Transformer HT secondary windings.

[RF Burn]

Interesting when given a little thought . . .

In audio use (push-pull circuits) using a centre tapped primary on the output transformer the resistance of the windings is virtually irrelevant as the impedance thrown back from the secondary will be orders of magnitude greater in comparison. There is also the inductance to take into account because of higher operating frequency range.

In the case of a centre tapped power transformer secondary however the difference in winding resistance between the half windings will give rise to an 'unequal' rectification of the two half-waves when any load current is drawn. The half-winding with the lowest resistance will be supplying more of the output current than the other half due to the IR voltage drop differential, also the ripple will be uneven for the same reason. So, maybe if there is significant difference in resistance then it might just something to think about?
Any attempt to 'correct' the 'problem' would simply involve using two different values of limiting resistor in the rectifier feeds (but I have never seen this done in practice).

Adrian

[Radio Wrangler]

In the case where a full-wave valve rectifier is replaced with two semiconductor diodes and resistors are added to simulate the impedance of the old rectifier, it would be possible to measure the resistance of the HT winding halves and to fit dissimilar resistors to equalise this.

It adds elegance though it may not be noticeable, but real audibility doesn't seem important in some audio circles... but then they would never go for solid state rectification.

David

[G6Tanuki]

Originally Posted by GrimJosef

The lower resistance half of the winding will also be doing more than its fair share of the reservoir cap recharging work, and this will cause it to suffer more Ohmic heating.

There's a counterbalancing effect though! The winding providing a bit more current will incur greater ohmic losses and so run hotter, which will lead to its resistance rising faster than the lower-current winding, and this could then provide a kind of 'automatic compensation' effect.

[Alternatively, we're all guilty of horribly overanalysing the problem and worrying ourselves unnecessarily about the outcome]

[Leon Crampin]

Definitely overanalysed, I think. Just to throw another joker into the mix, the halves of a bi-phase valve rectifier are seldom balanced either. The section passing the greater peak current due to the associated power transformer secondary lower resistance will lose emission more rapidly than its partner. At some point, balancing to some extent is likely to occur.

Of course, a bridge rectifier largely avoids this if the diodes are matched - and gives better utilisation of copper and iron in the transformer. Our German friends knew this; but in the UK I suspect the valve makers (some of whom made radios, eg. Philips and GEC) would have pushed their bi-phase solution.

Leon.

[joebog1]

For GJ.

When I make "hi-falutin" audiophool traffos I vacuum varnish the windings and bobbin. I then assemble the traffo ( insert lams) then dip the whole lot. My transformers DON'T make any noise and still get some cooling. They are virtually impossible to rewind however, (that is in fact very similar to Quad traffos ). I mostly use toroids for mains, and they are also quiet, but not as quiet as a vacuum varnished EI unless ,(as I do quite often ) I run very low flux densities in the toroid. Toroids BTW, have almost equal resistances on both halves of the secondary. As I insulate between layers with toroids, there is a slightly longer path on the outside winding.

Joe

[thermionic]

Originally Posted by GrimJosef

........... I'm currently repairing an Audio Innovations 500 and the hum from its mains transformer is substantially louder than the hum from its speakers....

Cheers,

GJ

I repaired one of there a couple of years ago now, whose mains transformer had shorted turns on the primary section.

Mike Barker re wound it, and commented that the original was not built to a very good standard. Needless to say, the replacement transformer was quiet!


SimonT.