about 130 turns of 10 ga magnet wire, and the thermal fuse mounted against the core. The last two pictures show the polypropelyne plastic film covering the primary winding for insulation/isolation and ready for the secondarys to be wound. The core size and desired flux transfer will determine the number of turns of the primary, volts/turn. The turns ratio and flux density will determine the secondary voltage and wire size. This big toroid has a large flux density, and about 0.9VAC/turn. If I reduce the number of turns in the primary, the flux transfer may cause the core to saturate which is very bad. More turns will limit the core flux transistion and reduce the available VA, also reducing the volts/turn.

Keep in mind a large toroid like this one must be slow started and cannot just be thrown onto the mains. Since toroids have very low leakage flux due to no gap, the core is prone to magnetic memory.