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Was wondering what effect adding resistance to lower the time constant would have on coil current. Plotted this curve. Not quite what I would have guessed. Thought the curve might be interesting to someone.
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Green,Originally posted by green View Post
Read the following web link to see how current rises according to the coil time constant. http://www.sayedsaad.com/fundmental/...20coil.%20.htm
To find the coil time constant in micro-seconds (uS) do the following: add the DC resistance of the coil wire, the MOSFET 0n-resistance, and any series resistance (coax wire resistance and/or any series resistor). Divide this total resistance into the coil inductance in micro-henries (uH) to find the coil time constant (TC). Example: a 300uH coil that has a total resistance of 5 ohms has a TC of 60uS. This means that in one TC the current rises to 63 percent of max current. The maximum current is the coil total resistance divided into the supply voltage on the coil. A 12V supply will have a 12V divided by 5 ohm maximum current or 2.4 amps, but a 60 uS pulse width (one TC) will raise the current to only about 63 percent of maximum current, a 120 uS pulse width (two TCs) rise to about 85 percent of maximum and a 180 uS pulse width (three TCs) rise to about 97 percent of maximum current.
Longer pulse widths also raise the amplitude of the flyback voltage and require a lower value damping resistor to properly damp. That is why most commercial TX pulse widths are adjustable to only about 10 percent of ther frequency range to eliminate being in sync with either local power frequencies (50Hz or 60Hz) and other harmonics that may cause interference with the PI RX circuit. If you vary the TX frequency too much, you may need to add a variable damping resistor trimmer pot to optimize the damping for the full range of your chosen TX pulse width.
I hope this helps?
bbsailor -
Interesting, if you add resistance the TC gets smaller (faster coil!) but the max current gets smaller too.
RDamp wants to be smaller too. So I wonder how to optimize the coil? Go for max current?Comment
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Silver Dollar,Originally posted by Silver Dollar View Post
A shorter TC does not mean a faster coil but only less time to reach one, two, or three TX pulse width TCs. A faster coil is how fast the coil can change from being a TX coil to being an RX coil and catch the quickly decaying signal from low TC targets like gold, nickels or other small lower TC targets. The value of the damping resistor (DR) determines the turn-off slope or the coil turn-off time constant. A 300uH coil with a 600 ohm (DR) has a 0.5uS turn-off TC. The optimum turn off speed is 5 times faster then the TC of the target you are trying to optimize to detect. Fast coils typically use higher value DRs due to less total capacitance in the TX circuit that consists of: (1) the coil turn-to-turn capacitance, (2) the MOSFET COSS capacitance, (3) the coax cable capacitance, and (4) the shield-to-coil capacitance.
There is no "one size fits all''. You should design and optimize for specific target TCs, battery draw (search time on a full charge) and ground conditions. Wet salt beaches start to react at or near 10uS but the higher dry beaches are usually OK at 10uS or even a little lower. If you think that more power equals more depth, you are right but to double the detection distance requires 64 times more power to achieve by power increases alone. Other techniques use high TX pulse frequencies (shorter TX pulse widths) in the 3,000 PPS range using an integration circuit to increase the RX sensitivity.
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