Trying to wind a mains isolation transformer.

[David_]

Hello.

I have two toroidal cores scavanged from 50Hz applications:
**broken link removed**

The little one is:
Hight(h) = 30mm.
Outer diameter(Od) = 94mm.
Inner diameter(Id) = 52mm.
Effective length(le) ≈ 222mm
Effective area(Ae) ≈ 630mm²
12turns = 700µH @ 100Hz.
AL = 4861.1

the big one is:
Hight(h) = 38.5mm.
Outer diameter(Od) = 98mm.
Inner diameter(Id) = 60mm.
Effective length(le) ≈ 243mm
Effective area(Ae) ≈ 731mm²
10turns = 250µH @ 100Hz.
AL = 2500

I used:
le ≈ pi(Od - Id) / ln(Od / Id)
Ae ≈ h * (Od - Id) / 2

Though I don't know where to go from here, one problem is the winding configuration.
Reading online I find some people saying that one should use two equal single windings but some say I should use some kind of of center-tap connected to PE-earth?

Is there anyone here that does know how I should wind a isolation transformer, and I am searching but have not jet found any hint on how to figure out the number of turns.
I have seen some design that used 2x 300 turns but I want to know how many turns and why.

I can't really give any absolute power requirement but the main use will be to drive ether a digital oscilloscope or a function generator, I'll look up how much current my equipment require, oh its for 230Vrms.

Any thoughts are very welcome.
Regards

 

[FvM]

Power transformer number of turns is mainly commanded by achievable core flux. Toroidal cores are made from corn oriented wound tape and have higher saturation flux than regular transformers. But you'll still end up with 4 or 5 windings per volt.

 

[The Electrician]

Power transformer number of turns is mainly commanded by achievable core flux. Toroidal cores are made from corn oriented wound tape and have higher saturation flux than regular transformers. But you'll still end up with 4 or 5 windings per volt.

That should be "grain" oriented.

 

[crutschow]

That should be "grain" oriented.

Well, corn is a type of grain. :razz:

 

[chuckey]

Can't help with the number of turns, but just to remind you, that its an ISOLATION transformer so the insulation between primary and secondary should withstand 2 KV or so. So I would think that means > 3mm tracking distances between the winding ends and 3 layers of Mylar in between the windings. It could be worth while to include a floating centre tap, so you could use it as a 230/115V transformer. If you earth the centre tap, then the secondary cannot accurately reflect what the mains supply looks like (Unless your mains is 110/0/110 V )
Frank

 

[FvM]

That should be "grain" oriented.

Yes, an interference of the german term "kornorientiert".

A safety ("isolation") transformer would never use an earthed center tap.

 

[David_]

Using the information obtained from:
https://www.giangrandi.ch/electronics/trafo/trafo.shtml
I've worked out primary turns(np) = 632, secondary turns(ns) = 664 for the smaller core.
But that is when i use (Od - Id) * h = core cross-section surfac(94-52*30=1260)
but if I use the estemated effective area then i get np=1264, ns=1327, which fits better with:

Power transformer number of turns is mainly commanded by achievable core flux. Toroidal cores are made from corn oriented wound tape and have higher saturation flux than regular transformers. But you'll still end up with 4 or 5 windings per volt.

I need to do more research on the matter in general though.

 

[FvM]

But that is when i use (Od - Id) * h = core cross-section surfac(94-52*30=1260)
but if I use the estemated effective area then i get np=1264, ns=1327, which fits better with:

The term core surface in the calculation tool is misleading. Consider how a transformer works and you'll know that cross section is meaned here.

 

[schmitt trigger]

On an isolation transformer, the primary and secondary turns are always equal. Unless you want to adjust the secondary voltage.

 

[David_]

But would it not be plausible that if one would wind the primary first with insulation in between to the secondary that the extra distance to the core(for the secondary) could require a couple of more turns than the primary?

I have read through much material about transformers but I don't really get it jet but I shall continue until I understand.

 

[FvM]

A typical 5 % surcharge for secondary windings count as suggested by your calculation tool also applies to isolation transformers. It's intended to compensate the windings series resistance voltage drop with rated output current.

 

[schmitt trigger]

I have read through much material about transformers but I don't really get it jet but I shall continue until I understand.

There are many good books about transformers.
But if you want a hands-on book that allows the neophyte to start building transformers right away, and gather the essential equations to design your own, very few books are better than Eric Lowdon's Practical Transformer Design Handbook.

https://www.amazon.com/Practical-Tr...F8&qid=1425585553&sr=1-1&keywords=eric+lowdon

To answer your question: ideally the primary to secondary ratio should be 1:1
Having said that the windings have resistance that cause the secondary voltage to drop with load.
The choice is yours whether to compensate or not, and how much to compensate.