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Treasure Hunting & Metal Detecting Forum Message Boards - Detector Hunter Whites Minelab Tesoro XP > Tech Talk > Metal Detectors (Moderators: Eugene52, xavier, GoldDigger1950) >

Problem with coil oscillation

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levettp Topic starter

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	Problem with coil oscillation « on: November 05, 2010, 12:52:47 am »

I am experimenting with various coil designs for a PI metal detector but experience a lot of problems with coil oscillation. I use a separate TX and RX coil. My problem lies with oscillations in the TX coil with resulting RX signal oscillations between +/- 0.5V (without any amplification) that continues way beyound 120 micro seconds with a damping resistor value of 680 Ohm.

I then installed a variable resistor as damping resistor on the TX coil but have to turn it down to such a low resistance for oscillations to reduce that I hardly get a signal then (most current running through resistor instead of coil).

Can I use a choke coil to reduce these oscillations?

Is there any theory that I can use that will give me the relationship of oscillation time vs pulse current, coil inductance, coil capacitance & damping resistance?

Any advice will be greatly appreciated!

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xavier

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	Re: Problem with coil oscillation « Reply #1 on: November 05, 2010, 02:36:10 am »

Hi Levettp

This is probably the best article on mono coils for PI detectors.

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http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=/projects/fastcoils/index.dat

enjoy

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	Re: Problem with coil oscillation « Reply #2 on: November 05, 2010, 03:00:59 am »

What type of Power Transistor are you using?
Mosfet? Hexfet? etc....
You might find another transistor with a faster "fall time" might help....... or I might be wrong.

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	Re: Problem with coil oscillation « Reply #3 on: November 05, 2010, 03:06:27 am »

Thanks Xavier

It is a great article!

Seems to me that the bottom line is that I must reduce capacitance in the total coil setup

Posted on: November 05, 2010, 04:02:52 AM

I'am currently using a PWM chip that is an existing chip on the PIC board I use. Will go check out the specs on this chip. Although "fall time" is important it appears from the article Xavier sent me that the OFF state capacitance of the chip is of great importance and this is what I will investigate first.

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	Re: Problem with coil oscillation « Reply #4 on: November 05, 2010, 08:17:31 am »

In the PI coils, that's the goal. Keep Cap down as low as possible. If you find your RX coil ringing, reduce the Damping Resister.

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	Re: Problem with coil oscillation « Reply #5 on: November 05, 2010, 12:43:56 pm »

Is there a quicker way to measure the capacitance than the resonant frequency method e.g. measuring it with some sort of instrument?

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	Re: Problem with coil oscillation « Reply #6 on: November 05, 2010, 12:58:04 pm »

Have a look at this

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http://www3.telus.net/chemelec/Projects/Reactance/Reactance.htm

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levettp Topic starter

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	Re: Problem with coil oscillation « Reply #7 on: November 06, 2010, 01:33:54 am »

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http://www3.telus.net/chemelec/Projects/Reactance/Reactance.htm

is based on resonant frequency.

I've read that some Fluke can measure coil capacitance but can't get any confirmation of this so far.

#UPDATE# This RCL instrument might do the job?

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http://assets.fluke.com/appnotes/it_products/An_rcl.pdf

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« Last Edit: November 06, 2010, 02:06:43 am by levettp »	Logged

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