Thursday, August 9, 2018

Coil size vs Q

  • You hear so much about Q, some give it a mystical almost magic quality but what is it?
  • When an inductor passes current it build a magnetic field. When the excitation is removed the field collapses and induces current. This inertia attempts to maintain a constant current. 
  • When the coil is charging the resistance of the wire limits current. The inertia maintaining the current at the charge level. Q = Xl/R 
  • Lower R produces high Q. We can use a larger wire to reduce R BUT that is not the only parameter at play. The inductance is determined by the number of turns AND the volume of the winding. 
  • The coil cross section can contain more turns of smaller wire which increases inductance and wire resistance. Fewer turns give lower inductance and lower resistance.
  • The length of the wire will vary R. The former diameter will effect inductance. So we can use a larger core and fewer turns but longer wire per turn.
  • Gee, does it have to be so complicated? No not really. So look at the chart.

    The groups of lines are for different core size. The vertical line is for the inductance I choose to look at. The horizontal lines indicate the Q you would expect for each coil. The Q varies from less than 50 to over 100. Which Q would be best? That depends on the circuit parameters. 

    Suppose you what to tune a 450Khz signal with a Band Width (BW) of 5 Khz. BW = Fo / Q. Assuming a Q of 100 
    BW = 450K / 100 = 4.5K
    Humm, We would be clipping our highs from our signal.
    Food for thought?
    More food for thought can be found here:

    If I make Q my only concern and set the C factor at minimum in order to raise L and Q what would that do to hand effect?
    If I make a coil with the Q=100,000 what effect would that have on signal quality? 


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