Inductors temporarily store energy within their magnetic fields, resulting in a current flow that “lags” the voltage across the inductor due to “back emf” opposing that current flow. An increasing current flowing through a coil generates a magnetic field. As that magnetic field is building, the flow of current is inhibited until the current reaches steady state when there is no longer resistance to the current flow. As the current flow decreases, the magnetic field decreases, releasing the stored energy back to the circuit, temporarily maintaining current flow. This property can be used to soften current surges or spikes within a circuit. An inductor that is used to pass direct current (DC) while blocking high frequency alternating currents is called a Choke, due to its increasing impedance at rising frequencies, thus suppressing high frequency “noise”. Inductors are used in power converters to store energy, to minimize and smooth out “ripple” current superimposed on the DC output and to filter EMI noise. By combining inductors with capacitors & resistors, tuned circuits and bandpass filters can be constructed. To contain the magnetic field & reduce radiated EMI, a toroidal geometry is a good choice. If additional shielding is desired, a ferrite sleeve or Mu-metal shielding can be incorporated.