Optimum component selection

[bbsailor]

I assumed the other fake opamps were 741s. That's crazy to fake such a cheap opamp.

Carl,

How can metal detector circuit builders ensure that the components they plan to buy are the most advantageous to the final performance? Example: MOSFETS used to drive the TX pulse have an on resistance that affects the total coil current as well as capacitance that affects the damping resistor value.

Thanks

Joseph J. Rogowski

[Carl-NC]

I moved this to a new topic.

You first have to set the design goals. This means determining what total turn-on series resistance you want as well as SRF. You may want a near-zero resistance so that the turn-on current is as close to a linear ramp as possible, ferinstance if you are wanting to use multiple pulse widths to do ground balance. Or, maybe you want a higher resistance to get closer to a flat-top current for a better high conductor response. Same with SRF; we always think that higher (faster) is better and for detecting sub-gram nuggets this may be true, but for a cache hunter it may provide little benefit. And a slower response allows for using a cheaper FET (lower BVDSS), if cost is important.

The overall design goals drive not only the selection of the FET switch but also the design of the coil and pretty much everything else. That's why the first order of business is define what it is you want to do. I started the AMX project with a thread called "Product Definition" in an attempt to nail down the project's goals. Too often people skip this step and just start building a circuit. You can do it that way, but you're far more likely to end up with an underdesigned or overdesigned circuit and using inappropriate parts.

[Detectorist#1]

Hi Carl,
I totally agree with that. The first step is to determine the purpose of the project: size and type of the targets, depth of detection and type of the soil.

[Detectorist#1]

For components. For DIY projects, the price is not very important - these projects are not for mass production. According me, more important for these projects is to use components that surely guaranteed end parameters without complex settings in final assembling. This will discourage the DIY assemblers.