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04-04-2025, 12:58 PM
Hi Guys
The transformers used in any application should be used in a manner that takes advantage of their capabilities without introducing loading imbalances that cause an increase in EMI (electromagnetic interference) noise output.
For example, a toroidal transformer is considered to be about as ideal as we can manufacture. One rule for winding them is that each winding must completely cover the core for there to be minimal noise escaping. Each winding acts to shield the core. Some windings may require so many turns of wire that it goes around the core more than one, but each layer must meet the requirement of covering the core completely.
If there is a tapped winding, then each side of the tap, or any portion between taps constitutes a separate winding and is made as such, then the related wire ends are connected together to wire lead-outs. Because each portion of the tapped winding fully screens the core, the loading on each portion can be independent of the other sections. For example, a center-tapped winding can have widely different loads on each side of the CT without a noise penalty. This is true for all transformer types and applications.
There will be gaps in the core screening where the lead-outs are made, so there will be some EMI.
Semi-toroidal transformers typically have a core that uses square-U laminations that can be inserted through bobbins holding the windings, plus straight pieces to close the U on that layer of the core. The Us are inserted alternately from each end to avoid accidently making a significant gap. This takes advantage of standard EI winding methods and can be very economical.However, there will be two large gaps in the core coverage, so a semi-toroid has inherently higher EMI than a true toroid.
For lowest EMI output, the semi-toroid must have balanced loadings through its two wire groups held within the two bobbins. Each group will have a primary and secondary, requiring that both primaries be used as primaries all the time and always used all the time - wired in series or parallel to suit the mains input - and that the secondaries must be used all the time at equal loading.
Hobbyists wanting to build a small PSU to support a tube may see a clever way to wire a standard semi-toroid to generate both plate and heater voltages. This only works in North America or Japan where mains is 120Vac or 100Vac, respectively. One primary ties to the mains. The second primary is used to provide high voltage. The two low-voltage secondaries support the heater. For hobbyists, the noise output may be acceptable to them, and they may be used to using / building noisy equipment? But for a discerning ear, this is not as good as it could be and it is of no use for the rest of the world using 240V mains.
Traditional EI transformers are inherently noisy and often use half-turns to generate the correct output voltage. Shielding the core is not a big concern as the windings are never extremely tight anyway. There are limits to how tight you can wind the wire without it breaking during the assembly, or breaking later in use. magnetic leakage will cause end-bells (metal coverings over the windings) to vibrate and buzz, but to what extent depends on the manufacturer's attention to detail and execution of design.
Most auxiliary function PTs will be smaller in their power rating (VA). The magnetising current is often of a similar order to the full-load current and core losses can be a few watts of heat coincident with a steep temperature rise. In general, EMI is in step with the core temperature and is something most hobbyists and even some designers do not notice or know about. Cooler operation at full load requires a larger, heavier, more expensive transformer, so you are buying a larger part to have better longevity and performance through cooler running, or specifying a custom PT with a low-temp rise.
Adding a small PT to an existing amp, or building a preamp using small PTs, we are lucky that "small" can be critically-sized or generously-sized for very similar cost.
The transformers used in any application should be used in a manner that takes advantage of their capabilities without introducing loading imbalances that cause an increase in EMI (electromagnetic interference) noise output.
For example, a toroidal transformer is considered to be about as ideal as we can manufacture. One rule for winding them is that each winding must completely cover the core for there to be minimal noise escaping. Each winding acts to shield the core. Some windings may require so many turns of wire that it goes around the core more than one, but each layer must meet the requirement of covering the core completely.
If there is a tapped winding, then each side of the tap, or any portion between taps constitutes a separate winding and is made as such, then the related wire ends are connected together to wire lead-outs. Because each portion of the tapped winding fully screens the core, the loading on each portion can be independent of the other sections. For example, a center-tapped winding can have widely different loads on each side of the CT without a noise penalty. This is true for all transformer types and applications.
There will be gaps in the core screening where the lead-outs are made, so there will be some EMI.
Semi-toroidal transformers typically have a core that uses square-U laminations that can be inserted through bobbins holding the windings, plus straight pieces to close the U on that layer of the core. The Us are inserted alternately from each end to avoid accidently making a significant gap. This takes advantage of standard EI winding methods and can be very economical.However, there will be two large gaps in the core coverage, so a semi-toroid has inherently higher EMI than a true toroid.
For lowest EMI output, the semi-toroid must have balanced loadings through its two wire groups held within the two bobbins. Each group will have a primary and secondary, requiring that both primaries be used as primaries all the time and always used all the time - wired in series or parallel to suit the mains input - and that the secondaries must be used all the time at equal loading.
Hobbyists wanting to build a small PSU to support a tube may see a clever way to wire a standard semi-toroid to generate both plate and heater voltages. This only works in North America or Japan where mains is 120Vac or 100Vac, respectively. One primary ties to the mains. The second primary is used to provide high voltage. The two low-voltage secondaries support the heater. For hobbyists, the noise output may be acceptable to them, and they may be used to using / building noisy equipment? But for a discerning ear, this is not as good as it could be and it is of no use for the rest of the world using 240V mains.
Traditional EI transformers are inherently noisy and often use half-turns to generate the correct output voltage. Shielding the core is not a big concern as the windings are never extremely tight anyway. There are limits to how tight you can wind the wire without it breaking during the assembly, or breaking later in use. magnetic leakage will cause end-bells (metal coverings over the windings) to vibrate and buzz, but to what extent depends on the manufacturer's attention to detail and execution of design.
Most auxiliary function PTs will be smaller in their power rating (VA). The magnetising current is often of a similar order to the full-load current and core losses can be a few watts of heat coincident with a steep temperature rise. In general, EMI is in step with the core temperature and is something most hobbyists and even some designers do not notice or know about. Cooler operation at full load requires a larger, heavier, more expensive transformer, so you are buying a larger part to have better longevity and performance through cooler running, or specifying a custom PT with a low-temp rise.
Adding a small PT to an existing amp, or building a preamp using small PTs, we are lucky that "small" can be critically-sized or generously-sized for very similar cost.
