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Old 6th October 2006, 08:20 PM   #1
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Default transformer confusion

Im very new to this stuff and have what may be a silly question but I'm going to have to ask it.

On a circuit which specifies AC and AC Zero from the transformer how do you determine which end of the winding is which?
The transformer is a talema 2x25 120va.

The label shows red/black and orange/yellow as being the pairing but doesn't say which is AC Zero?

I realise this isn't much to do with solid state but there isn't a transformers thread and it is valuable learning for when I build my first gainclone.
john
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Old 6th October 2006, 10:59 PM   #2
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Hi John,

Transformers have to do very much with solid state amplifiers!

As a precaution, I presume you are talking about the secondary windings; you have sorted out the primary.

The annotations AC and AC ZERO are relative, and not any specific end of a winding. It can be either end, but on two or more windings are meant to indicate relative phase. Thus, on the transformer you can connect the two 25V windings in parallel - that would enable you to draw twice the current for one winding. But they need to be in phase, otherwise you will get a short. This is just like connecting two torch batteries together. If you strap both together for 1,5V but more current (longer life), you obviously have to strap the negative ends together and on the other side the two positives. Reversing that will mean a short across both batteries with disasterous results. On the other hand, if you want 3V, you would tie your batteries in series. That means the positive of one to the negative of the other, and you use the other two ends which will give you 1,5 + 1,5 = 3V.

It is just the same for AC. But your description can be interpreted in two ways. If you mean that red/black is one winding and orange/yellow another, you will have to find out which sides of the windings are "in phase". That can be easily done with an ac voltmeter. You just connect say red to say orange, and measure across black and yellow. If the reading is 0v, it means that they are in phase; for 25V (at double the current) you use red and orange connected together, and black and yellow for the other end.

However, if you measured 50V across black and yellow, the two windings are in series. That means that there is 25V either side of the red/orange connection which is a kind of centre tap. You would use that configuration for e.g. a full wave rectifier as in a diode each at black and yellow, going to the load, with the centre tap (red/orange) to the other side.

If you understand this, note that you could also have tied together black and yellow for the centre tap and used red and orange for the extremities. This the same as reversing your previous two battereis - it does not matter which one is at the "bottom" and which at the "top". If they are in series they will still give 3V over the outer terminals.

I hope you understand this. Otherwise tell us which colour leads belong to which windings, so that it can be drawn out for a clearer image.

Regards.
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Old 6th October 2006, 11:40 PM   #3
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2 windings on the same transformer will give you 2 sine waves, 50% of time giving current one way 50% of time current the opposite way for each winding.

there is 2 possibilities for placing 2 windings in series:

a connection made so both winding are pushing the same way at the same time and you get twice the voltage.

otherwise 2 windings pushing one against the other and you ge no voltage.
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Old 7th October 2006, 12:32 AM   #4
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Default Transformer Confusion

Thanks for the input. By trade I'm a car mechanic so DC I'm fine with but AC is a bit of a mystery, as you can no doubt tell.
The transformer is to drive a Velleman K8060 power amp. Theres a post by a member by the name of "Sith" regarding the same kit under the heading "Is this any good" on the solid state forum. He has attached a schematic for the amp if that would help you.
I dont think the secondaries are wired in series, but I could be wrong. I'm assuming they feed 2 stages of the amp seperately.
The colours I listed are for the secondary windings, Red/black are one pair and Orange/Yellow are the other. The primary winding is Brown/Blue.
Once again thanks for the help as the smell of burning circuit board is never a pleasure!
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Old 7th October 2006, 01:27 AM   #5
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I saw the circuit, and yes they are connected in series.

if connected correctly you will have an ac voltage from the lower point to the mid point, and you will have same reading from midpoint to the upper point, but you will have twice the value if measured from lower point to upper point
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Old 7th October 2006, 01:36 AM   #6
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see below
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Old 7th October 2006, 02:13 PM   #7
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Default Many Thanks

Thanks a lot! I think I have a grip on it now. Thats the real joy of the internet, total strangers giving their time for free to help newcomers.
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Old 7th October 2006, 05:32 PM   #8
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Hi John,

There was a time when all of us were newcomers. Others then helped us. It is an example we are all tasked to honour.
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Old 8th October 2006, 06:15 PM   #9
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An admirable and rare attitude in this "whats in it for me?" world.
So much for the internet being populated only by sex fiends and scam merchants!
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Old 10th October 2006, 11:11 AM   #10
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apologies for slightly hijacking the thread, but its very much related,

i see you can parallel the secondary windings of an xformer, but if this were for an amplifier, how could you obtain a split rail, with a centre 0v rail?

i have a 45-0 0-45 and want to run a p3a but it will rectify to around 63v per rail...

cheers

matt
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