The figure of merit or quality factor of an AC device is termed Q. The Q is associated to the selectivity of a circuit. The higher the Q better the circuit capacities in separate near frequency signals. This factor is defined by the next formula:
Formula 1
Q = XL / R
Where:
Q is the quality factor or Q factor
XL is the inductive reactance of the coil in ohm (Ω)
R is the resistance of the coil in ohm (Ω)
Note: the same concepts can be applied to any AC device when the factor XL becomes X (the total reactance in Ω).
Derivated Formulas:
Formula 2
Q = (w x L) / R
Where:
w is 2 x 3,14 x f where f é is the singal frequency in hertz (Hz)
Formula 3
R = XL / Q
Formula 4
XL = Q x R
Application Example:
A 2 mH choke has a resistance of 10 Ω and a 1 MHz impedance of 500 Ω. Calculate the Q factor of this choke at the given frequency.
Data:
XL = 500 Ω
R = 10 Ω
Q = ?
Applying formula 1:
Q= 500/10 = 50
