Confuse decay versus resistor damp

[Wahyu]

Hello all,

I have made 2 coil ;

1. Coil made from wire wrap awg 30 kynar, inductance 304uH, resistan 7.4 ohm, resistor damp 700 ohm, good decay curve 6uS
2. Coil made from litz wire 0.04x50, inductance 314uH, resistan 4.2 ohm, resistor damp 470 ohm, good decay curve 5uS

Why litz wire less value resistor damp but less decay time?

I dont measure SRF, same coax, same shield

[Qiaozhi]

Hello all,

I have made 2 coil ;

1. Coil made from wire wrap awg 30 kynar, inductance 304uH, resistan 7.4 ohm, resistor damp 700 ohm, good decay curve 6uS
2. Coil made from litz wire 0.04x50, inductance 314uH, resistan 4.2 ohm, resistor damp 470 ohm, good decay curve 5uS

Why litz wire less value resistor damp but less decay time?

I dont measure SRF, same coax, same shield

When you say "good decay curve|", where are you looking at the RX signal?
Directly at the coil, or at the output of the preamp?

[Wahyu]

Output of the preamp, same machine too

Thank Qiaozhi for reply

[Qiaozhi]

Output of the preamp, same machine too

Thank Qiaozhi for reply

I just wanted to check, as it's quite difficult to judge when the signal is critically damped if you monitor the coil output directly.
Measuring the SRF may provide a clue to the answer, as it looks like the second coil should be the slowest, based on the numbers you've provided.

[Wahyu]

Ok Qiaozhi, i will measure SRF

Thank

[bbsailor]

Hello all,

I have made 2 coil ;

1. Coil made from wire wrap awg 30 kynar, inductance 304uH, resistan 7.4 ohm, resistor damp 700 ohm, good decay curve 6uS
2. Coil made from litz wire 0.04x50, inductance 314uH, resistan 4.2 ohm, resistor damp 470 ohm, good decay curve 5uS

Why litz wire less value resistor damp but less decay time?

I dont measure SRF, same coax, same shield

The answer that Qiaozhi provided about measuring SRF is correct as higher self resonance indicates less coil capacitance and less retained energy needing to be damped thus a higher damping resistor value. There is one more thing to consider: coil current and coil energy needing to be damped. Try putting a 3.2 ohm resistor in series with the Litz wire coil and see what damping value you need when both coils draw the same current. Let us know what you find.

Anything you can do to reduce any capacitance in the TX circuit will speed up the damping process and make a faster coil. Use the shortest coil cable possible to make a big capacitance minimization. Use coil wire insulation with the lowest dielectric constant. Use a MOSFET with the lowest COSS and highest voltage to not clamp the flyback peak. Litz wire is good at low delays because the strands are so thin that eddy currents will not last very long in the coil wire itself.

I hope this helps?

Joseph J. Rogowski

[Wahyu]

Hello bbsailor, i reading your post http://www.geotech1.com/forums/showt...4670#post19467

With the higher current, the flyback voltage will be higher and require a lower value damping resistor to allow the earliest sampling.

Diffrent resistan coil, litz has less resistance and and more energy current than kynar

Ok i will make SRF as your post and add series resistor with litz coil,

Thank bbsailor for reply

[Wahyu]

Hello Qiaozhi and Joseph
I have been test SRF

- Coil wire wrap awg30, SRF 800khz, with coax and shield
- Coil litz, SRF 580Khz with coax and shield, I think litz have more interwinding capacitance

I have question, when measure value inductance, coil only, or with coax and shield?


Thank

[Qiaozhi]

Hello Qiaozhi and Joseph
I have been test SRF

- Coil wire wrap awg30, SRF 800khz, with coax and shield
- Coil litz, SRF 580Khz with coax and shield, I think litz have more interwinding capacitance

I have question, when measure value inductance, coil only, or with coax and shield?


Thank

For case 1, the capacitance calculates as 130pf, and case 2 as 240pF.
The damping resistance then becomes 764R and 572R, not taking into account additional capacitance presented by the MOSFET, other components and the PCB layout.

Measuring SRF with the coax cable and shield attached is the correct method. In this way you can capture a large percentage of the total capacitance (probably around 84%).
If you have correctly achieved critical damping using 700R (case 1) and 470R (case2), the total capacitance appears to be 158pf, and 285pf respectively.

[Wahyu]

Thank Qiaozhi

[green]

Hello all,

I have made 2 coil ;

1. Coil made from wire wrap awg 30 kynar, inductance 304uH, resistan 7.4 ohm, 130pf, resistor damp 700 ohm, good decay curve 6uS
2. Coil made from litz wire 0.04x50, inductance 314uH, resistan 4.2 ohm, 240pf, resistor damp 470 ohm, good decay curve 5uS

Why litz wire less value resistor damp but less decay time?

I dont measure SRF, same coax, same shield

A spice model to see why litz coil decays faster. It's slower with spice if you enter coil specs. Circuit probably not the same as yours. Don't see how changing the circuit would make the litz coil faster.

coil capacitance from Qiaozhi reply #9(For case 1, the capacitance calculates as 130pf, and case 2 as 240pF)

[Wahyu]

Hello green, Thank for spice, i will test

[green]

Hello green, Thank for spice, i will test

Hi Wahyu, the circuit is something to play with(change circuit values, coil to + or - input, jumper MUR460 diode) to see what happens and compare with scope traces from your detector circuit. Spice has the litz coil over damped with the 470 ohm resistor, 572 ohm(Qiaozhi calculation reply #9) comes close with spice. I'm confused with you why the litz coil damps sooner with your circuit. Hope you find why and let us know.

[pustareka]

A spice model to see why litz coil decays faster. It's slower with spice if you enter coil specs. Circuit probably not the same as yours. Don't see how changing the circuit would make the litz coil faster.

coil capacitance from Qiaozhi reply #9(For case 1, the capacitance calculates as 130pf, and case 2 as 240pF)

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