Chance PI coil...

[homefire]

Yes, you're are probably quite right! I guess I can always cut it off flush with the coil housing and seal it over with glass if it not effective. It would be interesting to see if the un-connected shield still has any effect.

{{{Also, I would be interested to know if you measure the resonant frequency of your coils and, if so, how you do it. I found an earlier post in this section describing an oscillator to test coils and may have a go at building it when time permits.}}}

Just inject a signal from a Function Generator while monitoring the voltage . Sweep the coil until it spikes. Scope is best but can be done with Good DVM.

Unconnected Shield will cause issues.

HF

[Rinkerdink]

Hi guys,

im thinking of using some hook up wire from jaycar and the enamelled copper I already have to make 2 coils to test against each other, the hook up wire is pvc insulated but the insulation is pretty thick relative the the wire diameter, now wondering about the former material, I'm concerned about making a big capacitor depending on what I use, I've got some stiff (ish) card that's used to segregate electronic circuits and is an insulator, not sure what it's called but hopefully you'll know what I'm talking about? Would this be ok?

Cheers

chris

[baum7154]

Yes, you're are probably quite right! I guess I can always cut it off flush with the coil housing and seal it over with glass if it not effective. It would be interesting to see if the un-connected shield still has any effect.

Also, I would be interested to know if you measure the resonant frequency of your coils and, if so, how you do it. I found an earlier post in this section describing an oscillator to test coils and may have a go at building it when time permits.

-------------------------------------


I always measure the Self Resonant Frequency (SRF)of my coils as it is very important to know. Actually what I measure is the SRF of my coil and its integral feed line made of a continuation of the coil wire twisted at about 3 turns per inch.

There are two methods:

1. Use a frequency generator and an O'scope to determine the point where the signal peaks as Homefire describes.

OR

2. Use another PI detector to excite your undamped coil and a scope to display the resonant ringing of the test coil. You can determine pretty well the SRF of the coil/feedline using the scope time base i.e. if one full cycle of the ringing frequency is 1us then the SRF is 1mHz (1/1us)

Dan

[baum7154]

Hi guys,

im thinking of using some hook up wire from jaycar and the enamelled copper I already have to make 2 coils to test against each other, the hook up wire is pvc insulated but the insulation is pretty thick relative the the wire diameter, now wondering about the former material, I'm concerned about making a big capacitor depending on what I use, I've got some stiff (ish) card that's used to segregate electronic circuits and is an insulator, not sure what it's called but hopefully you'll know what I'm talking about? Would this be ok?

Cheers

chris

-------------------------------------------------------

Chris,

Your PVC insulated wire should be considerably faster than the enameled wire of the same gauge and coil size due to the extremely thin enamel insulation that doesn't give much inter-wire separation. It is this lack of separation and dielectric constant that raises capacitance in a coil, and kills coil speed.

Any good dielectric material can be used as a coil former. I my development of 3DSS coils I often used 1/8" masonite fiberboard for prototypes because it is easy to fabricate. For durability in the field I use .093" Lexan or polycarbonate. Material thickness should about .1" in most cases with slot widths about the same as the coil former material thickness. This will result in the wires crossing through the slots at 90 degrees to the previous wire in the slot and this minimizes parallelism of the wires and hence inter-wire capacitance.

One note: The capacitance we talk about is distributed capacitance within the coil and cannot be measured directly as capacitance. It can be calculated from the measured coil inductance and the measured SRF ( Self Resonant Frequency).

Best of luck

Dan

[green]

-------------------------------------


I always measure the Self Resonant Frequency (SRF)of my coils as it is very important to know. Actually what I measure is the SRF of my coil and its integral feed line made of a continuation of the coil wire twisted at about 3 turns per inch.

There are two methods:

1. Use a frequency generator and an O'scope to determine the point where the signal peaks as Homefire describes.

OR

2. Use another PI detector to excite your undamped coil and a scope to display the resonant ringing of the test coil. You can determine pretty well the SRF of the coil/feedline using the scope time base i.e. if one full cycle of the ringing frequency is 1us then the SRF is 1mHz (1/1us)

Dan

I use method (2). Usually connect scope probe to PI bench circuit amplifier out exciting the test coil. A test with the scope probe connected to some different locations on the test coil to show effect on measuring resonance. Test coil, 8 inch diameter, graphite shielded, 31 inches twisted pair lead, 285uH. Have tried laying probe next to test coil, connecting scope probe and ground lead to twisted pair lead wire insulation looks a lot better.

[Rinkerdink]

Hi Dan,

thanks for the reply, as with most of this stuff, the answer to one questions leads to 10 other questions lol. I had a first try with enamelled copper and ultimately failed but was a useful experience.

ill keep at it.

cheers

chris

[Goaty]

Thanks to everyone (all the gurus) for their suggestions for measuring PI coil SRF. I agree that it is an important parameter to quantify and document along with DC resistance, inductance and wire characteristics (stranded/solid/litz, insulation type and thickness and conductor cross section area so that other coil builders can compare their efforts.
Where the difficulty arises, especially for new constructors like me, is with the variety of techniques and equipment used to measure it and the potential for error and variation in the results which can make comparisons meaningless.
For example, the feedline type and length can make a large difference to the measured SRF and many constructors include it as it is relevant to what the control circuit 'sees'. Also, the feedline is often made as a continuation of the coil windings as a twisted pair to reduce capacitance and avoid joins at the coil so it would be difficult for them to quote a figure for just the coil alone.
Another problem which gets in the way of uniform measurement is the complexity and expense of the equipment required compared to the other coil measurements and errors can creep in if allowance is not made for things like scope probe lead capacitance in the function generator/scope technique.
And lastly let's not forget the effect of shielding.
So, after all this rambling, I guess what I am getting at is the need for a list of these methods in one place on the forum and their strengths and weaknesses so they can be easily referred to when someone quotes SRF for their new coil.

[green]

Thanks to everyone (all the gurus) for their suggestions for measuring PI coil SRF. I agree that it is an important parameter to quantify and document along with DC resistance, inductance and wire characteristics (stranded/solid/litz, insulation type and thickness and conductor cross section area so that other coil builders can compare their efforts.
Where the difficulty arises, especially for new constructors like me, is with the variety of techniques and equipment used to measure it and the potential for error and variation in the results which can make comparisons meaningless.
For example, the feedline type and length can make a large difference to the measured SRF and many constructors include it as it is relevant to what the control circuit 'sees'. Also, the feedline is often made as a continuation of the coil windings as a twisted pair to reduce capacitance and avoid joins at the coil so it would be difficult for them to quote a figure for just the coil alone.
Another problem which gets in the way of uniform measurement is the complexity and expense of the equipment required compared to the other coil measurements and errors can creep in if allowance is not made for things like scope probe lead capacitance in the function generator/scope technique.
And lastly let's not forget the effect of shielding.
So, after all this rambling, I guess what I am getting at is the need for a list of these methods in one place on the forum and their strengths and weaknesses so they can be easily referred to when someone quotes SRF for their new coil.

I don't know how anyone could repair or experiment without at least an oscilloscope and multi meter with capacitance measurement. Tried documenting a coil build this morning. I've been building IB coils so when I added the shield I grounded the shield only so all the numbers are without the coil grounded except when the scope is connected to the coil wires. After charting the data thought I should have grounded both the shield and one side of the coil for a mono coil. Maybe should have for an IB Rx coil. Should have for an IB Tx coil. My scope displays frequency. Recorded cycles/10div and time/10div to calculate frequency to compare with frequency reading if the scope didn't display frequency. Adjusted time base to get 10+- 5 cycles for the 10 div. Coil inductance is calculated from resonance and added capacitance. Some observations. Connecting the scope to the bare coil wires effects the resonance for a fast coil. Connecting the scope to the coil wire insulation doesn't. Not a good clean signal but usable. Wasn't expecting the not twisted lead wires to effect resonance as much as they did. Not sure how important knowing how fast the coil is but I think it's interesting knowing what effect shielding, adding the lead wires and others has on coil speed. Don't have a signal generator to compare procedure. Instrument calibration would effect comparing data person to person. Each time I try some of these PI measurements I see something I should have done different. I agree it would be nice to have a test procedure to follow. Amplifier out is the signal from the exciting PI.

[baum7154]

Thanks to everyone (all the gurus) for their suggestions for measuring PI coil SRF. I agree that it is an important parameter to quantify and document along with DC resistance, inductance and wire characteristics (stranded/solid/litz, insulation type and thickness and conductor cross section area so that other coil builders can compare their efforts.
Where the difficulty arises, especially for new constructors like me, is with the variety of techniques and equipment used to measure it and the potential for error and variation in the results which can make comparisons meaningless.
For example, the feedline type and length can make a large difference to the measured SRF and many constructors include it as it is relevant to what the control circuit 'sees'. Also, the feedline is often made as a continuation of the coil windings as a twisted pair to reduce capacitance and avoid joins at the coil so it would be difficult for them to quote a figure for just the coil alone.
Another problem which gets in the way of uniform measurement is the complexity and expense of the equipment required compared to the other coil measurements and errors can creep in if allowance is not made for things like scope probe lead capacitance in the function generator/scope technique.
And lastly let's not forget the effect of shielding.
So, after all this rambling, I guess what I am getting at is the need for a list of these methods in one place on the forum and their strengths and weaknesses so they can be easily referred to when someone quotes SRF for their new coil.

____-------------------------------------------------------------

Regarding the capacitance contributed by a given length of twisted pair...I have measured a length of 24awg stranded, 600 volt Teflon insulated twisted pair and it comes to about 1pf per inch of length. From this procedure you can approximate the capacitance added by your feed line if you have a capacitance meter. Just leave the other end of the twisted pair open during measurement.

[garyq]

Aluminium Foil Test

Tested detector with 1110 ohm coil damping on 1/2' target tonight over wet grass and saw no degradation in signal strength from last night's test over wet grass. Got the same 8 signal strength units at maximum proximity to target. Coil is 8" diameter.

Other targets were tested in air in the basement as follows:
TARGET DISTANCE
1 sq in x .001" thick aluminum foil 9.5"

1/2 sq in Aluminum foil 8"

1/4 sq in Alum foil 7"

1 cm sq Alum foil 4"

Hello baum7154,

In regards to your Alum foil tests that I have included in my quote from your post#23 test results I am confused in regards to the 1 cm sq Alum foil result at 4" compared to the 1/4 sq in result at 7".

The reason for my confusion is 1/4 sq in (which is 6 mm sq) produced more depth at 7" then the larger 1 cm sq (which is 10 mm sq) at 4"

Therefore I assume you have made an error for the size of the sq Alum foil piece at the 4" depth?

Did you mean to say 1/16 sq in (which is 1.5 mm sq) Alum foil 4"?

Thanks in advance.

[baum7154]

Hi Garyq,

1/4" square is as you say '6mm square' (actually 6.35 mm)...However '1/4 square inch' is 0.5" square or 12.7mm square, much larger.

Hope this clears the confusion.


Dan

[Goaty]

Some more progress photos on my 3DSS coil build.

The foam layers were wrapped with a layer of paper (masking) tape



Then a layer of glass cloth. I used strips cut from some 4oz offcuts. I wasn't too fussy about the width, wanting to experiment a bit to see what was most effective. I used a 50% overlap so I effectively ended up with a double thickness all-over. A small piece of masking tape was used to hold the start of each length so it did not slip while winding.



The resin used was "Kinetics" epoxy which is a 2:1 resin/hardener normally used for surfboards (I do ding repairs for the local surfers so have plenty on hand). A few points about glassing may help anyone who has not had much experience with it before -
(1) Measure quantities very accurately. I use a 30ml plastic syringe for the resin and a 10ml one for the hardener.
They can be used repeatedly as the epoxy will not attack the plastic and rubber - just don't mix them up!
(2) Check the resin/hardener ratio for your brand of resin. They vary considerably for different brands - check the label! Hardeners are usually available in different setting speeds, unlike polyester resins where setting speed can be varied by the amount of catalyst added. For laminating any speed hardener is OK - I use 'fast' as I am impatient but slow must be used for potting coils to allow the heat to dissipate.
(3) Mix the two components very thoroughly by stirring for several minutes. Don't beat the mixture or it will have lots of air bubbles in it.

To apply the resin I placed the coil on a flat board covered with a piece of building dampcourse membrane (poly-ethylene) and spread it on with a strip of old pc board. Don't bother with a brush - it's too hard to clean.
Just drizzle it on, one side at a time starting with the bottom, and allow it to soak through the cloth with a bit of light squeegeeing with the pc board strip. Work with the lap direction so you can smooth down the edges instead of lifting them. I used 30ml of mixed epoxy to do this size coil.





When the glass is completely 'wet out' leave it to set bottom side down.

When set it will easily peel off the plastic sheet leaving a nice flat bottom.



Leave it a day or two to thoroughly harden before sanding off the rough edges. Trying to sand it 'green' will just clog the abrasive paper.



I use a 4" fibre sanding disk in an angle grinder for 'roughing down' then finish by hand sanding although the angle grinder can be a bit fierce if you are not used to using it.





The coil at this stage weighs 115g and already seems quite strong, light and, hopefully, waterproof.
When time permits I plan to measure its characteristics and try it out on the minipulse I have almost finished. I would also like to experiment with a graphite paint shielding layer but may scrap this if it does not prove to be of any benefit.
I will publish results here as I progress but hopefully the above information may be of use to others experimenting with coil building using epoxy composites.

[Virdi_248]

Hello again. I'm in the process of tuning the coil and I have a question. Is an uncoated coil better in this detector?

In the image you can see a coil similar to mine.

http://www.silverdog.co.uk/images/coil_cpi2.jpg

[baum7154]

If you are planning to use this coil for short sample delays of about 10us or less it needs graphite shielding but will not operate at that short delay. I have tried and abandoned this coil design for detecting small gold nuggets. It is probably fine for coins and rings.

Are you asking if it needs protective coating such as fiberglass I'd say yes.

Dan

[Virdi_248]

I want to get a coil that detects a € 1 coin in 20cm depth and ignores small pieces of iron.
I got a depth of 15cm for a poor € 1 coin.
With iron I have problems to avoid it at 100% in mask 2 and 3. My goal is 20cm and avoid small pieces 100% iron.
My goal is coins and rings, in my country gold in nature almost does not exist.
My question is if it is advisable graphite as shielding considering that the land where I live has a lot of iron.