Fast coil idea

[ivconic]

[ATTACH]50543[/ATTACH]

OK, here is the picture of the fast-slow coil, TX&RX again

That's somewhat close to what i wanted to experiment with coils from induction cooker.
Either to use it as TX, either to use it as RX. Not as monocoil.
But in both cases coil is not having wanted features.
In case of TX; would be tough to drive it and keep fet alive longer.
In case of RX; received signal would decay too fast.

[Tinkerer]

That's somewhat close to what i wanted to experiment with coils from induction cooker.
Either to use it as TX, either to use it as RX. Not as monocoil.
But in both cases coil is not having wanted features.
In case of TX; would be tough to drive it and keep fet alive longer.
In case of RX; received signal would decay too fast.

Here is an idea to try:

connect the 2 coils in series.
place the coils beside each other at a distance of about half of the diameter.
place RX coil between the TX coils with equal overlap
damp each of the 3 coils with about 500 Ohms
pulse with square voltage wave, about 2kHz, using minimum 500V Mosfet.
observe target response on RX coil output. Use large targets.
If you observe anything interesting, add preamp
play with distance of the coils and turning one of the TX coils upside down, using coupling in opposing and aiding configuration.

[Tinkerer]

Interesting trace. Usually the first half of the decay would be faster than the last half. Only way I can get a trace that looks similar, is to underdamp and let the return to zero go to -.6V across the MOSFET. Would be hard to see with 200V/div. Wondering how you get the mirror image critical damped.

I have noticed deformations of the flyback half sine, like you mention, at other times. Sometimes it is the type of Mosfet, sometimes it involves shielding or cables.
The present coil and TX setup is the result of making hundreds of coils and trying dozens of different Mosfets and dozens of different cables and hundreds of different timing schedules.

This coil is not shielded yet.

This result is only attained when everything is just right.

Attached is the 1000V test. Damped at about 3us.
Also the 50V div. picture. I can not do lower than that.

[green]

Underdamped Tx off with spice.

[Carl-NC]

For those who tinker (pun intended) with coils, here are a few thoughts to consider when making "fast coils".
1. A fully stimulated target needs a coil discharge time constant (TC) five times faster than the targets TC. The coil discharge TC is based on the coil inductance divided by the damping resistor value and any other resistors such as the op amp input resistor being grounded when the clamping diodes are on.

5x works well, it provides for a 90.6% efficiency. Other efficiencies are:

1x = 63.2%
2x = 78.7%
4x = 88.5%
8x = 94.0%
50x = 99%

So even 2x faster than the target tau works pretty well.

Also, the discharge TC is 0.5*L/R, not L/R... assuming critical damping.

[Tinkerer]

Underdamped Tx off with spice.

Looks excellent

[green]

5x works well, it provides for a 90.6% efficiency. Other efficiencies are:

1x = 63.2%
2x = 78.7%
4x = 88.5%
8x = 94.0%
50x = 99%

So even 2x faster than the target tau works pretty well.

Also, the discharge TC is 0.5*L/R, not L/R... assuming critical damping.

Numbers look like what I remember from the past. Tried with spice again to work my brain. Don't know if it looks right, thought coil 2x faster than target would look different. Anyone see what I might be doing wrong?

[Elliot]

... The optimum coil discharge TC to fully stimulate the target is 2/5 or .4 uS and would need the damping resistor to be 750 ohms (300/750 is .4). If the delay is set at 10us, the stimulated target eddy current energy will be zero at this delay so a delay of 7 or 8 us would be needed to detect this target. As targets get smaller, tinkering with coil designs need to keep these things in mind (no pun intended).

Joseph J. Rogowski

I think you mean "The optimum coil discharge TC to detect a fully stimulated target is 2/5 ..."

[Carl-NC]

I think you mean "The optimum coil discharge TC to detect a fully stimulated target is 2/5 ..."

No, he said it correctly... it is the coil discharge that stimulates the target. Faster is better, up to a point.

[Elliot]

No, he said it correctly... it is the coil discharge that stimulates the target. Faster is better, up to a point.

That's what I said a few weeks ago, but was howled down by another person! People said that it was during the pulse TX period that charged the target. Now you are stating that it is the coil discharge that stimulates the target (which is what I always thought). The only thing that I can deduce is that you are both right - i.e. the coil and target is charged during the Tx pulse period, but the good work done to the target during the 60-100uS square pulse stage is negated, and overcome. by the coils back EMF spike, and then a greater charge of opposite direction is then applied to the target. Now, if this is the case, then all the talk about charging the target for 3-5 TC's during the square wave portion (before the back EMF kicks in) is a load of nonsense, as it all gets undone by the back EMF. I'll let greater minds fight this one out.

[dbanner]

Eddy currents are induced by changing magnetic fields, either expanding or collapsing.
They do not appear when the field is in a steady state.
At tx switch on, there is an expanding magnetic field, during which time Eddy currents appear in the target, but these are transient currents and will decay to zero when the field reaches a steady state and filters through the entire target. At which point the current in the coil is switched off, which causes a sudden collapse of the magnetic field, and as a result of electromagnetic induction, Eddy currents once again appear in the target, this time of reverse polarity.
The collapsing field is also responsible for the so called back EMF generated in the coil.

[dbanner]

Due to recent posts by Carl, It ihas become clear to me that the collapsing magnetic field at tx switch off is responsible for stimulation of the target for the purpose of target detection. However it is also clear that the type of target bears some relationship to the initial Eddy currents decay time after tx switch on, and can subtract from the stimulation presented by the collapsing field if current switch off occurrs before the Eddy currents have had sufficient time to decay to zero.

[Carl-NC]

That's what I said a few weeks ago, but was howled down by another person! People said that it was during the pulse TX period that charged the target. Now you are stating that it is the coil discharge that stimulates the target (which is what I always thought). The only thing that I can deduce is that you are both right - i.e. the coil and target is charged during the Tx pulse period, but the good work done to the target during the 60-100uS square pulse stage is negated, and overcome. by the coils back EMF spike, and then a greater charge of opposite direction is then applied to the target. Now, if this is the case, then all the talk about charging the target for 3-5 TC's during the square wave portion (before the back EMF kicks in) is a load of nonsense, as it all gets undone by the back EMF. I'll let greater minds fight this one out.

Sorry, either I missed that, or decided to address it in my separate "Optimizing Target Responses" thread. dbanner's previous two posts pretty much nail it down. Target eddies are generated during both the TX pulse period and during the turn-off slew, but in a traditional (mono coil) PI it is the turn-off slew that does the useful work. I've been busy the last 2 nights but will try to get the next "Optimizing Target Responses" entry done this weekend. Things will become clearer.

[ivconic]

What if we make coil from golden wire?
Or printed coil with gold plated prints.
The way they gold plated old IBM PC XT mainboards... back in the good old days!