TGSL Experiments

[golfnut]

For Q

Hi for Q, simply.

Q= wL/r

w=2 *PI *freq


r in ohms

so if you got a 5ohm coil, L is 6mH, frequency of 14kHz

Q= 2* 3.14 *14x10^3 * 6x10^-3 / 10 (Q has no units)

Steve

[salih]

Simon,

The latest noise measurements of Pin 6 on my LM308s.. Far away from the house, the only local source of noise would be my scope, and one incandecent light bulb.

U104, about .02v p-p
U105, about .01v p-p

Measurements are somewhat subjective.. but my best estimate is above.
The noise seemed to be random, nothing to sync on.
Offset was ever so slightly positive, maybe .005v or less.
Results were reproducable.. In fact, I moded TGSL#2 (the version with TL071s instead of LM308s). Both circuits function identically. Bench test of 40cm+ with no chatter... even two different coils.

Now for the real results: No increase in detection depth on buried objects.. just an impressive bench test. It DOES however improve the audio somewhat on detectable targets, so increasing sensitivity does offer some benefits in my opinion...

Overall, a setting where a 1e coin can be detected in air at 30 to 35cm seems optimum. Nothing I have can do much better in my dirt. Maybe better in sand? Anyway.. Don't have to "scrub" the surface to pick up shallow objects!!

Here is an easy mod I did with TGSL#2. Just clip one wire to the 10k pot and move it to the closest jumper with -5v. (Look close at the 2nd pic.)

Don

hello dfbowers
I swear to God you are a genius.
When I change Wire key sensitivity Became my MD(tgsl) More sensitive to coin 2.2cm. Now I can Detected in air 2.2cm cion on 37cm
Now I am going to search for gold.

Greetings

[simonbaker]

I am pretty sure that at the frequencies we play with, skin effect is not taking place significantly.......

I looked here:-

http://en.wikipedia.org/wiki/Skin_effect

and found this:-

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth may be much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.

I was going to suggest silver wire, but it appears to be the same as copper, but Aluminium is about the same as Gold wire.......you never know!!!

Regards

Andy

I wouldn't think so either; however, because the wires are bundled into a thick "loop", I wonder if they act somewhat like a very large conductor where the inner conductors are experiencing the "skin effect". Litz wire is very specially organized so the bundle of wires weave through different radii -- don't know how well my coils do that.

The skin depth at 14.5 kHz is about 27/1000 inch. My coil bundle has a radius of about 125/1000 inch, so it is plausible that maybe some kind of skin effect is going on, but I don't know.

It's also possible my Q measurements were affected by something else and I didn't get a real estimate. Wish I had one of those "Q" meters.

Regards,

-SB

[ivconic]

Hey Andy. Cast is off and things are still pretty stiff.
I'm thinking about making more coils soon. Do you need any shells? I need one for my Barracuda now.
Guess what.. Detectors find titanium pretty easily

Sheeesh!
Few more steps like that and you will turn to Terminator!
Sorry... not intend to mock!
Wish you fast recovery!
All the best!

[Jerry]

I am pretty sure that at the frequencies we play with, skin effect is not taking place significantly.......

I looked here:-

http://en.wikipedia.org/wiki/Skin_effect

and found this:-

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth may be much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.

I was going to suggest silver wire, but it appears to be the same as copper, but Aluminium is about the same as Gold wire.......you never know!!!

Regards

Andy

Very interesting article Andy. However I think their is a typo or at least something misleading in the way they presented 8.5 mm for the skin depth at 60 hz. I think they may have written micro meters out as mm in the first part of the article which is a whole different thing than Milli meters. There are a few charts later in the article that show that.

I tried cutting and pasting those charts here but it did not work for some reason.

Another table shows how skin effect goes up for a twister pair phone line with
distance. It is interesting that that C stays constant with frequency at the same time the resistance increases.

The only place I have seen or used litz wire is for vlf radio coils where we were trying to get a high Q circuit.

Jerry

[der_fisherman]

Skin effect

Hi der_fisherman --

Yes for sure the inductive/reactive impedance dominates the total impedance of a coil at higher frequencies. However, the coil still has a resistive component and can be modeled as a pure inductor with a series and/or parallel resistance, as well as series and parallel capacitance. The pure resistance determines the Q of the coil which directly determines the amplitude of the voltage or current (depending on how you drive the coil) at resonance. The pure inductor and capacitor determine the resonant frequency. If there was no resistance, the voltage and/or current would go to infinity at resonance.
I was estimating the Q of my coil and it seemed to behave like a pure inductor with series resistor that was about 4 times greater than the DC resistance of the coil. So I was wondering what the physics was that represented the resistance, possibly skin effect, radiation, or perhaps something else.
I figured it was significant because I was using heavy 24 AWG wire hoping to get a high Q when in fact the physics was frustrating my attempt -- although I'm sure I gained some over 30 AWG wire, just not as much as I expected or hoped. This seems to be often the case in this business!
Cheers,
-SB

I am pretty sure that at the frequencies we play with, skin effect is not taking place significantly.......

I looked here:-

http://en.wikipedia.org/wiki/Skin_effect

and found this:-

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth may be much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.

I was going to suggest silver wire, but it appears to be the same as copper, but Aluminium is about the same as Gold wire.......you never know!!!

Regards

Andy

[der_fisherman]

I guess it stays in.. for now

Be thankful, if you ever get accidentally buried under tons of sand or the like, we can search for you with our Golden Sabre DIY metal Detectors!!!!

regards

Andy

[simonbaker]

Did anyone eventually answer your question about resistance? I could not find a direct answer so assuming not.

The answer is that a coil with any form of AC on it does not have resistance, it has impedance.

That is how much impedance/resistance it presents to the AC signal over a DC signal.

Think of the action of a choke, put in a circuit to oppose fluctuations in voltage.

I hope this helps.

Regards

Andy

Hi der_fisherman --

Yes for sure the inductive/reactive impedance dominates the total impedance of a coil at higher frequencies. However, the coil still has a resistive component and can be modeled as a pure inductor with a series and/or parallel resistance, as well as series and parallel capacitance. The pure resistance determines the Q of the coil which directly determines the amplitude of the voltage or current (depending on how you drive the coil) at resonance. The pure inductor and capacitor determine the resonant frequency. If there was no resistance, the voltage and/or current would go to infinity at resonance.

I was estimating the Q of my coil and it seemed to behave like a pure inductor with series resistor that was about 4 times greater than the DC resistance of the coil. So I was wondering what the physics was that represented the resistance, possibly skin effect, radiation, or perhaps something else.

I figured it was significant because I was using heavy 24 AWG wire hoping to get a high Q when in fact the physics was frustrating my attempt -- although I'm sure I gained some over 30 AWG wire, just not as much as I expected or hoped. This seems to be often the case in this business!

Cheers,

-SB

[dfbowers]

Then your next project is to learn how to reject Titanium!!! LOL!!

Will the Titanium eventually be removed, or will you retain it for the rest of your life?

Interesting pictures by the way.....

regards

Andy

I guess it stays in.. for now

[der_fisherman]

Hey Andy. Cast is off and things are still pretty stiff.
I'm thinking about making more coils soon. Do you need any shells? I need one for my Barracuda now.
Guess what.. Detectors find titanium pretty easily

Then your next project is to learn how to reject Titanium!!! LOL!!

Will the Titanium eventually be removed, or will you retain it for the rest of your life?

Interesting pictures by the way.....

regards

Andy

[dfbowers]

Hi Don,

how is your wrist getting on, are you back making coils again? You haven't said anything recently about it.....

I hope its now OK.

Regards

Andy

Hey Andy. Cast is off and things are still pretty stiff.
I'm thinking about making more coils soon. Do you need any shells? I need one for my Barracuda now.
Guess what.. Detectors find titanium pretty easily

[der_fisherman]

Hi Don,

how is your wrist getting on, are you back making coils again? You haven't said anything recently about it.....

I hope its now OK.

Regards

Andy

[der_fisherman]

As mentioned in previous post, my 7 ohm coil seems to act like a 27 ohm coil at resonance (about 13 Khz).

Where does the extra resistance come from? Is it radiative losses? Is it "skin effect"? Seems much more than I expected.

Does anyone have data on coil resistance at different frequencies?

Regards,

-SB

Did anyone eventually answer your question about resistance? I could not find a direct answer so assuming not.

The answer is that a coil with any form of AC on it does not have resistance, it has impedance.

That is how much impedance/resistance it presents to the AC signal over a DC signal.

Think of the action of a choke, put in a circuit to oppose fluctuations in voltage.

I hope this helps.

Regards

Andy

[simonbaker]

We already know that the TGSL oscillator is a low power design but since I started playing with this I always wondered "how low" in comparison to other detectors.

There was a link posted elsewhere in the forum that detailed making a field strength meter so I thought that I would go ahead and make use of some of the authors ideas:

http://www.5volt.eu/archives/20

Well as it turns out, you don't really need the amplifier circuit for making induced test coil voltage measurement as the signal strength is quite large enough to be accurately measured with a scope or DVM. Assuming that my scope is not loading the signal too much the figures should be a good ball-park. I just measured the peak voltages and calculated the RMS values. Then, I made use of the equations in the article to calculate "H" field magnetic strength in "microTeslas" @ 12".

Maybe Aziz or Carl can tell me if my numbers are even close or not.. But at least I can get a relative idea between detectors. Maybe I'm way off!!!

One thing that became immediately obvious is that coil size and shape has a HUGE bearing on "H field" strength at any given distance. So, I tried to group coil sizes and types together just to compare like geometry. The bottom equation shows the relationship between coil size and field strength.
Don

Thanks for putting that together and including the reference equations. Always interesting to see how MDs compare.

It would be interesting to include measurements at, say, 3 inches depth also, although I would recommend a smaller sniffer coil (maybe 3 to 4 inches diameter) just to grab a smaller region of the field to better compare intensity.

-SB

[dfbowers]

TGSL & TX power

We already know that the TGSL oscillator is a low power design but since I started playing with this I always wondered "how low" in comparison to other detectors.

There was a link posted elsewhere in the forum that detailed making a field strength meter so I thought that I would go ahead and make use of some of the authors ideas:

http://www.5volt.eu/archives/20

Well as it turns out, you don't really need the amplifier circuit for making induced test coil voltage measurement as the signal strength is quite large enough to be accurately measured with a scope or DVM. Assuming that my scope is not loading the signal too much the figures should be a good ball-park. I just measured the peak voltages and calculated the RMS values. Then, I made use of the equations in the article to calculate "H" field magnetic strength in "microTeslas" @ 12".

Maybe Aziz or Carl can tell me if my numbers are even close or not.. But at least I can get a relative idea between detectors. Maybe I'm way off!!!

One thing that became immediately obvious is that coil size and shape has a HUGE bearing on "H field" strength at any given distance. So, I tried to group coil sizes and types together just to compare like geometry. The bottom equation shows the relationship between coil size and field strength.
Don