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Offline mykaitchTopic starter
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« on: January 12, 2013, 06:10:58 am »
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I have resurrected my DIY Bandido. I have read in many places how to make coils and commonly, it is recommended that the coil be wrapped in tape. DON'T. When you do this the turns are tightly pressed together and the coil reactance changes - sometimes a lot. Try tuning that when covered in tape! I have soaked my latest one in varnish. This holds the turns together and it is easy to gently pull the last turn away for tuning - invariably the frequency will drop. Had a lot of trouble with a Tx bucking coil so I am having a go at the Rx type.
Q ? Anybody know which type the uMaxII uses ?
Q? What batteries/life for the uMaxII ? (I have 6AA and its a bit heavy, thinking of a PP3 which will give around 10 hours with my version. About right ?
Q? Shielding ? I have seen methods that leave a break in the shield so that it does not orm a closed loop. Seems right to me. Answers please Huh?
Finally...many designs show a common Tx/Rx wire, ie ground. The uMaxII has a balanced input (like a low Z mic) so you cannot do that here. Cheesy


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Offline GoldDigger1950
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« Reply #1 on: January 12, 2013, 08:32:12 am »
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Quote:Posted by mykaitch
I have resurrected my DIY Bandido. I have read in many places how to make coils and commonly, it is recommended that the coil be wrapped in tape. DON'T. When you do this the turns are tightly pressed together and the coil reactance changes - sometimes a lot.

That's calculated in. This is, unfortunately, bad advice that you are giving. Taping keeps the coil impedance from changing. The reactance changes with normal operation and is not a measurable quantity since that has to be done in circuit and in operation. Reactance is a real time measurement of the frequency, interwire capacitance and the inductance. The AC current in the circuit has the greatest effect on reactance and thereby has to be measured by theoretical calculations. Also, the movement of the wires in air can cause much more change than taping every could. That's why the wires at taped, to help keep them motionless. When making a coil, the end result should always be to make it waterproof and that requires it to be embedded in plastic or epoxy. Prior to that, taping or tying the coil is the normal practice.

Try to refrain from giving out advice unless you are absolutely certain it is correct advice. Well meaning as it was, it was wrong.

Posted on: January 12, 2013, 04:26:08 PM
Quote:Posted by mykaitch
Shielding ? I have seen methods that leave a break in the shield so that it does not orm a closed loop. Seems right to me. Answers please Huh?
Finally...many designs show a common Tx/Rx wire, ie ground. The uMaxII has a balanced input (like a low Z mic) so you cannot do that here.

Depending on your requirements, shielding can be good or bad. If you use full Faraday shielding, the result is that you can use your machine in areas of high audio range interference like an ultrasonic detection system at a park. Happens a lot these days. The Faraday shield also stabilizes the receiver tuning making up for the transmission losses by a factor of 5 to 1. This is due to the receiver sensitivity being so great. Gains of 100,000 or more in the receiver preamps are common.

There is no common ground in a PI detector since the coil output goes above and below ground in a single coil PI detector. The Faraday shield is commonly grounded to the RX circuit common ground to keep the noise down in the RX side.

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Offline mykaitchTopic starter
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« Reply #2 on: January 12, 2013, 01:22:40 pm »
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Thanks for the reply. Here's a conundrum then; if you tape up the coil you cannot then fine tune it although its easy to mod the osc so it can be adj. If however, you seal the coil with varnish (or shellac as was), the turns will be fixed in place and waterproof and it can still be fine tuned. THEN, you can tape it too if you must since the varnish will set hard - and the turns will not move. And then add expoxy too if you like- I am going to try plastic bumper filler; its cheaper.

'Try to refrain from giving out advice unless you are absolutely certain it is correct advice. Well meaning as it was, it was wrong', no not totally. OK I used the term reactance in the wrong context which ,as we know, is a function of f L and C. (and as such LCorF need to change if reactance is to change).
Criticisism is fine but perhaps a less sharp tone ? Fight







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Offline GoldDigger1950
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« Reply #3 on: January 12, 2013, 02:18:00 pm »
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Quote:Posted by mykaitch
Thanks for the reply. Here's a conundrum then; if you tape up the coil you cannot then fine tune it although its easy to mod the osc so it can be adj. If however, you seal the coil with varnish (or shellac as was), the turns will be fixed in place and waterproof and it can still be fine tuned. THEN, you can tape it too if you must since the varnish will set hard - and the turns will not move. And then add expoxy too if you like- I am going to try plastic bumper filler; its cheaper.

'Try to refrain from giving out advice unless you are absolutely certain it is correct advice. Well meaning as it was, it was wrong', no not totally. OK I used the term reactance in the wrong context which ,as we know, is a function of f L and C. (and as such LCorF need to change if reactance is to change).
Criticisism is fine but perhaps a less sharp tone ?

If you want a less sharp tone, don't be so unequivocal or didactic in your presentation of what you see as correct advice. Take a look at the top of the forum. I'm one of the moderators here. For good reason. Just ask any of the other moderators why I am on the DIY forums as a moderator.

Look, one thing you are forgetting here is that a coil is not a DC device and does not conform to the DC rules. AC rules are different. Your idea is fine for open aire coils but not for use where moisture or water is an issue. Absolutely wrong, in fact. What you have to do is make your design work so that it can be sealed in either epoxy resin or acrylic for stability. Potting compound also works if it is the rigid kind, not the kind that stays soft.

Designing a detector is often done backwards from the coil to the control box. In your case, you built a kit and were given the parameters of the coil. You must have decided to play with the design recommendations or you wouldn't have had issues like you describe. That's one of the drawbacks of DIY building. You must follow the instructions TO THE LETTER or suffer the consequences of a failed project. If the project says to tape the coil, by golly you'd better tape it. Despite common conceptions, a coil is NOT designed to be at resonance with the TX circuitry but tuned to 6db down. You would never, ever in a billion years get a transmitter field from a resonant circuit. You would get NO energy outside of the coil at all. Zero. To get a signal, you need to de-tune the coil from the TX and tune it to the RX. That's the name of the game in metal detectors. The resistance from an imperfectly tuned circuit is what gives the field its size and shape. This is an AC resistance combined with capacitance and inductance then referred to as reactance.

You can take all of your normal formulas and chuck them out the window. You are in a decidedly different design zone here.

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« Last Edit: January 12, 2013, 02:24:45 pm by GoldDigger1950 »
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Offline xavier
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« Reply #4 on: January 12, 2013, 02:36:50 pm »
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GD beat me to it but just for the hell of it here is my reply to you

I don't think that you know the difference between inductance, impedance, reactance or internal capacitance nor do you know how to treat coils. Coil making happens to be the most complex part in metal detector building, it therefor requires a very good understanding of their behavior.  
Quote:Posted by mykaitch
Here's a conundrum then; if you tape up the coil you cannot then fine tune it

You do have a point but there is one thing that you are forgetting, the coil is first made then the electronics are build around it Not the other way round which is exactly what you have to do when you want to clone a detector.  And here is another point for you, you do not have to tape a coil if you are going to epoxy it nor do you have to directly shield it as you can shield the casing and for underwater detectors I would not recommend any shielding.    

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So many questions so little time

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