The best location for the damping resistor?

[kt315]

i had gave this example but people do not want take attention. this coil seems VERY perfect and MUST fit.
but does not fit and does not work. SIC.

https://www.findmall.com/read.php?34...19#msg-2385319

I bought this coil because it looked like an Eric Foster coil and that it would work on my Headhunter Pulse machine.
I metered the coil, and the inductance is 996 uh and the resistance is 10 ohms. I changed the connector to a HHPI connector.
The problem is it will only detect a very large object an inch or two from the coil.
The coil looks brand new, with no wear whatsoever and since the inductance and resistance are reading correctly, it seems like the coil should not be defective.
So, I would like to know if this coil is compatible with the Headhunter Pulse, and if so, what can I do to that might get it to work?

nobody know the cause why coil does not work.

[Qiaozhi]

There is one consideration to note concerning putting the damping resistor in the coil, and that's if you're being aggressive with early sampling. There's been quite a lot of talk in the forum about using the correct type of wire for the coil in order to reduce eddy currents in the wire itself. Minelab also have a patent concerning solder connections in the coil, and ways of preventing these looking like a target. The same will apply to putting the damping resistor in the coil.

Unless you're pushing the envelope of a design, the bottom line is that it's basically up to your personal preference how you solve the problem. The solution a commercial company uses may be different to that required for personal use.

[Carl-NC]

nobody know the cause why coil does not work.

I think that coil was probably made for the Goldquest which runs a short TX pulse at 10kHz. Most likely with standard damping the preamp is still saturated when sampling occurs.

[dbanner]

The more common place for RD is on the PCB, the reason seems obvious. Everything beyond this point becomes a function of the damping circuit. The coil characteristics are measured from the point of the connector at the end of the cable and not from inside the search head of the coil. This seems to be the overriding factor.

[nonkapo]

Thanks all for the usefull replys. So after dbanner's comment i would say its better to keep few damping resistors on the female socket of the control box. At least for me it does the job as i'm not planning to use more than three coils and at the same time it will be less of headache to solder on the socket rather than spoiling the pc board.
The Qiaozhi's comment about the soldering patent get me thinking what kind of a solution they may have? If it is just the wires i do not think it present a problem, cause it's tiny piece(unless it is the basket design KT315 mention somewhere which requires UTP pairs soldering). But if 1 watt resistor is there together with the soldering we have different movie;-).

[dbanner]

In the case of a dual field coil where two coils are coplanar, there are two damping resistors, where the value of critical damping resistor is determined by the electrical parameters for the total coil assembly and the value of second damping resistor( which I assume is placed within the coil assembly) is calculated from parameters of the isolated smaller coil.

[dbanner]

For this reason I believe a whites dual field coil should work on a whites surf master PI without mods(except for changing the 390 ohm RD on PCB), the critical damping resistor being on the PCB, the second damping resistor within the dual field coil housing.

[bbsailor]

i had gave this example but people do not want take attention. this coil seems VERY perfect and MUST fit.
but does not fit and does not work. SIC.

https://www.findmall.com/read.php?34...19#msg-2385319

I bought this coil because it looked like an Eric Foster coil and that it would work on my Headhunter Pulse machine.
I metered the coil, and the inductance is 996 uh and the resistance is 10 ohms. I changed the connector to a HHPI connector.
The problem is it will only detect a very large object an inch or two from the coil.
The coil looks brand new, with no wear whatsoever and since the inductance and resistance are reading correctly, it seems like the coil should not be defective.
So, I would like to know if this coil is compatible with the Headhunter Pulse, and if so, what can I do to that might get it to work?

nobody know the cause why coil does not work.

If my memory is correct, this coil has 30 turns of AWG 30 wire (single strand) to make the 1mH mono coil. Eric Foster used this type coil on the CS6PI metal detector that he designed. This is a very low power PI machine as there is a 39 ohm (again, if I remember correctly but look it up) in series with the coil and only produces a flyback peak of 80V. The delay of this machine was between 15 to 17 uS and was good for wet ocean beach hunting.

Damping resistors work by suppressing the flyback pulse oscillation as soon as possible. These oscillations occur because the energy in a charged coil by the TX pulse current at turn off time creates a flyback pulse that can peak near 1000V for high power PI designs. Add to this flyback model, the capacitance added by:
1. Coil turn-to-turn wire capacitance
2. MOSFET COSS (output capacitance)
3. Coax cable capacitance (about 30 pf per foot)
4. Coil to shield capacitance

The rule of thumb is to keep the coil alone capacitance about one half the total measured capacitance at the end of the coil cable. This is done by measuring the self resonant frequency of a known coil inductance and then doing the math.

Then you have some collective value of capacitance that tends to absorb the flyback energy and causes that flyback energy to oscillate. Lower total capacitance has less energy to oscillate and requires less damping so a higher value damping resistor can be used. The sooner the TX pulse oscillations can reach near zero volts, the faster the RX circuit can be turned on. This is called critical damping. During this delay time any eddy currents remaining in the stimulated target are decaying. If you have a small target with a 2 uS time constant, in 5 time constants of that target's charged eddy currents will be at zero and have little or nothing to detect at a 10 uS delay. If you could drop the delay down to 7 or 8 uS you could begin to detect that target with some remaining eddy currents.

As you make your delay lower other factors begin to become involved.
1. Operational amplifier time to come out of saturation. Lower gain multiple stage op amps work faster than single stage high gain op amps.
2. Sensitivity to detecting environment. Wet beach hunting is typically done at about 15 uS delay or higher. Shielding a coil isolates the coil from ground capacitance but also adds some additional coil capacitance.
3. External noise at some multiple of local AC current frequency: 50Hz in some countries and 60Hz in some other countries requires a shielded coil.
4. Coil wire itself (including wire soldered joints). How long are eddy currents being retained by the coil wire itself. If your wire holds the eddy currents any longer than the minimum delay, the wire will act as a target.
5. Full stimulation of the target: Theory tells us that the TX pulse discharge time constant (TC) should be 5 times faster than the desired target TC to fully stimulate it. This coil discharge TC is measured by coil inductance divided by the damping resistor value. If a 300 uH coil has a damping resistance of 750 ohms than the discharge TC will be 300/750 or .4uS and would be good to stimulate a target with a TC of 2 uS (.4 X 5).

If you build a coil and can detect small gold and when you reduce the delay and suddenly your PI machine locks up, you are beginning to detect something in the coil itself.

As you can see, when you attempt design a coil to reduce the delay to detect smaller targets, other factors come into play. It is a real balancing act. Choose your battles wisely!!!

Joseph J. Rogowski

P.S.

In keeping with the topic: where is the best location for the damping resistor. It depends on how many coils you plan to use and the range of damping resistor values that will critically damp each coil. If you have a range of coils that will critically damp from 800 ohms (faster) down to 600 ohms (slower coil) put a metal film (1Watt to 3Watt) 800 Ohm damping resistor on the PI circuit board. Make sure that it is not getting too warm as its resistance will get a little higher when warm. Then, when using coils that critically damp at lower resistance values you can add a smaller resistor in parallel to the circuit board value by locating it in the coil connector. Size is critical but since its value will be near 2500 ohms in parallel with 800 ohms its power absorption will be much less and you could get away with using a fraction of a Watt resistor. Use a metal film type.

[Qiaozhi]

Here's the Minelab patent ->

[dbanner]

Incidentally, the Fisher impulse schematic shows damping resistors (162R fixed plus a 500R trimmer) located on pcb at loop connector. There are also internal resistors in the
Search head. I suppose this is the best of both worlds.

[nonkapo]

Here's the Minelab patent ->

So it sounds like shielding the soldering joints?!?!? I was expecting something more sofisticated;-).

[Tim]

Another fabulous insight from BB Sailor - Thank you Joseph. Although I get good results with my SMpro(DD Tacoma) with a ML commander DD, it was fitted with a 390R. I think it was Homefire or 6666 that alerted me to the fact that the Commander has an internal damper, so it will be under-damped with both connected. Therefore next time I dismantle it I will fit a switch on the 390R, so I can use the ML/Coilteks as well as Homemades.
I think I remember reading here somewhere that a wirewound pot would be no good as an inbuilt variable damping resistor because of changing inductance, but a 2(?) watt wiper type - would that work? Perhaps 1K in series with a 1K2x2W resistor to reduce current through the pot??

[dbanner]

I used damping tool circuit on my home built surf and just left it in as permanent. So I can vary from 381 to 1.6k depending on coil I use. I'm always making new coils for experimental purposes, trying to find what works and what doesn't work best, so variable damping is convenient when intending to use multiple coils. I can return to a preset position on tool for any previous coil.

[Ferric Toes]

I mostly put the damping resistor on the circuit board and only occasionally in the connector on low power PI's. I don't put them in the coil housing for the reason shown in the plot. The fast decaying trace is for no object in the coil, while the top trace is for a 1 watt metal oxide resistor, which is the type I generally use for damping. As you can see the resistor generated quite a substantial signal due to its internal metal endcaps. Higher wattage resistors are considerably worse. I like to start off with as flat a response as possible out of the preamp, without any superimposed metal signals. The Vallon VMH3CS detects this 1W resistor a couple of inches off the coil.

Eric.

[kt315]

in reality a resistor is spiraled track done around a ceramic base. it is inductivity, but with very low value.
and yes, it can receive EMF and retransmitted it back.