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Problems with search coil 40 cm.

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  • Problems with search coil 40 cm.

    Hello, I built many sandbanks detector coils 21 cm. diameter without any inconvenience. use magnetic wire 0.5 mm. in diameter and 22 turns. put tape around electrical installations. The output of the operational amplifier LM318 pin 6, the curve on the oscilloscope is perfect !!!!. Now comes my problem, now a coil construct 40 cm. in diameter and I use the same magnetic wire of 0.5 mm. diameter, 16 turns and all within a heat shrinkable cable. The problem is that when I look in the oscilloscope and gives me the signal on saturated wave, it is as if in the coil of 21 cm. I have a metal coil playing ,. I am puzzled. Test again with 8 cables UTP cable and make two turns, this time the signal is a bit more like the ideal, without metal wave it is observed without the normal decay but managed to adjust the threshold of sound and has good sensitivity, someone I can say that I am making mistake ?. Another day will show photos. Carlos

    Hola, construi muchos detectores sandbanks con bobinas de 21 cm. de diametro sin ningun inconveniente. utilice alambre magnetico de 0,5 mm. de diametro y 22 giros. coloque alrededor cinta para instalaciones electricas. A la salida del operacional pin 6 del amplificador LM318, la curva en el osciloscopio es perfecta!!!!. Ahora viene mi problema, construyo ahora una bobina de 40 cm. de diametro y utilizo el mismo alambre magnetico de 0,5 mm. de diametro, 16 giros y todo dentro de un cable termo contraible. El problema es que cuando miro en el osciloscopio la señal me da ya sobre saturada la onda, es como si en la bobina de 21 cm. tengo un metal tocando la bobina,. estoy desconcertado. Realice otra prueba con cable UTP de 8 cables y realice dos giros, esta vez la señal es un poco mas parecida a la ideal, sin metal la onda se observa sin el decaimiento normal pero logro ajustar el umbral de sonido y tiene buena sensibilidad, alguien me puede decir que error estoy cometiendo?. Otro dia mostrare fotos. Carlos

  • #2
    A 21cm diameter coil, with 22 turns of 0.5mm magnet wire, should have an inductance of 303uH.
    A 40cm diameter coil, with 16 turns of 0.5mm magnet wire, should have an inductance of 355uH.
    You can use the calculator here -> Coil Calculator

    Have you adjusted the value of the damping resistor to achieve critical damping?

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    • #3
      I think it's very small resistance that attacks the two diodes 1N4148 220 ohms. Be installing a 1K resistor and resistance accords and see the wave damping in the oscilloscope. Carlos

      Creo que es muy pequeña la resistencia que ataca a los dos diodos 1N4148 220 ohms. Instalare una resistencia de 1K y ajustare la resistencia damping y vere la onda en el osciloscopio. Carlos

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      • #4
        Originally posted by Carlos View Post
        I think it's very small resistance that attacks the two diodes 1N4148 220 ohms. Be installing a 1K resistor and resistance accords and see the wave damping in the oscilloscope. Carlos
        That's probably too high a resistance for the SandBanks PI. I would suggest trying either 680R or 470R.

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        • #5
          Hola carlos , has probado hacer alguna bobina grande para el idx?

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          • #6
            Originally posted by hameg_5000 View Post
            Hola carlos , has probado hacer alguna bobina grande para el idx?
            Please read the forum rules -> Basic Rules of the Forums
            and make your posts in English.

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