A little clarity amongst the hum, maybe?
Mlloyd1, you are correct, a big cap in series with the primary will take care of a DC offset, it will also produce other problems, higher supply impedance and resonance amongst others. The offset killer that I was referring to was the MOV, (I'm guessing) in the picture that djk referenced. I think Nelson Pass might be suggesting that it is a PTC, (thermistor) and if it is, I fail to see how it could fix the problem, (but omniscient I'm not).
Unbalanced simple AC loads in the power reticulation system do not, as Nelson Pass stated, produce DC offsets, some switched or complex loads certainly do and these loads are becoming more common. I've yet to see a case where they produce saturation in a well designed transformer but I can see that it could happen. What I do have trouble with is in understanding why they would be of more concern to toroidal transformers, I don’t buy the soft clipping argument. If the transformer is suffering from saturation due to DC offsets then use a more conservative design, (EI or toroidal) or better still deal with the offset.
Let’s not forget that the primary current is not sinusoidal but very peaky under normal conditions.
I still strongly suspect that some of the information, (smoke) given by manufacturers, (the marketing departments) is just trying to justify why they use cheaper transformers, (generally the most expensive component that they have to buy, (apart from the shiny box maybe
).
By the way the concept of “dual rectifiers” is lost on me, are we talking about full wave rectification here?
One other point off correction, I referred to the cause of transformer noise as magnetostriction, the term was not used strictly correctly but in the power industry its use is common.
Let’s enjoy the music, not the transformer hum, take care, regards WALKER
Mlloyd1, you are correct, a big cap in series with the primary will take care of a DC offset, it will also produce other problems, higher supply impedance and resonance amongst others. The offset killer that I was referring to was the MOV, (I'm guessing) in the picture that djk referenced. I think Nelson Pass might be suggesting that it is a PTC, (thermistor) and if it is, I fail to see how it could fix the problem, (but omniscient I'm not).
Unbalanced simple AC loads in the power reticulation system do not, as Nelson Pass stated, produce DC offsets, some switched or complex loads certainly do and these loads are becoming more common. I've yet to see a case where they produce saturation in a well designed transformer but I can see that it could happen. What I do have trouble with is in understanding why they would be of more concern to toroidal transformers, I don’t buy the soft clipping argument. If the transformer is suffering from saturation due to DC offsets then use a more conservative design, (EI or toroidal) or better still deal with the offset.
Let’s not forget that the primary current is not sinusoidal but very peaky under normal conditions.
I still strongly suspect that some of the information, (smoke) given by manufacturers, (the marketing departments) is just trying to justify why they use cheaper transformers, (generally the most expensive component that they have to buy, (apart from the shiny box maybe
).By the way the concept of “dual rectifiers” is lost on me, are we talking about full wave rectification here?
One other point off correction, I referred to the cause of transformer noise as magnetostriction, the term was not used strictly correctly but in the power industry its use is common.
Let’s enjoy the music, not the transformer hum, take care, regards WALKER
I have some toroidal transformers in my projects that are designed for 220V line, not for 230V that it is today here. These hum often. Especially when someone is welding in nearby workshop (harmonics and dc offset).
In transformer construction one reason why E-I doesn't hum so easily is that these have what's called distributed airgap in the core, while toroids are usually wound from single sheet of electrical steel and have practically no effective airgap.
Additionally flux coupling is not as tight between primary and secondary, leading to increased stray flux near saturation, limiting further saturation.
Thus all other things being equal, toroid core will saturate more easily under overload or dc offset conditions.
Well designed, neither should hum or saturate, of course.
Sometimes toroid transformers are made with ferrite core, or powdered metal core. Neither is good idea for audio. Losses in high frequencies are a good thing.
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