Transformer unwinding question.

[GloBug]

OK So I hacked into a 35-0-35 Power transformer. I want to lower the voltage for a chipamp project.

I was able to unwind the top layer of the secondary to get down to 18.6v.
Now to get to the 2nd secondary I will have to unwind the rest of the "18.6"v and rewind it, or..........
........I also have a second transformer I could also just unwind one of the secondaries on it and voila I have dual rails via dual transformers. Any advantage to this or just a complete waste of iron?

I also understand the VA rating will go down from 250VA to some lower value, any guestimates?

I also am aware that a higher voltage would use thinner wire and lower voltage uses thicker wire. Am I OK to use the "35v" secondary wire for "18v" use? It's diameter is 1.63mm / 0.253".

I thought I should ask before I went to far.

[KatieandDad]

Yes your VA rating will go down.

Your 250VA transformer was rated to produce approximately 3.5A.

Reducing the number of turns will certainly reduce the secondary voltage as you expect but the windings are still only rated at 3.5A.

If you want to go the whole hog. Strip the secondaries completely and rewind two new secondaries.

If you've reduced Vs from 35V to 18.6V then the VA rating will be dropped in the same ratio - approx 130VA.

There are other factors involved that may increase this somewhat but they are there for safety in the original design.

UNLESS YOU KNOW WHAT YOU ARE DOING - DO NOT UNWIND OR TOUCH THE PRIMARY WINDING (S)

[GloBug]

OK say if I strip the secondaries and rewind, should I go to a bigger gauge wire to fill the space left from removing the turns? (I don't actually have larger)

I will try it with the original wire, my gap will increase a bit more when I unwind the other secondary.

I won't touch the primary.

Thanks for your time.

[GloBug]

I am starting to lean towards using the second transformer for the second rail rather then risk unwinding and rewinding the rest. I am not confident I could strip the wire and have it completely intact for rewiring. I played wit a scrap piece, it is possible for the enamel to chip off with the potting lacquer.

[DF96]

Yes, generally you can't reuse enamel covered wire because of the risk of insulation damage. Wire voltage depends on number of turns. The maximum voltage depends on insulation thickness, which may be on the wire or added extra when winding. Maximum current depends on wire thickness and how easily the heat can get out. If you want to get similar VA to the original transformer, then rewind the new lower voltage secondary with thicker wire which will just fill the space occupied by the old winding. Unless you have the necessary machinery you are unlikely to fully achieve this at home.

[GloBug]

I have a bit of a plan B

I have a line on a dual secondary 25V, 3.2A per secondary. Does this sound OK for a Daniel's LM3875 Chipamp? I'm not sure how to calculate that back to a regular "VA" rating.

[GloBug]

At 25V is a little higher then I would like but I think I could live with it.
Here is what it looks like.

[DF96]

25V at 3.2A, twice, is 160VA.

[GloBug]

That simple eh? Lol That was my guesstimate but things are never usually that easy.

They are not much bigger VA wise then what I am using already. I do like the shape of them though.

[AndrewT]

The 0-35 & 0-35Vac windings could be split to give four windings of ~17.5Vac

Now add same diameter wire turns to one of the windings until your get the open circuit output voltage you require.

Do the same for the next winding. Ensure that both windings give exactly the same output voltage. You can series connect them out of phase and the voltage measured across the open ends will be 0.0Vac. This proves that both windings have the same number of turns. Connect these two windings in parallel.

Repeat this exercise for the other two 17.5Vac windings.

You end up with dual secondary 18Vac 7Aac, transformer. Or 19Vac 7Aac, or 20Vac 7Aac

That will give a VA rating that is higher than the original transformer.
What this is telling you is that the VA rating for each winding is as 20Vac & 7Aac is 140VA , but the total VA that can be supplied is limited to the original VA value.

There are three big advantages to doing it this way.
1.) you do not have to find the exact centre point of each of the two existing 35Vac windings. You can correct for an error in where you actually split the windings by adding on different numbers of turns to arrive at your final open circuit voltage.
2.) you have doubled the area of copper in the secondary winding. When you reduce the current rating of all the windings to that which results in the same VA as was the original you will have less I^2R losses in the secondary winding. The transformer will run very slightly cooler.
3.) because you can split the two 35Vac windings at anywhere near the centre of the winding you can get access to the buried winding after you unwind a couple of turns of both sides of the snipped winding.

Before you snip:- look and see how far apart the turns are. Wind on sticky tape to one side of the snip region and wind on sticky tape to the other side of the snip region. Space these two sticky tape binders far enough apart to allow two full turns of windings to be undone on both sides of the snip. Sticky tape is quite difficult to feed through the toroid, but this binding must be tight and preferably 3 or 4 turns thick to prevent the 35Vac windings coming undone.