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I thought I read somewhere that a PI coil needs to be low resistance like less than one ohm so it helps to discharge the coil after TX turn off , what are peoples thoughts on this ?
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Coil resistance affects the turn-on characteristics, has no effect on the turn-off. Turn-off is affected by parasitic C and damping.
The turn-on current is a rising exponential with a tau that is set by the coil inductance and the total path resistance: coil + battery/cap + switch. The lower you make the resistance, the closer the exponential gets to a linear ramp. It is generally beneficial instead to flat-top the current, but that requires higher R, which then results in a lower final current. You can then jack the supply up to compensate, but you quickly get into power consumption problems. Another approach is a kickstart circuit to get the current going fast, followed by a low hold voltage. This has been discussed in other threads. In that case, low R helps reduce power consumption (generally, low TX coil R always benefits power consumption, whether PI, VLF, or BFO). -
Made my target response tester with .5A peak constant current or 1A peak constant rate adjustable time Tx. Same average Tx current. Test not effected by coil resistance. Constant rate gives highest signal if average current is the same. Controlling current in a loop doesn't help with power supply efficiency.
Tried simulation with spice. 0R5 and 6R coil resistance with 300uH and 600uH coils. Adjusted supply volts for 2A peak current. Was surprised at difference in volts for what appears to be same signal strength. Maybe I'm doing something wrong? 600uH coil appears to have twice the signal strength the 300uH coil has.Comment
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Thanks Carl and Green , I have been giving this some reflective contemplation and have been trying to find where I thought I read it, however I could not find it, so I have concluded that I've remembered it incorrectly, and have been chasing up some flat top threadsCoil resistance affects the turn-on characteristics, has no effect on the turn-off. Turn-off is affected by parasitic C and damping. ETC
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