Just a question: Is ferrite bead actually an inductor? Or a filter?
The wire threaded through the bead (either once or a number of times)
becomes (or already is depending on frequency) an inductor.
Inductors in the presence of AC current through the wire have reactance
(often designated as Xl) which is measured in ohms. The higher the
frequency the greater the reactance.
In one sense an inductor alone is not a 'filter'. (Draw a line across piece
of paper and put some coils/squiggles in the middle of it to indicate
inductance and it should demonstrate that is it not 'filtering' anything in
Filters are traditionally thought of as circuits designed to;
a) Allow all signals/voltages above a certain frequency - High Pass.
b) Allow all signals/voltages below a certain frequency - Low Pass.
c) Allow all signals within a certain bandwidth - Band Pass.
d) Prevent or bypass a certain specific frequency signal/voltage which is
desired or causing a problem etc. - Often called by some such name as a
'Stop' or 'Spike' or 'Notch' Filter.
True 'filters' require design of all the components and impedances involved.
Filter circuits can be very simple comprising little more than one inductor
and/or one capacitor, or very complicated with many components.
However if the requirement is to stop or attenuate higher frequencies while
allowing lower ones to pass an inductor including one using a ferrite bead
as the magnetic core could be considered to 'filter out' those higher
Thus a ferrite bead on the AC power lead of a computer monitor would allow
the 60 hertz AC power to go through unimpeded. But high frequencies which
might be causing interference to/from nearby equipment could be greatly
Inductance 'L' is measured in Henries. Reactance is measured in ohms.
Frequency is measured in hertz.
Reactance Xl = 2 x pi x frequency x L.
Thus Xl at 1000 hertz for 1 henry inductor is; 2 x pi x 1000 x 1 = 6284 ohms
Xl at freq. 20,000hz for a 0.01 henry inductor is; 2 x pi x 20,000 x 0.01 =
At very, very high frequencies the inductive reactance of the wire itself,
without any extra inductors (beads etc.) can be most significant.