series resistor in coil on P.I.

[william]

What is the purpose of a series resistor in coil circuit of P.I.? I know to limit current but is there another purpose? Thanks William

[bbsailor]

Re: series resistor in coil on P.I.

See some Eric Foster or Reg posts on the Findmall PI classroom. Search on the words "series resistor", "coil" "coil design", "power". Research these same words on this forum also. This topic was covered pretty well.

I may not do justice to his explaination but, in a nut shell, a series resistor is used to limit the current in the coil which allows for a faster receive sampling.

There are two distinct types of PI TX circuit design; (1) high power/current, low resistance coils and (2) higher resistance coils, including adding series resistance to the coil to reduce the high current back emf.

You can obtain greater sensitivily by high current meaning the "brute force" method but if you want high sensitivity to gold, you can get increase sensitivity to this low conductive target by sampling faster, around 10uS. This 10uS delay is difficult to achieve so creative design parameters are needed, including doing the counter intuitive, increasing of the coil resistance (including adding a series resistor) to allow the receive circuit to sample faster. This also has the practical consequence of reducing the weight of battery capacity to something convenient to carry.

It takes about 64 times more power to get about a 50% improvement in depth. Depending on what you are looking for, there are PI designs (meaning: high power, or fast) that can accomodate you. If you can handle a large battery, the a high current PI may be what you need for the nature of the metal you are seeking.

I'm in the middle of building the Hammerhead PI (on this web site) so this stuff is pretty fresh in my mind. Next week, I may not even know what PI stands for.

bbsailor

[Eric Foster]

Re: series resistor in coil on P.I.

Hi BBSailor and William,

Good summary of the two techniques BBS. The other factor in adding a series resistor, is that it shortens the coil time constant L/R. With a low resistance and high current, you are struggling to use much more than 300uH and shut the field off in time to sample at 15uS. With 20 - 30 ohms series resistance you can go up to 1mH and still sample at 15uS. Part of the resistance can be in the coil, so you can use thin wire and save weight. Although the pulse current is smaller, the higher inductance compensates for loss of signal amplitude and you can run at a considerably higher pulse rate so that you are integrating over a larger number of samples. The net results is that you can have a detector whose transmitter takes 1/10th of the current of a brute force type and still achieve similar ranges on small targets.

Eric.

[william]

Re: series resistor in coil on P.I.

Thanks BBSailor and Eric, I enjoy the forums very much and have learned lots, thanks again. William