I have been studying the plans for a few AC to DC converters and I understand that the basic conceptual design is one where the AC power is transformed to lower the voltage and is then passed through a bridge rectifier to convert that power to what I call hopping DC. The power is then smoothed with capacitors and is then regulated to a constant DC output using voltage regulators. In my efforts I took it upon myself to take one of these kinds of power supplies apart to see what is inside of it and I came upon a question that I do not have the answer to. I have attached a picture of the inside of the power supply. Between the square component with the yellow electrical tape and the four small capacitors there is some type of circular device with a helix coil of copper wire around it. I think that this is some type of solenoid however I am inquiring to the purpose of this component and why it would be inside of this circuit? All of my research tells me that solenoids are used to create motion most of the time however I don't see this solenoid attached to anything unless it is somehow being used to power the fan inside the unit. I see these coils inside of every computer power supply I have ever looked inside and I have become exceedingly curious about their intended purpose inside of AC to DC converters like the one I am planning to build out. Thanks in advance for any comments or answers!