Measuring Inductors


Often you find yourself
in the position of needing to wind your own coil for a project, or maybe
you come across an unmarked coil in the junk-box. How can you best find
out its inductance? An oscilloscope is all you need. Construct a
resonant circuit using the coil and a capacitor and connect it to a
square wave generator (often part of the oscilloscope itself) Adjust the
generator until you find the resonant frequency f.

When C is known (1000pF) the inductance L may be calculated from:
L=1/(4π2.f2.C) If you are also interested how good the coil is i.e. what
is its quality factor or Q, you can use the oscilloscope again. If the
level of the damped oscillation drops to 0.37 (= 1/e) of the maximum
after about 30 periods, then the Q factor of the coil is about 30. The Q
factor should be measured at the intended operating frequency of the
coil and with its intended capacitor. The coupling capacitor should by
comparison be a much smaller value.


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