Toroid or Solenoid?
A toroid is similar to a solenoid but is constructed with a circular profile, creating a continuous structure. Toroidal cores are used to make toroidal power transformers, while solenoid cores are for UI and EI transformers.
An ideal solenoid would be infinitely long with tightly packed coils, where each coil falls into a plane perpendicular to its central axis. The generated magnetic field is concentrated inside the solenoid coil. The intensity of the magnetic field outside the solenoid coil is theoretically zero. However, in the actual case, the magnetic field lines always form closed loops, thus resulting in some flux escape at the end of the coil to complete the loop, so with nothing to contain the escaped flux, it will produce power losses and interference with other adjacent components.
To contain the escaped magnetic flux in solenoid cores, a return path such as limbs is necessary, making the construction of a transformer larger.
Due to its ring shape, the magnetic flux of a toroid is greatly reduced. The coils on a toroid core are uniformly and tightly wound onto the toroidal core to form a loop. Because a toroid is circular in construction, the magnetic field easily completes a loop, so there are no ends or corners for the magnetic flux to escape, obviating the need for a return path, which is why a toroid transformer is compact and light in weight as well as highly efficient compared to traditional transformers like EI transformers.
Although solenoids are often used, toroids are preferred by electrical engineers because of the benefits they offer.
Conclusion
Toroid and solenoid transformers have their own pros and cons, so they should not be used interchangeably. Selecting the right transformer for your application is very important.