PI Coil Resistance

[rickodetrader]

Hello All,

I've finally got my Surf PI Pro going! on the bench

Have been trying some different coils I made and fine tuning the damping resistor using a 1K 0.75W potentiometer and attaching CRO probe to output of NE5534. All good.

Now, The ones using a thicker gauge wire (lower resistance) seem to create the classic ring when not properly damped.

Specifically a 300 uH, 0.65 ohm spiral coil and a 88.6 uH 0.65 ohm solenoidal (0.5mm enamelled copper). MOSFET does definitely get hotter!!! so these were run only for a few minutes of testing.

The 326 uH, 4.4 ohm basket weave and 436 uH, 4.13 ohm solenoidal coils created using 0.25mm enamelled copper wire don't "ring".

I suspect I'm missing out on some depth and/or sensitivity

So most of the coils which are built on here seem to be 0.5mm or higher gauge wire so maybe not many of you have seen this.

Are lower resistance coils better (faster) ? (but cause your mosfet to run hotter). A quick look at the inductor Q equation. Q = (inductive reactance)/resistance would indicate so but...

What do you reckon? Yeah, no, maybe??? Am I reading too much into it... again

rickodetrader

[Carl-NC]

Larger wire = higher parasitic capacitance = more ringing. Ringing isn't necessarily bad, as long as it doesn't prolong preamp recovery. That is, if the preamp output is in its valid linear range when you want to sample, it won't matter if there is ringing or not.

Larger wire also reduces R which can result in a higher peak current, but how much depends on the turn-on pulse width vs the turn-on tau. Generally a higher peak current gives a higher flyback voltage, and the MOSFET will get hotter if it's avalanching.

[satdaveuk]

Also takes alot more juice out of your battery so worth checking current draw and way up the pros and cons for a couple of mm depth.
Also bare in mind the you need to check in real conditions as you could have to projects in air tests one catching 25cm the other 30cm then do real tests and they well maybe reversed in depth or often the same.
With the coils you have already made would be good for all of us to take them into the real world and let us know how you get on with them advantages/disadvantages.

Regards

[georgi.g.zhelev]

My experience with PI shows: R no more than 1 ohm !!

[satdaveuk]

My experience with PI shows: R no more than 1 ohm !!

Does that applie to all PI projects? im suprised, what size wire are you using because oviously you need to get the correct inductances which vary from one project to another , well i presume they do, ive not had alot of experience with PI, certainly IB format is very critical with both inductance and resistance.

Regards

[rickodetrader]

Hi Carl and others,

Thank you for explaining. It sounds like the higher current in the heavier gauge wire is causing the greater fly back voltage causing the heat as you say.

The detector does work still as you say because the sample period is obviously not in the ringing zone.

Just need to keep asking questions as searching for the specifics can be kinda tricky if no one else has really come across these things...

[Tec]

Gate resistor and coil resistance

A gate resistor can help.
The gate of the mosfet,..make sure there is a 10 or 20 ohm resistor on the gate.

Also, I don;t know why the obsession with low resistance and high current in the coil.

Rather than thinking of the system as a high-current pulse and a reflection from a target.

I like to think of it as a simultaneous interaction.

The target and the coil are like a transformer.

The target is a small single-turn loop and the coil is a multi turn loop.

Energy is being transferred from the target to the coil as in a step-up transformer.
A tiny voltage in a low resistance, is being converted to a higher voltage in a higher resistance.

There will be maximum energy transfer when there is an optimum resistance in the coil.
I believe.
It is the same as matching an audio amplifier output to speakers. If you simply make the speakers as low resistance as possible and connect them to a 4 ohm amplifier output, there will be mis-match, inneficient power transfer and distortion.

So, I think the coil should have a decent resistance, 4 ohms, maybe even more.
The target and coil will INTERACT more efficiently at a certain sweet spot.
Too little resistance, or two much resistance will mean less power transfer between coil and target, target and coil (all happening simultaneously).

Of course targets are all different.
But, my point is, a mega high current, fast decay, wont necessarily mean a strong signal from a target.

[bklein]

This sweet spot thing seems very real with my gp extreme and I'd like to better understand it and have bench tools to optimize it rather than just make a whole bunch of experimental coils. In another thread we just talked about how on the gp a mono is both the tx coil and rx coil - like on a surf pi. But I would think that one could design the tx coil for tx and have a separate coil for rx - as the DD coil is this way but the rx coils are differentiated. I'm thinking to get my dip meter working at lower freq to maybe help with optimization. That and use a separate rx coil at bench level to optimize tx coil design first to create the highest level of eddy currents in the target. Is this a common or worthwhile design practice? Perhaps things are simpler than they appear and a single tx/rx coil has been proven good to go with.(?)