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Old 1st May 2003, 12:40 PM   #1
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Default odd transformer design

Hello everybody!

I would like to introduce myself as a electronic dumb-*** ungraduated in any kind of EE but being an addict hobbyist. At some point I am starting being bored with replicating things - I would like to understand them. This will be the case of another thread elsewhere, now I need the help of you, electronic freaks.

I've recently salvaged huge three coil inductor from some kind of military radio and I want to try use it as transformer in hugeous bench supply, which I am missing. Of course after rewinding. In the image is the structure of the inductor. The Sd of the core is 3,5 cm * 5,5 cm. Core looks like from standart plates like in common EI transformers. The point is, that i know formulas for evaluating transformer design on classic EI core, but i would like to have primary in the centre, and +/- rails of secondary on side. How to compute it? Is my idea wrong?
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Old 1st May 2003, 05:44 PM   #2
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if you have the primary on the centre leg, this is where you have the highest flux density (Teslas), which should determine number of turns on the primary (rule of thumb < 1 T, preferably 0,9)

the magnetic flux divides however into the two outer legs, meaning that you cant use output /input voltage
= primare turns/secondary turns. Flux outer = Flux center/2

You will have to recalculate output voltage using the areas , flux ,number of turns.

I hope you understand what I mean...

I have seen a transformer winded like that once, but it was for having a large insulation potential between primary and secondary.

/rickard
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Old 2nd May 2003, 10:32 AM   #3
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Thanks rikkitikki, that help a lot. Actually it makes sense to me, I had some physics lessons.
It means, that basic turns/voltage ratio formulla is simplified due sharing same core. So I only need to find right, more general formullas with flux regards.

Ok, lets have another thing. You mentioned, that the flux density should be kept under 1T. It is the point, when the core saturates and stops conducting energy, resulting in winding resistance decrease with posible meltdown? Or it is any relation to hysteresis curve?
Im sorry, i didn't found any book of physics in my flat so I hope this trivia question will not hurt...

One thing olso comes to my mind. Till your reply many people viewed the post, but no one answered. Am I that dumb or this is not basic knowledge?

Thanks again.
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Old 2nd May 2003, 10:49 AM   #4
dhaen is offline dhaen  Europe
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Hi,

Although I have been in various branches of the electronics industry for 35 years, I've never seen a power transformer of this design.
I have been waiting to see if someone recognised it.

Are you sure it is a transformer as such? What I have seen (in the distance past) is small device wound this way. On this device, the outer windings were connected in series, and a DC current passed through them. This gradually saturates the inner core, causing the inductance (and therefore the impedance) to reduce.
Could it have been designed for this? A DC controlled reactance?

Just an idea...
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Old 2nd May 2003, 11:03 AM   #5
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I am convinced that it is not transformer. Surely not clasic power transformer. It was salvaged from old mil. radiostation and with its dimension, it looks like some antena accesory. I would only like to use core and coils skelets and rewind it for dual power supply. A hard task for beginner like me.

I only received the thing from friend and anything he knows about current is that it flow, so i cant give any details of the things previous operation.

Thanks for interest, it looks like becoming a mystery...
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Old 2nd May 2003, 11:23 AM   #6
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If each of the three poles has a primary and secondary, then you have a three phase transformer. The primarys and secondarys can be wired into a delta or wye config. If you also had three phase service at your house (unlikely) you could make a very nice dc supply with very little ripple. Like comparing a 2 cylinder to a v6.
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Old 2nd May 2003, 09:50 PM   #7
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I go for Brians explaination (if its spelled that way)
I have som 3phase transformer taht can be hooked up in different ways depending what its for.
Normally in my case its fed with 3x400V (380) and is Y-winded on the primary . Sec. is in my case 3x24V D (delta)
But if that is your case I dont know.


cheers
björn
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Old 3rd May 2003, 11:32 AM   #8
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Default no way

Well, I cant rembember how the winding is done and I dont have tje transformer right now, but that was the first thing that came to my mind when i saw it - 3phase transformer. Somehow I didn't proove it.

In either way it is not an option for me - I can't gain access to 3phase mains in my flat although it would beef me to pro.

Rikkitikki has answered original question, but it seems that design of this like is really odd. This fact forces me to ask, if it is way to go, to rewind it with rikkitikki considerations. Any ideas?
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