- 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.Example:Suppose you what to tune a 450Khz signal with a Band Width (BW) of 5 Khz. BW = Fo / Q. Assuming a Q of 100BW = 450K / 100 = 4.5KHumm, 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?
Thursday, August 9, 2018
Coil size vs Q
Posted by Reset at 8:08 AM