Coil time constants, L/Rd vs wire TC

[green]

Did a test awhile back with different size coil wire. I have a spool of AWG19 magnet wire that I have been wanting to make a coil with. Thinking of replacing coil C with the AWG19, 23 turn flat radial and retesting A, D, and E with C. Don't remember what Rd was, probably around 1k. Do a better job of documenting test this time. Wondering which TC controls when amplifier comes out of saturation. Maybe he longest TC? 250uH/1000=.25us TC. If it was the longest TC then any wire smaller than AWG29 wouldn't effect result. Think I should be able to calculate time for the coil to decay to a current for the amplifier to come out of saturation since I know peak current. Don't know starting amplitude for wire decay? Wondering how decay is effected by current flow direction, coil current decays length wise and wire current around the wire. Maybe I'm looking at it wrong? Any guesses how long it will take for the amplifier to come out of saturation with AWG19? Any thoughts or suggestions? Wondering if wire size matters with a figure eight coil?

[green]

Finished the coil, 23 turns, AWG19 solid magnet wire, 200mm ID flat spiral coil. Recorded no target signal, came out of saturation sooner than I expected(about 10us).

[bbsailor]

Did a test awhile back with different size coil wire. I have a spool of AWG19 magnet wire that I have been wanting to make a coil with. Thinking of replacing coil C with the AWG19, 23 turn flat radial and retesting A, D, and E with C. Don't remember what Rd was, probably around 1k. Do a better job of documenting test this time. Wondering which TC controls when amplifier comes out of saturation. Maybe he longest TC? 250uH/1000=.25us TC. If it was the longest TC then any wire smaller than AWG29 wouldn't effect result. Think I should be able to calculate time for the coil to decay to a current for the amplifier to come out of saturation since I know peak current. Don't know starting amplitude for wire decay? Wondering how decay is effected by current flow direction, coil current decays length wise and wire current around the wire. Maybe I'm looking at it wrong? Any guesses how long it will take for the amplifier to come out of saturation with AWG19? Any thoughts or suggestions? Wondering if wire size matters with a figure eight coil?

Green,

The coil discharge TC is governed by the coil inductance divided by the damping resistor value (Rd). To get an Rd of 1000 ohms you need a very low capacitance coil. It is a coil discharge TC of .25us and you will best stimulate a small target with a TC five times higher or a target with a 1.25 us TC. Since it takes 5 TCs for a fully stimulated target to fully discharge it would take 6.25 us for the 1.25us target to fully discharge so you would need to sample at 5 or 6 us to be able to hear any target signal.

If you sample at 5 us then you want to use a coil wire with less than a 5 us rating so the wire does not act as a target when low sampling at 5 us.

Typically you want to turn the current off when the charge curve is at about 3 TCs or at about 95 percent of max current. This places the curve on a more horizontal portion of the charge curve and the best place to turn off the current. If you turned off the current when the charge curve is more vertical you will not fully stimulate the target as the rise and fall are so close that some of the target charge effect is cancelled.

When designing a PI metal detector for low delays you need to do a very good balancing act to include:
Fully stimulating your low TC targets
A preamp that comes out of saturation very fast
A coil that has the potential to operate at low delays
Creative ways to amplify very small signals such as through integrating many RX signals

Joseph J. Rogowski

[green]

Green,

The coil discharge TC is governed by the coil inductance divided by the damping resistor value (Rd). To get an Rd of 1000 ohms you need a very low capacitance coil. It is a coil discharge TC of .25us and you will best stimulate a small target with a TC five times higher or a target with a 1.25 us TC. Since it takes 5 TCs for a fully stimulated target to fully discharge it would take 6.25 us for the 1.25us target to fully discharge so you would need to sample at 5 or 6 us to be able to hear any target signal.

If you sample at 5 us then you want to use a coil wire with less than a 5 us rating so the wire does not act as a target when low sampling at 5 us.

Typically you want to turn the current off when the charge curve is at about 3 TCs or at about 95 percent of max current. This places the curve on a more horizontal portion of the charge curve and the best place to turn off the current. If you turned off the current when the charge curve is more vertical you will not fully stimulate the target as the rise and fall are so close that some of the target charge effect is cancelled.

When designing a PI metal detector for low delays you need to do a very good balancing act to include:
Fully stimulating your low TC targets
A preamp that comes out of saturation very fast
A coil that has the potential to operate at low delays
Creative ways to amplify very small signals such as through integrating many RX signals

Joseph J. Rogowski

If I critical damp a coil in spice and look at the discharge rate it's closer to twice as fast as calculated. Should it be or am I doing something wrong?

I'm wondering if there is a way to select largest wire size to have little effect on when the amplifier comes out of saturation based on the coil TC(L/Rd). (Coil TC)L/Rd=1/(pi*circuit resonance). (.318us for 1MHz resonance) (.212us for 1.5MHz resonance) Think it would be difficult to get less than .2us TC. Wondering if Litz wire with TC 1/10 of the coil TC is necessary?

[green]

New measurements for coils A, C, D and E. Some different than before, used different procedure. Coil D was labeled pvc coating, it's wire wrap wire so probably kynar.

Scope picture reply#2 was recorded with 1A peak coil current, higher peak current would increase time for amplifier to come out of saturation

SRF measurement_coil only no lead wire. Circuit resonance_ shielded coil with 33inch twisted pair lead, Tx includes a MUR460 in series with coil.

[green]

Looking at my test procedure reply#5 I wondered why I didn't measure circuit resonance with the loop same as SRF. Tried with a coil I was using to excite resonance. Rd and Rin disconnected, loop 1.28MHz, probe to hot lead insulation 1.28MHz and X10 probe to hot lead 1.13MHz.