What are the uses of hollow coils? MagTop told you. The air core coil has three functions: choke coil, filter and oscillation.
1 choke coil
In low-frequency circuits, it is used to prevent low-frequency AC; from pulsating DC to pure DC circuits; it is often used in the middle of two filter capacitors at the output end of the rectifier circuit, and the choke coil and capacitor form a π-type filter circuit. In high frequency circuits: It is a high frequency choke coil used in old regenerative radios to prevent high frequency current from flowing to the low frequency side.
Same as the above theory, it also prevents the rectified pulsating DC current from flowing to the pure DC circuit composed of two capacitors (electrolytic capacitors) of the choke coil (in order to simplify the circuit and reduce the cost, the choke coil is replaced by pure resistance).
The pure DC current can be obtained by blocking the AC current by using the capacitor charging and discharging function and the DC current of the choke coil.
We say that rectification is to convert alternating current into direct current, so oscillation is the reverse process of converting direct current to alternating current. We call the circuit that does this "vibrator". Oscillator waveforms: Sine, Ramp, Trapezoid, Square, Rectangle, Peak. Frequency ranges from a few hertz to tens of hertz. It is widely used in wired power and radio fields.