The toroidal transformer double-line is to make the midpoint voltage of the output or input just half of the output voltage at both ends of the head and tail. The double line is generally not used for the primary winding because the voltage difference between the two lines is too high and easy to break down. B and A1 are scraped and twisted together as a center point by means of a double wire and four wire ends (generally A, B and A1, B1) which are generally secondary (i.e., low pressure) and wound. A and B1 are both ends. For example, a 200W 220V to 5V-0-5V toroidal transformer. You can use two lines and spare.

There are three advantages to the toroidal transformer's two-wire winding:

1. The two windings have symmetrical DC resistance and the AC impedance is symmetrical.

2. Satisfy the phase relationship required by the circuit (when the first end of the two-wire winding is connected, the current directions of the two coils and the junction point are opposite, and the phase sequence of the two coils is opposite at the same time), so that the two coils are completely balanced.

3. Wrap the two wires together, and connect the two head ends together to make the center tap. After the two ends are used as the output head (for the center tap), the two sides are symmetric to the center tap voltage.