fra0300 Basic Electroincs

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The Q of an inductor is the measure of its quality. The more 'perfect' the inductor, the higher its Q value.  

Q = XL / ri


    Q = quality (unitless) 
    XL = inductive reactance in ohms 
    ri = the internal resistance of the inductor

A 'real-life'  inductor can be regarded as a 'perfect' inductor connected in series with a resistance (ri). 


The Q of a coil is also known as the merit of a coil. 

A 'perfect' inductor would have a Q of infinity. Although it is possible to achieve Qs on the order of 2000 or so, most inductors have Q values that are less than 100 when used in their intended frequency ranges.

The basic equation for Q shows that the Q of an inductor is proportional to its reactance: 
  • Increasing the value of XL increases the Q of an inductor.
  • Decreasing the value of XL decreases the value of Q.
Since the reactance of an inductor is proportional to the applied frequency (XL = 2pfL): 
  • Increasing the applied frequency (f) increases the value of Q.
  • Decreasing the applied frequency decreases the value of Q.



Internal Resistance of an Inductor 

The internal resistance (ri) of an inductor is made up mainly of the DC resistance of the wire that makes up the inductor, itself. At radio frequencies, however, the value of ri is increased by significant amounts of eddy currents and RF skin effects. 

There is an inverse relationship between the Q of an inductor and its internal resistance: 

  • The higher the value of resistance, the lower the value of Q.
  • The lower the value of internal resistance, the higher the value of Q.

For most practical purposes, the internal resistance of an inductor is negligible when the Q value is greater than 100 (the reactance is more than a hundred times greater than the internal resistance). 




David L. Heiserman, Editor

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Revised: June 06, 2015