Fast coil with teflon wrapping wire

[nick_f]

Hi All,

I decided to build a coil using wire-wrap insulated with teflon. The coil has 29 turns on 20cm and an inductance of 461uH. At this stage it doesn't have any shielding. Initially when I connected the new coil I was expecing to see some reduction in the width of the pulse. I was surprised to see that nothing changed! I realised from my previous experiments, that the key to get a narrow width, is to increase the resistance in parallel with the coil as much as possible. At this stage, I have a 1.5 meter coax cable (50 ohm) and the coil. The total resistance in parallel with the coil is 1150 ohm. I got the pleasant surprise to see that my pot R42 is set to the minimum, and I need to bring the pulse at TP6 even closer! I decreased the value of cap C24 to 5.6nF and now I can adjust it very close.
It can detect the 1 inch aluminium foil at around 8cm. The voltage of the flyback pulse was 290V. I increased it to 400V, but it didn't increase the detection of the aluminium foil. There was little difference in the width of the pulse at TP4 by changing the flyback voltage.
Based on the CRO image, is this defined as a 6us coil or as a 10us coil?

Regards,
Nicolae

[nick_f]

In the second stage, I've put Aluminium shielding over the coil. Before that, I covered the coil with some cheap electrical tape. The change in inductance has been small, now the inductance is 467uH. The capacitance coil-shield is 262pF. Because of it, I had to lower the coil damping resistor from 1150 ohm to 900 ohm. The sensitivity to 1 sq inch Al foil is about 7 cm. In the future I will buy some spiral winding and place the shield on top of that. I am not sure if Aluminium shielding affects too bad the detection of small targets. If I get some better shielding material, I will try.

Regards,
Nicolae

[bbsailor]

In the second stage, I've put Aluminium shielding over the coil. Before that, I covered the coil with some cheap electrical tape. The change in inductance has been small, now the inductance is 467uH. The capacitance coil-shield is 262pF. Because of it, I had to lower the coil damping resistor from 1150 ohm to 900 ohm. The sensitivity to 1 sq inch Al foil is about 7 cm. In the future I will buy some spiral winding and place the shield on top of that. I am not sure if Aluminium shielding affects too bad the detection of small targets. If I get some better shielding material, I will try.

Regards,
Nicolae

Nicolae,

Normally when a coil has less capacitance the damping resistance is increased. Conversely, higher coil capacitance requires lower values of damping resistors. Think of it as coil circuit capaitance stores energy and must use lower values of damping resistance to acheve the best damping for early sampling.

Aluminum foil is not a good shield at lower delays as the foil is detected as a target. Try to obtain some Scotch24 mesh shield. This is a tubular material that is 1" wide. Cut it so it is one thickness layer and cut it wide enough to fully go around the thickness of your coil wire bundle that would be somewhere between .125 to .25" in diameter, depending on wire size and spacer thickness. This should drop the coil-to-shield capacitance to about 50 to 60 pf.

If you want shorter delays for detcting small gold, drop the coil inductance to about 300uH to 350uH.

Measure the coil self resonance before and after you put on the shield and post the results.

Use only a coax long enough to reach from the coil to the control box to minimize coax capacitance.

These tips should get you moving in the right direction.

bbsailor

[nick_f]

Hi bbsailor,
You are right, when the coil has less capacitance, the damping resistor is increased. I didn't say differently. In the first step, I found the damping resistor to be 1150 ohm for the coil without shielding (= no shield capacitance at all). After placing the Al shielding (= more capacitance, added 262pF), I had to lower the damping resistor to 900 ohm.
By the way, thanks for the excellent article about making fast coil, I think most of us are using it as a reference when building MD Coils.
I will make a coil with an inductance within 300-350 ohm and I will post the self-resonance values before and after shielding. At this stage I will still be using Al foil, because I can't find a supplier in Australia to sell at a decent price.

Regards,
Nicolae

[nick_f]

Optimal inductance for detecting gold

Nicolae,
If you want shorter delays for detcting small gold, drop the coil inductance to about 300uH to 350uH.
bbsailor

bbsailor,

I am curious why the optimal inductance for detecting small gold is 300uH to 350uH. What happens when the inductance is higher or lower than that? What would be the optimal inductance to detect large gold?

Can you confirm my theory? I suspect it is a matter of compromise, if the inductance is lower, maybe the current through the coil would be too high (transistors required to have high current, but they could have increased capacitance) or the flyback pulse won't have enough amplitude (therefore the detection depth would be shallow). If the inductance is larger, it will also have higher interwinding capacitance and the delay will get worse and coil won't be good at detecting gold.

Regards,
Nicolae

[Carl-NC]

There is no optimal coil inductance, only optimal speed. You simply want the decay of the coil to be faster than the fastest target you are looking for. Speed is largely dominated by the effect of interwinding capacitance, which is why "fast" coils top out at around 300uH.

- Carl

[nick_f]

Speed is largely dominated by the effect of interwinding capacitance, which is why "fast" coils top out at around 300uH.
- Carl

Yes Carl, but I am sure a 100uH or 200uH coil would have even less interwinding capacitance, shouldn't that be even faster? It must be another reason why we can't (or don't want to) reduce the inductance of the coils under a value.

Nicolae

[bbsailor]

There is no optimal coil inductance, only optimal speed. You simply want the decay of the coil to be faster than the fastest target you are looking for. Speed is largely dominated by the effect of interwinding capacitance, which is why "fast" coils top out at around 300uH.

- Carl

Nicolae,

Carl summarized it pretty well.

You are right about PI coil inductance, pulse width, delay, peak coil current and PPS rate being a compromise. I will try to explain using the Hammerhead coil parameters as an example. You can change the values in my examples below to match you final design to see how it works on paper first.

Lets assume that the Hammerhead coil is 500 uH with about 2.5 ohms of total resistance, including the coil wire, MOSFET on resistance and coil cable lead resistance and uses a 680 ohm damping resistor (Rd). Since the maximum Hammerhead Pulse Width is 70uS, the coil current will not get too high.

The coil discharge Time Constant is governed by the coil inductance divided by the value of Rd or in this case 500/680 or 0.735uS. But wait, there is more to this!

Coil Discharge Stage 1: If the MOSFET is conducting, meaning that the flyback voltage is above the voltage rating of the MOSFET, then the flyback peak will appear to have a flat top, thus extending the total coil discharge time by the length of the flat top. The flyback peak voltage gets higher with wider pulse widths and higher inductance coils. This should not be a problem in Carl's Hammerhead design.

Coil Discharge Stage 2: The mono coil is used for both TX and RX. The RX signal amplifier has a 1K ohm input resistor connected to two clamping diodes. When these diodes are conducting above 0.7V the input resistor value is effectivly in parallel with Rd so the coil discharge TC is now 680 ohms in parallel with 1000 ohms or 405 ohms. Now the coil discharge TC is 500/405 or 1.234uS. This is why balanced DD coils that separate TX and RX can sample a little faster (about 2uS to 3uS faster) as there is less voltage on the input of the amplifier stage to clamp and less time for the opamp to come out of saturation.

Coil discharge Stage 3: This stage occurs when the diodes are no longer conducting thus defining the discharge TC below 0.7V as being Rd alone or 680 ohms. Add a little more time to this stage.

Add the time it takes for the NE5534 op amp to come out of saturation based on the power supply rail-to-rail voltage and gain, then you can see that sampling at very low delays can be a challenge. So if you sample at 10uS, a 0.5 gram 30uS total discharge time nugget has lost almost half of it's signal. Higher current pulses take longer to damp and extends the sampling time (longer delays).

All targets have a Time Constant. A 0.5 gram gold nugget has a total discharge time of 30uS. Divide the total discharge time by 5 to get the target TC. This 0.5 gram nugget has a 6uS TC. In order to fully stimulate a target the coil discharge TC should be 5 times less than the target TC.
The 6uS nugget TC should have a coil discharge TC of 6/5 or 1.2uS. If your nugget was any smaller, with a shorter TC, it would be harder to fully stimulate and detect.

Since the value of the damping resistor is governed by the total TX circuit capacitance, you can now appreciate how minimizing the capacitance can help raise the value of Rd to allow the coil to discharge fast enough to stimulate small, low TC targets. This is what fast coils are all about.

Here are some techniques that can be used to speed up the coil.
1. Make the coil in the 300uH to 350 uH inductance range. A 10.5" diameter coil with 19 or 20 turns will put you there.

2. Use stranded but not silver plated wire. You want the coil wire to discharge eddy currents faster than you are sampling. Silver plated wire is too conductive and allows eddy currents to develop in the coil wire. Tin plated is better as it is a little more resistive between the strands and breaks up the eddy currents better.

3. Use a MOSFET with lower COSS as the value of Rd is partly determined by damping the MOSFET output capacitance.

4. Try using a fast recovery, high voltage diode between the MOSFET drain and the coil with a 56K ohm resistor to ground to discharge the high voltage between pulses. This will allow you to increase the value of Rd by one or two hundred ohms which speeds up the coil discharge slightly.

5. Use a coax cable that has a capacitance rating of from 16pf per foot to 25pf per foot. Only use about 32" to minimize capacitance. Typically, higher impedance coax has a lower capacitance rating.

6. Use a shield that is not detected at the minimum delay that you are using. Aluminum foil is detected at low delays. The coil-to-shield capacitance adds approximately 20% of that capacitance to affect lowering the coils self resonant frequency which then lowers the required value of Rd. Distributed capacitance is very hard to calculate but is much easier to measure. Just note the coil self resonant frequency without the shield and then again with the shield and derive the coil capacitance from a wed-based LC resonance calculator. The difference is what the shield added.

Remember, Carl designed the Hamemrhead to have variable almost everything as a learning platform. Typically, commercial detectors target particular PI applications such as nugget hunting, coin hunting, relic hunting, mineral surveying and underwater uses and only predesign coil inductance, resistance, diameter and other operating parameters to focus on that application. When you are thinking about a particular application for the Hammerhead, you must start with the nature of the targets you are seeking such as target size, conductivity, depth and work backwards to optimize the PPS rate, pulse width, battery life, and coil size for your application. Short pulses in the 30uS to 50uS range can be used to detect small gold but it will still detect coins but not as well as using a 200uS to 300uS pulse width.

I hope this answers your questions and allow you to better understand the Hammerhead and PI detectors in general.

Look for a free program on the web called miscEl.zip. This program will help you see the coil charge TC and discharge TC and see the effects of changing the value of Rd on the coil discharge time. It wil be most educational and match my examples above.

bbsailor

[bbsailor]

bbsailor,

I am curious why the optimal inductance for detecting small gold is 300uH to 350uH. What happens when the inductance is higher or lower than that? What would be the optimal inductance to detect large gold?

Can you confirm my theory? I suspect it is a matter of compromise, if the inductance is lower, maybe the current through the coil would be too high (transistors required to have high current, but they could have increased capacitance) or the flyback pulse won't have enough amplitude (therefore the detection depth would be shallow). If the inductance is larger, it will also have higher interwinding capacitance and the delay will get worse and coil won't be good at detecting gold.

Regards,
Nicolae

Nicolae,

The inductance range of 300uH to 350uH for mono coils is a practical value that works for a wide range of targets while keeping the flyback pulse within reason to accomodate common MOSFET voltage values and desired delays.

Also, the number of coil turns is a compromise between the TX pulse strength and the RX coil sensitivity. Smaller coils have more turns and are more sensitive to smaller targets.

The flyback pulse is not what induces the eddy currents into a target. It is the fast turn off of the coil current that builds slowly when the pulse voltage is turned on. But when the current turns off the coil current discharge induces eddy currents in any target near the coil. After the delay, the TX coil becomes the RX coil and listens for the eddy currents decaying in the target.

The current rises by the current charge TC which is the coil inductance divided by the total TX circuit resistance. A 500uH coil that has a 2.5 ohm resistance has a current rise TC of 200uS. So a 70 uS Pulse width will not raise the current too high. The max current is defined by E/R or 12/2.5 or 4.8 amps. But in one TC of a 200uS pulse width the current will only raise to about 63% of 4.8 amps or about 3 amps. With a 70 us pulse width the max current will be a faction of this.

A 300 uH coil that is 5.5 ohms with a 0.5 MOSFET on resistance has a TC of 300/6 or 50 so a 50uS TX Pulse Width raises the current to 63% of 2 amps or 1.26 amps. Another 50uS or a total of 100 uS pulse width raises the current to about 85% of 2 amps or 1.7amps. Add another 50uS or 150 total and the current rises to 95% of 2 amps or 1.9amps. The last two time constants only add about 5% to the current.

When you run the pulse rate up around 2000PPS the time between pulses is 1/2000 or 500uS. If the TX pulse is 50uS long, the duty cycle is 10 percent and consumes about 126ma. The TX pulse rate and pulse width defines the duty cycle and that defines how much power is being drawn from the batteries. How heavy do you want your batteries to be? What weight can you carry? For how long? All of these answers go into the balancing act of designing a PI machine to fit a particular application with resonable performance.

Lower current pulses typically have a higher resistance TX circuits with a resistor in series with the coil to reduce the coil charge TC, turning off the current after about 3 TCs or about 95% of max current. These lower current TX circuits typically operate at higher frequencies in the thousands of PPS (pulses per second) rate while large coils (3 to 6 ft diameter or square) operating with several amps of coil current operate in the low hundreds of PPS TX pulse rate.

The pulse rate also defines how fast or slow you swing the coil without missing a target. Large coils move more slowly while small coils typically use a higher PPS rate because they move faster over the ground.

Larger coils tend to operate at longer delays because the coils have a larger circumference with a higher coil-to-shield capacitance and with this these coils typically use a 7 ft long cable to connect to the PI control box. many large coils operare with delays beyong about 30uS and do not need shields at these longer delays. Any shileding helps to quiet the coils from external noise but adds more capacitance and this requires sampling at later times. The largest practical mono coil in the 300 to 350 uH inductance range is an actual 11" to 11.3" coil in a 12" housing operating at a 10uS delay.

When detecting for gold, you want to find both small pieces as well as large pieces. You may need to change coils and cover the same ground a second time for both the small and large size nuggets. However, the larger pieces will be fewer and farther between!

Designing a PI machine is a real balancing act!

bbsailor

[nick_f]

Dear bbsailor,
Thanks again for the very informative two-part tutorial. I have to read it a few times to sink in better all the information presented, it is extremely useful for people who are interested in the subject. I will add some questions after I will make sure you haven't already covered that topic.

Best regards,
Nicolae

[Carl-NC]

Yes Carl, but I am sure a 100uH or 200uH coil would have even less interwinding capacitance, shouldn't that be even faster? It must be another reason why we can't (or don't want to) reduce the inductance of the coils under a value.

Nicolae

Lower inductance = less windings = weaker B field. The B field strength increases proportionally with number of windings, but coil "speed" decreases with number of windings. So you want the highest inductance you can get and still have a fast enough coil for the fastest target you are looking for.

If I were designing to detect tiny gold nuggets, I would make a low inductance super-fast coil. But if I only want large deep meteorites, I might go for a higher inductance coil that is slower.

- Carl

[nick_f]

Thanks Carl, now everything makes sense to me.

[nick_f]

Hi All,
I found some interesting discussion between Eric Foster and Dave Johnson, at http://www.findmall.com/read.php?34,129541

Regards,
Nicolae

[bbsailor]

Hi All,
I found some interesting discussion between Eric Foster and Dave Johnson, at http://www.findmall.com/read.php?34,129541

Regards,
Nicolae

Nicolae,

If you want a good education about PI detectors, use the search feature on the Findmall PI forum. Make sure you select "all dates" for the PI technology forum. Consider searching on the following words or phrases posted by "Eric Foster" to get you started. Search this web site also for the same words and phrases.

Pulse Width
Time Constant
Coil Inductance
Sensitivity
Pulses Per Second
TX Frequency
DD Coil
Mono Coil
Battery
Delay
Coil Size
Coil Diameter
Coil Resistance
Series Resistor
Peak Current
Noise
Gain
B Field
Inductance
Dielectric
Dielectric Constant
MOSFET
Coil Shield
Shield Spacer
High Power
Low Power
Sample Width
Sample Window
Second Delay
Flyback
Flyback Pulse
Eddy Current
Eddy Currents
Target Time Constant
Target TC
Coax Cable
Coax Capacitance

When you read something that interests you, look for other words and phrases then search on those. There is a wealth of technical information on Eric's Findmall web site and Carl's Geotech1 web site. You must become activly involved to extract the information and connect the dots yourself.

Go to the web and search on "inductor time constant" to fully understand how and why current lags voltage in an inductor. This is fundamental to understanding the other stuff.

Reread Carl's Hammerhead construction article a few more times and make your own list of additional words and phrases that you need to better understand.

Get a notebook and collect the following information about specific PI model designs.

Coil Inductance
Coil Resistance
Coil Series Resistance
Total TX Circuit Resistance
Battery size
Battery life
Peak Coil current
PPS
Pulse Width Range

I spent many hours disecting the discussions on Eric's and Carl's Geotech1 web sites to attempt to understand how all these terms and phrases connect to each other. Learning is all about having the right context upon which to hang and then connect new facts. By building the Hammerhead, you have demonstrated an interest in the technology, now you need to use your interest to fully stimulate what you need to know. Learning is easier when you have a reason to learn.

Read the many patents that are on Carl's web site or if not posted yet, note the patent number and go to Google Patents then download and read them. You will soon see how all of this stuff comes to knit itself together.
Carl V. Nelson has some interesting patents that you should read after building a good background. You will also see how Carl's Hammerhead design is a synthesis of many commercial designs with common circuit fragments such as the RX signal amplifier, integration stage, first delay, second delay, NE555 timer, charge pump and MOSFET coil driver.

This should get you moving in the right direction and be very productive.

bbsailor

[nick_f]

Dear bbsailor,
I am reading a bit at a time from what you wrote and it all makes sense. I am surprised to discover I had quite a different view (not too accurate) in a few instances. For example, I didn't think that having more resistance in the Tx circuit will do any good, but that is not true. As R directly affects the TC, the higher R is, the shorter TC is. At the moment, what I learnt about TC is that it affects the battery life. I wonder what would happen if we would use a constant current source to charge the coil (a constant current source has internal resistance very high, and based on this, TC = L/R would be very low).
I can see the advantages of using a DD coil. How about using a large TX coil with a diameter D and four Rx coils inside it, each with a diameter of D/2? The Tx coils would have more turns (= more sensitive), and having 4 of them, would not decrease the scanning surface. I consider the disadvantage of concentrical coils is the reduction of scanned area (despite better pinpointing).
For people who want to know more about time constants, there is plenty to read here: http://en.wikipedia.org/wiki/Time_constant
Now I understand what are you talking with all that 63% and the rest of it

Regards,
Nicolae