damping resistor

[Waikiki_Sweep]

When I see the comment "If you slightly under-damp the coil, then sensitivity is improved" is there a percentage of the damping resistance that is typical to run in the under-damped condition? For example if a coil requires 1000 ohms for critical damping to be met then is the under-damped condition perhaps 10% more than 1000 ohms or 1100 ohms? Or is it more like 2% or 1020 ohms? Anyone have an idea on this? I know it probably depends on individual coils but I'm just trying to get an idea of how much more resistance under-damping might be.

Regards,

Dan

Test it with osciloscope. Presence of target changing that shape of underdumped decay. Probably ground will change it too. So your underdamped in air coil may be will work as regularly dumped in presence of target or real ground.
Too much work to test because you need that oscilloscope to pull outdoor or box with soil or salt water at home.

[baum7154]

I actually saw the effect of ground proximity when originally setting coil damping on my detector last summer. The detector seemed to give best sensitivity to a 1/2" X 1/2" .001 foil target on wet ground at a damping resistor value of 980 ohms. However when the detector was raised away from ground it would not operate in open air at a '10' Guard Interval setting. At that time I asked the question about if there is another procedure for setting coil damping not in open air but in proximity to the ground because of it's effect on the coil. As a result the damping was raised to 1110 ohms until this weekend when it was raised to 1260 ohms.

Dan

[Waikiki_Sweep]

I actually saw the effect of ground proximity when originally setting coil damping on my detector last summer. The detector seemed to give best sensitivity to a 1/2" X 1/2" .001 foil target on wet ground at a damping resistor value of 980 ohms. However when the detector was raised away from ground it would not operate in open air at a '10' Guard Interval setting. At that time I asked the question about if there is another procedure for setting coil damping not in open air but in proximity to the ground because of it's effect on the coil. As a result the damping was raised to 1110 ohms until this weekend when it was raised to 1260 ohms.

Dan

It is very good results! As dumping raising sensitivity improving greatly.
Only a problem is how to operate with it on the field.
No automatic dumping correction will be expected in Chance so may be a pot that will change dumping + - 100 Ohms
to adjust it in fields will help to get more depth. Sure it is not easy to operate but after some time probably
it is possible to remember what dumping was the best for particular ground conditions.

Also a small pocket digital oscilloscope ($66 on eBay) may be installed into detector case to watch decay curve in fields.



Why not?

Display Color 65K
Display 2.8" Color TFT LCD
Display Resolution 320×240
Analog bandwidth 0 - 1MHz
Max sample rate 1Msps 12Bits
Sample memory depth 4096 Point
Horizontal sensitivity 1uS/Div～10S/Div (1-2-5 Step)
Horizontal position adjustable with indicator
Vertical sensitivity 10mV/Div～10V/Div (with ×1 probe)
0.5V/Div～100V/Div (with ×10 probe)
Vertical position adjustable with indicator
Input impedance >500KΩ
Max input voltage 80Vpp (by ×1 probe)
Coupling DC
Trig modes Auto, Norma, Single, None and Scan
Functionalities: Automatic measurement: frequency, cycle, duty, Vpp, Vram, Vavg and DC voltage
Precise vertical measurement with markers
Precise horizontal measurement with markers
Rising/falling edge trigger
Trig level adjustable with indicator
Trig sensitivity adjustable with indicator
Hold/run feature
Test signal Built-in 10Hz～1MHz (1-2-5 Step)
Power supply 3.7V Chargeable Lithium battery / USB
Dimension (w/o probe) 105mm X 53mm X 8mm
Waveform storage SD card PC connection via USB as SD card reader Upgrade by boot loader via USB

[Tepco]

This trick with underdamped coil works well on analog detectors, processing signal by integration. Chance is different, completely digital, it uses ADC with sample&hold, no integration takes place. When conversion command arrive, chip will immediately store current voltage at input, then convert it to digital and send to uC. In this case, underdamping wont work, detector expect “by the book” damped waveform. I suppose coil is actually well damped or even underdamped, this coil type when unshielded can be extremely fast, used in analog detector can probably go below 10uS. Proximity of ground will somewhat change setting for unshielded coil due to capacitive effect, this is normal. Interestingly, Chance can work perfectly in salt water with shielded coil, possibly even with unshielded, (using some sort of soil\water switch to change damping resistor value). Due to nature of software used to process decay signal, it just reject any response below certain TC and happily ignore salt water influence.

[Waikiki_Sweep]

Interestingly, Chance can work perfectly in salt water with shielded coil, possibly even with unshielded, (using some sort of soil\water switch to change damping resistor value). Due to nature of software used to process decay signal, it just reject any response below certain TC and happily ignore salt water influence.

That is good news! I am waiting for a Chance PCB that has been sent to me already, to build a Chance just to use it in salt water.
Only problem I expect that Chance needs a very low capacitance coil. So coil have to be bulky to has a space between wires.
If so it will be crazy bad to move coil in water because of high drag.
Even extra spacer between shield and coil will make that drag.
If there is no shield so salt water acting like a shield delivering that extra coil to shield capacitance and making coil slow.
I will try flat spiral to make it in more hydrodynamic shape.
Anyway I have to try!

[baum7154]

Good luck on your CHANCE PI build! If I can help please let me know.

Dan

[baum7154]

This trick with underdamped coil works well on analog detectors, processing signal by integration. Chance is different, completely digital, it uses ADC with sample&hold, no integration takes place. When conversion command arrive, chip will immediately store current voltage at input, then convert it to digital and send to uC. In this case, underdamping wont work, detector expect “by the book” damped waveform. I suppose coil is actually well damped or even underdamped, this coil type when unshielded can be extremely fast, used in analog detector can probably go below 10uS. Proximity of ground will somewhat change setting for unshielded coil due to capacitive effect, this is normal. Interestingly, Chance can work perfectly in salt water with shielded coil, possibly even with unshielded, (using some sort of soil\water switch to change damping resistor value). Due to nature of software used to process decay signal, it just reject any response below certain TC and happily ignore salt water influence.

A comment on the CHANCE PI designer's original spec of a 390 ohm damping resistor...In my experience this value was far too low to be an effective value for both the original coil of 400uh and 1.7 ohm and for any other coil I have tried on the CHANCE. As you say this detector runs best with a 'by the book' damped coil and 390 ohms isn't even in the ball park.

Question: What Guard Interval setting would you expect to use for a 15us or slightly higher value for use in saltwater? Has anyone mapped the GUARD INTERVAL settings to microseconds? If so how do they correspond?

Thanks,

Dan

[green]

[ Has anyone mapped the GUARD INTERVAL settings to microseconds? If so how do they correspond?] CHANCE PI

Don't know the answer, but If you have a scope I think you should be able to plot it. Measure the A-D MCP 3201 pin 5 with one channel, Measure or trigger the scope across the amplifier input diodes D7 D8 with another channel. The diodes should go negative at flyback. Pin 5 goes to ground when taking a reading. Have you tried measuring resonance with a scope across the input diodes with R damping disconnected? I think it should work.

[Tepco]

A comment on the CHANCE PI designer's original spec of a 390 ohm damping resistor...In my experience this value was far too low to be an effective value for both the original coil of 400uh and 1.7 ohm and for any other coil I have tried on the CHANCE. As you say this detector runs best with a 'by the book' damped coil and 390 ohms isn't even in the ball park.

Question: What Guard Interval setting would you expect to use for a 15us or slightly higher value for use in saltwater? Has anyone mapped the GUARD INTERVAL settings to microseconds? If so how do they correspond?

Thanks,

Dan

I will try to answer this “from memory”, I don't have assembled detector anymore, this was time ago... (I have all parts and board, will make another at first opportunity). Original 390 Ohm resistor is bit too low, even for original coil, so coil is bit overdamped, intentionally. In earlier software, delay was set fixed to around 17uS, and unshielded coil spec. and resistor value was quite right for this setting. Coil can be faster, using different value resistor, depending on detector design and pulse parameters. Chance power stage is specific, cannot be compared with “classic” PI design directly. There is very significant difference between old and newer version of software, different pulse sequence etc. In older software, very few samples are taken, and everything done “as per Minelab” approach, one long pulse, sample before (EF) and after, then, in another cycle, burst of shorter pulses, without flyback release Two fets are used, one to effectively short circuit the coil and prevent flyback between short pulses, this happens only at the end of sequence, so coil is “charged” with same energy for both long pulse and burst. In newer version, “burst” consist of slightly more pulses of unequal length, and more samples, taken after different period of time after them (no idea what software does with this). I measured ,with scope, shortest period , closer to TX end and ADC sample command from MCU, as far I can remember, at guard setting 30 it is still 17us, at 10 around or somewhat less than 12uS. There is no need to increase guard interval (to get 15 or so uS) for salt water if coil can be made fast enough. Software reject very fast TC response, due to some reason, and completely ignore salt water influence. My friend tested one of my builds successfully in slat lake of Balaton, Hungary, having salinity greater than in ocean water, (and get fined, naturally), everything worked perfectly. Only trick is to build properly shielded coil capable for fast enough sampling. I managed to find some old backup of scope pictures from v.8 software, can post if this can be of any interest, unfortunately, I never recorded operation of new software version.

[baum7154]

Thanks TEPCO for your response. It would be good to know how V1.2.1 actually is programmed as that is what I am running now. I suppose that all we can do is try to document what we see with a scope and this is the drawback of not knowing the source code. It is good that CHANCE with V1.2.1 does see 4.5 & 8 grain pieces of gold, and somehow I'd like to determine exactly what delays are represented by the Guard Interval steps from 10 through 50.

Thanks again

Dan

[baum7154]

[ Has anyone mapped the GUARD INTERVAL settings to microseconds? If so how do they correspond?] CHANCE PI

Don't know the answer, but If you have a scope I think you should be able to plot it. Measure the A-D MCP 3201 pin 5 with one channel, Measure or trigger the scope across the amplifier input diodes D7 D8 with another channel. The diodes should go negative at flyback. Pin 5 goes to ground when taking a reading. Have you tried measuring resonance with a scope across the input diodes with R damping disconnected? I think it should work.

Yes I do have a dual trace Tektronix 7603 scope with 7A18, 7A26, & 7B53A plug-ins. I have not yet tried to measure resonance with the Rd disconnected but will soon. Also will try to plot the GI vs Delay and post it on the Forum.

Thanks for the info!

Dan

[baum7154]

Yes I do have a dual trace Tektronix 7603 scope with 7A18, 7A26, & 7B53A plug-ins. I have not yet tried to measure resonance with the Rd disconnected but will soon. Also will try to plot the GI vs Delay and post it on the Forum.

Thanks for the info!

Dan

Regarding measuring the resonance across the input diodes I think the capacitance of the scope probes would interfere with the measurement so I think it is better to go to the output of the op amp to isolate the probe from the coil. Does that make sense?

[Tepco]

Better disconnect coil completely from the circuit and measure it separately. Measurement of resonant frequency using detector power stage and disconnected damping resistor may lead to significant error, due to specific operating mode of TX circuit, and unpredictable behavior of transistor parasitic capacitance, effect of clamping diodes etc. This can form highly nonlinear circuit, influenced by detector switching waveform and other factors. Probe capacitance is very low, it will not influence result significantly, just use 10-100K resistance in series with signal source and probe parallel to coil to find resonant peak and calculate.

[Waikiki_Sweep]

Also depending of what you need and what you are measuring for different sets you will have different capacitance and resonant frequencies:

1. coil capacitance
2. coil capacitance + cable capacitance
3. coil capacitance + cable capacitance + TX driver capacitance

Probably you need all components together to measure capacitance and get proper dumping resistor value for whole schematic, not only for coil.

And shield will increase capacitance if you have shielded your coil. But if coil is not shielded ground will act like capacitor a little bit and especially water do this significantly.

[green]

[Regarding measuring the resonance across the input diodes I think the capacitance of the scope probes would interfere with the measurement so I think it is better to go to the output of the op amp to isolate the probe from the coil. Does that make sense?]

I think you will get the same answer either location. Measuring resonance in circuit includes the added driver capacitance and should measure lower than coil resonance. I think this is the frequency you are trying to damp. Is the resonance what you were trying for when you wound the coil? You still adjust damping for best response. If you can't adjust the delay to the minimum you want, knowing the resonance might explain why.