What resistances is acceptable for ot transformer?

[Disabled_shredder]

So I'm building a micro amp. I got a 125c transformer. I got it because the price was right and I didn't want to pay good money before proof of concept. I have heard off the shelf hammonds weren't all that great but this is a bit much I think. I got 156.2r on one side and 120.7 on the other. I'm not good at math much less percentages but that's wayy more than 20%. I called ce and they sent me a replacement and the new one is further off. Does anyone else get measurements like these from ot transformers or did I get the bad 2 in the batch?

[cxx]

So I'm building a micro amp. I got a 125c transformer. I got it because the price was right and I didn't want to pay good money before proof of concept. I have heard off the shelf hammonds weren't all that great but this is a bit much I think. I got 156.2r on one side and 120.7 on the other. I'm not good at math much less percentages but that's wayy more than 20%. I called ce and they sent me a replacement and the new one is further off. Does anyone else get measurements like these from ot transformers or did I get the bad 2 in the batch?

I think it has to do with the length of wire needed to get the # turns right. The later winds are longer. Most likely OK.

[Phil_S]

It seems a bit more than what I would imagine, but it is not unreasonable. As noted, more wire is required on the outer part of the winding the make the number of turns. R is directly related to the length of wire.

If you are uncertain, run a low a/c voltage through it and see if the output is the same on both legs. Likely you'll find it is OK.

[katopan]

Not unreasonable if it's a simple wind with no interleaving.

[roberto]

It is normal. Even the first one was good.

Resistance of a winding is resistivity of the wire multiplied by the lenght of the wire. The latest turns around the transformer have an higher radius than the first ones, so the lenght of each turn is

That's why the two resistances are different.

[VacuumVoodoo]

It's the number of turns not resistance that counts. I'd be concerned if inductance of primary sides differed significantly.

[tubeswell]

What Roberto and VacuumVoodoo said. Transformers are AC-transforming devices. The Pr:Sec VAC-ratio depends upon the Pri:Sec turns-ratio. The number of turns are the same on both sides of the primary, so that the VAC ratio for each side of the primary (in relation to the secondary) winding will be the same. One side of the primary winding is wound over the top of the other, and the wire has to be longer on the top side to keep the number of turns the same. So a difference in the DC resistance is normal.

[Disabled_shredder]

Everything I've read talks about resistance of the sides being within 20% the actual Performance of the transformer is decent the voltage between the plates is only off by 1-3 volts. I can live with that as I'm only trying to prove the concept before I have custom wound transformers made.

[Phil_S]

In a custom wind, it may be possible to specify interleaved windings to balance the length of wire. I'm not sure it's necessary or even desirable.

[tubeswell]

In a custom wind, it may be possible to specify interleaved windings to balance the length of wire. I'm not sure it's necessary or even desirable.

Yes I've noted this before. The greater the numbers of primary and secondary interleaving the better the high frequency response, but the benefits of interleaving are also dependent on OT size. A physicaly large transformer has increased capacitance between windings which causes phase shift and restricts the high frequency response. A smaller tranny with less turns favours the high frequency response.

[katopan]

Desirable depending on the amp. But the interleaving is often simple with 3, 5 or at the most 7 layers. Not like some of the hifi stuff.