PI SAMPLE DELAY QUESTION

[baum7154]

Another way is to isolate the scope from the coil by looking at the output of the preamp...the ringing is shown there but the probe should not influence it isolated by the amp.

[green]

Another way is to isolate the scope from the coil by looking at the output of the preamp...the ringing is shown there but the probe should not influence it isolated by the amp.

I should have said to disconnect the damping resistor and the input resistor. Anywhere you see the oscillation works.

[mschmahl]

Thank´s a lot Green and Baum7154, I will try your instructions.

[daverave]

i wound my coil tonight but because my coil shell is only about 5mm thick i had to lay the turns side by side and my inductance has now dropped to 140 uh which i guess is far too low...so not sure what to do now

[mschmahl]

I have meassured my coil how baum7154 has discribed it. Don´t know, if all is right. Here are some pictures.First with 20uS, second with 2uS and third with 0.5uS (5us with X10 on oszilloskope). So i have around 377kHz SRF. Could this be?
It is a simple coil AWG24 stranded wire (PVC isolated) 295 uH, 1,8 Ohm, shielded with coppertape.

[Davor]

This looks more like a MOSFET sustaining some kind of relaxation oscillation. Could you see what happens at a MOSFET gate?

[daverave]

Ive made a big mistake with the coil im making !!! to get the coil into the shell i had to wind the turns side by side and also reduce them to 12 turns...i now have a coil which is only 140uH and i feel that ive made a big mistake....can this coil still be used for beach work for rings and coins without problems ?????

[deemon]

I can follow your argument, but your conclusions are incorrect.
Although making the TX-on pulse longer gives the result of providing a larger RX signal, this is not caused by any target charging. The first reason why the RX signal is stronger is because any eddy currents that were generated during the initial on-time will be reduced or will have completely dissipated. These initial eddy currents have a negative impact on the target response. The second reason is because the magnetic field will have reached the maximum possible for that coil/circuit configuration.
If you use a shorter TX on-time, the coil current may be lower when the TX pulse is turned off (reducing the strength of the resultant magnetic field), and any eddy currents in the target will not have decayed away. Result: a lower RX signal, and nothing to do with any fictitious target charging.

Of course it may be right , if we stop the current in the coil when the current is still rising . And when we stop it later - we get more current and thus stronger magnetic field before current shutdown , so we have a stronger reply signal from the target . But there can be another situation when the coil has a little inductance and a relatively high DC resistance . In such a condition the current will grow very rapidly and then reach its maximum ( steady state ) very fast - in a few microseconds , for example ... and what we should find if we try this coil with the target that has a relatively large TC ( 50 microseconds , for example ) ? We'd notice that although the current becomes constant very early - we are still having more and more signal from the target when we increasing the ON pulse duration .... so the only explanation I can give is that we need some time to charge our target in order to get a maximum signal from it . And if we make the ON pulse duration longer than (2-3)*target TC , we'll notice that the target signal stops rising , so we can say that the target is "charged" ( or magnetized ) enough , and we don't need to spend more power .

Obviously it is always good to discuss these things. But ... if target charging is a reality, then what physical mechanism is responsible?

As for me , I can explain it only one possible manner . I mean that the coil energy is contained in the field that coil produces to the air but not in the coil itself , and when we stop the current - this energy returns to the coil , and even can be recuperated if we make a special circuit . But if the field mowing in the space meets any closed wire loop or any kind of conductive material ( metal target ) - it looks like some portion of this energy can be "delayed" or "arrested" near and inside this contour . So when we collapse the initial field - almost all the energy of the field returns to the coil , but this delayed field - comes later , being slowly released from the target . And this delay mechanism works equally in both directions , so we need a time to "push" this energy to the target , and the same time to release it . But quite a similar situation we have when we charge and discharge a capacitor , for example . And this is why I think that when we talk about processes in the target - it's possible to use the words "charge" and "discharge" anyhow ...

[Qiaozhi]

In such a condition the current will grow very rapidly and then reach its maximum ( steady state ) very fast - in a few microseconds , for example ... and what we should find if we try this coil with the target that has a relatively large TC ( 50 microseconds , for example ) ? We'd notice that although the current becomes constant very early - we are still having more and more signal from the target when we increasing the ON pulse duration .... so the only explanation I can give is that we need some time to charge our target in order to get a maximum signal from it . And if we make the ON pulse duration longer than (2-3)*target TC , we'll notice that the target signal stops rising , so we can say that the target is "charged" ( or magnetized ) enough , and we don't need to spend more power .

Have you confirmed this with a practical experiment?
If so, was the target ferrous or non-ferrous?

[green]

I have meassured my coil how baum7154 has discribed it. Don´t know, if all is right. Here are some pictures.First with 20uS, second with 2uS and third with 0.5uS (5us with X10 on oszilloskope). So i have around 377kHz SRF. Could this be?
It is a simple coil AWG24 stranded wire (PVC isolated) 295 uH, 1,8 Ohm, shielded with coppertape.

Some traces I posted in another thread awhile back. The first is without a diode in series with the coil. The next three are with different diodes in series. I think the coil was 280uh. The 344 khz without the diode is close to what you got. Looking at the upper trace it looks kike I had the scope probe on the coil, 450 volt flyback. The lower traces are are the ringing at a lower volt setting.

[green]

Originally Posted by deemon
In such a condition the current will grow very rapidly and then reach its maximum ( steady state ) very fast - in a few microseconds , for example ... and what we should find if we try this coil with the target that has a relatively large TC ( 50 microseconds , for example ) ? We'd notice that although the current becomes constant very early - we are still having more and more signal from the target when we increasing the ON pulse duration .... so the only explanation I can give is that we need some time to charge our target in order to get a maximum signal from it . And if we make the ON pulse duration longer than (2-3)*target TC , we'll notice that the target signal stops rising , so we can say that the target is "charged" ( or magnetized ) enough , and we don't need to spend more power .



Have you confirmed this with a practical experiment?
If so, was the target ferrous or non-ferrous?

LTspice simulation. I think I did it right. The output signal is the difference of the charge and discharge signal. Once the charge reaches zero the discharge stays the same. Would you expect it to be different with ferrous or non-ferrous?

[baum7154]

I have meassured my coil how baum7154 has discribed it. Don´t know, if all is right. Here are some pictures.First with 20uS, second with 2uS and third with 0.5uS (5us with X10 on oszilloskope). So i have around 377kHz SRF. Could this be?
It is a simple coil AWG24 stranded wire (PVC isolated) 295 uH, 1,8 Ohm, shielded with coppertape.

It is difficult but looking at the ringing portion of the scope screen shot at 20us /division, and assuming 7 cycles in 20us I get a ringing frequency of about 350 kHz. Your construction details of 24 awg wire with PVC insulation and a copper shield, plus the capacitance of the detector electronics i.e. mosfet etc. make it very possible that 350kHz is your true IN CIRCUIT ringing frequency. 24 awg wire will have more capacitance than smaller gauges, PVC has more capacitance than PTFE/TEFLON insulation and a solid copper shield contributes more capacitance than no shield or a more sparse shield. This all adds up to a slow coil. Another question is how much space is there between the coil and the shield and what is the spacer made of? Also if the coil has been flooded with epoxy between the windings the capacitance will be even higher, lowering the resonant frequency even more. I believe the coil is operating at about 350kHz.

Assuming the damping resistor was disconnected for this test...With 350kHz X PI X 295uh damping resistor should be about 324 ohms. In other words the coil appears to be under damped using a 390 ohm damping resistor.

Regards,

Dan

[Qiaozhi]

Would you expect it to be different with ferrous or non-ferrous?

I was just asking an exploratory question.

If deemon sees a difference in a practical [real world] experiment, then it may be due to magnetic reluctance. In lateritic soils, the ferrous particles can become magnetized (as the soil has a low reluctance and a high permeability) which could be classed as storing energy. However, I suspect this stored energy would have a negative effect, and is the reason why bipolar pulsing is often employed to cancel out any unintentional magnetization of the soil.

[bbsailor]

It is difficult but looking at the ringing portion of the scope screen shot at 20us /division, and assuming 7 cycles in 20us I get a ringing frequency of about 350 kHz. Your construction details of 24 awg wire with PVC insulation and a copper shield, plus the capacitance of the detector electronics i.e. mosfet etc. make it very possible that 350kHz is your true IN CIRCUIT ringing frequency. 24 awg wire will have more capacitance than smaller gauges, PVC has more capacitance than PTFE/TEFLON insulation and a solid copper shield contributes more capacitance than no shield or a more sparse shield. This all adds up to a slow coil. Another question is how much space is there between the coil and the shield and what is the spacer made of? Also if the coil has been flooded with epoxy between the windings the capacitance will be even higher, lowering the resonant frequency even more. I believe the coil is operating at about 350kHz.

Assuming the damping resistor was disconnected for this test...With 350kHz X PI X 295uh damping resistor should be about 324 ohms. In other words the coil appears to be under damped using a 390 ohm damping resistor.

Regards,

Dan

Dan, and all interested

There is another way to measure the Self Resonant Frequency (SRF) of a coil. Place a coil to measured (coil under test) on top of a fully functioning PI machine's coil. Attach the scope to the leads of the coil under test. Turn on the PI machine and through inductance the coil under test will be stimulated by the square wave pulse of the TX pulse. In the coil under test will be a ringing that tapers off. Measure the space between the peaks of the two highest rining pulses. That distance represents the time of the resonant frequency unloaded by the MOSFET COSS and any other circuit capacitance. Make sure that there is no damping resistor in the coil as the SRF ringing will not be easily seen.

This test will reveal the result of the PI coil under test inductance in parallel with:

1. The coil turn-to-turn capacitance
2. Capacitance based onthe Dielectric constant of the wire insulation and space between windings
3. Capacitance based on the dielectric constant of the spacer material and thickness of the space between the coil wire bundle and the shield
4. Capacitance of the length of coax wire between the coil and the PI control box.
5. Scope probe capacitance

Joseph Rogowski

[baum7154]

Thanks Joe,

That is indeed another good method to measure true SRF of a coil and /or coil /feed combination. From a practical standpoint I would say that the measurement made with the coil in the detector circuit gives the best opportunity to properly damp the coil with all variables in play. I would recommend that a variable resistance network like you outline in your monocoil paper be placed in circuit and adjusted to give the least ringing with maybe a bit of residual overshoot. This would be the shortcut to the critical damping that is the real objective here. It will be interesting to see what damping resistance actually is needed and how much it varies from the original 390 ohm damping.

Thanks again,

Dan