How to simply make a DD coil?
Manufacturing of coils for IB detectors represents certain complexity for those who make them for the first time. As a rule, people buy coils made at the factory for the certain type of metal detectors. But it is simply enough to make and adjust DD coil in domestic conditions. It is especially useful for not very rich people, from former USSR. Set of circuits of IB metal detectors work, using clock quartz on 32768 Hz. Frequency 8192 Hz (1/4 of 32768 Hz) is the basic frequency for the future coil. Well, let's start manufacturing coils.
In the beginning on a sheet of paper we draw a rectangular of 14.5 cm on 23 cm. After that, from the left top and bottom corner we lay 2.5 cm aside and connect it with a line. We make the same thing with the right top and bottom corners, but lay aside 3 cm. We put a point in the middle of the bottom line and two points to the right and to the left on the distance of 1 cm. We take a suitable plank, impose our sketch and drive in small nails (diameter 2 mm) in all the points mentioned earlier. After that we tear the paper away, bite nail heads off and put cambrics on them (insulation tubules). Cambrics protect the wire from the damage at the corners and allow to remove finished coil easily, by shifting them upwards. That's all - the pattern is ready!!! Check figure 1. Now we draw a direction of winding on our pattern. Then we take multi-colored tubules 1.5 - 2 cm long (insulation from a thin multicore wire). They have two purposes: 1. You will always know where the beginning and the end are (when the coil is ready). 2. They also protect tips from being broken. We take wire d = 0.35mm, pass the first tubule and fix the tip on the bottom nails, reel 80 coils of wire, put on the cambric of other color and fix the end of wire on the nail. Winding needs to be conducted on the middle of the nails (it is easier). We wind the coil with a thick thread (as plaits of a wire are winded) without removing from the pattern. After that we cover the coil with a furniture lacquer (straight sections, not the nails). When the coil is dry, we remove the coil from a pattern, accurately shifting cambrics upwards. We squeeze corners of the coil little bit and cover them with lacquer.
The following stage is a winding of the coil with insulation (I used a scotch tape). Then it is a winding of the RX coil with a foil, TX the coil can be left without foil. Do not forget to leave a break in the screen, in the middle of the top part of the coil, equal 10mm (it is shown by red color on fig. 1). Then it is a winding of the foil with a tinned wire (diameter 0.15-0.25mm). We begin to wind the coil, starting at a place of break of the foil, on two sides (from the break) up to an initial wire of the coil (wire with a red tubule in our case) and braid them together there. This wire with an initial wire will be earthen. The last stage is a winding of the coil with a black (fabric) insulating tape. The reason for using of it is availability and good coupling with epoxid pitch.
Practice has shown that none out of ten equally made coils differed strongly in inductance. Inductance of coils equals 3.680mH + - 0.005mH (at the winding with a wire d = 0.35). The capacity turns out to be about 0.1 uF (100n). In the same way we make two more coils but without screens.
Now I shall explain how to unite both coils in one construction and adjust them. We take a sheet of getinax "Emmas" (3mm) 30 X 27cm. Look fig. 2. In 6 - 8 places we drill thin apertures for fastening of the RX coil. We attach the coil to it using threads and pour the epoxid pitch over the coil. Our RX coil is situated at the bottom of the sheet now (closer to a ground). We expose TX coil so that the centers of branches of two coils were on distance of 1 cm away from each other and fix it temporarily (it is possible to use an adhesive tape). Now we take two pieces of a thin coaxial cable (approximately 1.5 m each) and connect coils. I usually use thick "noodles" (a cable of LF and Video for videorecorders) and if coils are used in parallel insertion, it works very well. Conclusions of the coils, connected to a tinned wire, twisting the coil, we connect to screens of coaxial cables, and distant (on winding) ends of coils - to the central veins of coaxial cables. We fix 4 coins (2 kopecks (USSR) or similar) from above the coil (it is possible to use plasticine); look fig. 2.
Now we proceed to adjustment. The given adjustment is suitable for any MD, but here it is described with the reference to the circuits that work on frequency of 8192 Hz. The third coil without screen is used for this purpose. We establish it on a piece of plywood, solder 0.1 uF and coaxial single-wire cable with screen, and a socket for the board (this coil we shall name "standard"). It is not necessary to adjust it in a resonance, simply put 0.1 uF. We hang the coil "standard" parallel to basic DD coil, the distance ~ 1-1.5 meters (the cable should be 3 meters for convenience). Now we insert coil "standard" in TX plughole, and one of DD coils in RX plughole. We measure amplitude on an output of the first RX amplifier (it can also be done with an oscillograph) - its value is not important, its precise visibility is important at this moment (it should be torn off noise a little bit); you can reach this condition by changing the distance to the coil "standard". We select capacity in DD coil on the maximum amplitude on an output of the first RX amplifier. We insert next DD coil in RX plughole and repeat adjustment on it. Now when all capacities in DD coil are selected, we disconnect the coil "standard" and connect DD coil on its place. We reduce it to "0" a minimum of a signal on an output of the first RX amplifier. If the minimum is more than 1-2 mV we repeat tuning capacities with coil "standard". And thus, repeating stages of adjustment (resonance - minimum) we lead the coil to the minimum indications 0 (avoid using different metals for tuning coils, whenever possible).
One more advice: After the first stage, fix coils (using pitch) constantly, leave only middle parts of coils unfilled (select 0 precisely). It is also possible to adjust coils on other frequencies, but all calculations you will have to do by yourself.
Posted on: February 17, 2015, 08:44:38 am
and i want to share this link for coils and types..
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http://www.gold-prospecting-wa.com/metal-detector-coils.html
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http://www.thunting.com/smf/index.php/topic,59743.msg291869.html#msg291869
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