# Coil Impedance

by FredGarvin
Tags: coil, impedance
 Mentor P: 41,096 The magnitude of the impedance of an inductor is $$|Z| = 2 \pi f L$$ where f = frequency and L = inductance of the coil. It takes a lot of inductance to give you much impedance at 50/60Hz powerline frequencies. Power transformers, for example, use many turns around ferrous iron plate cores to get the inductance up enough to work at AC powerline frequencies. An air core (or other non-ferrous core) coil will have a low inductance, and hence a low impedance at powerline frequencies. If the DC power supply spec is for a ripple voltage of 10% or less, that means that the AC ripple waveform that is superimpressed on the DC will have 10% or less of the DC value. So the output would look something like this in the worst case: $$V = V_{dc} (1 + 0.1 sin(\omega t))$$