Finishing two 3DSS coils

[6666]

Here you go:
https://www.diyaudioandvideo.com/Cal...uctorDesigner/

I made the coil length very short. Say about 0.25? or something

Thanks

[Roo2]

Hi,
indeed the right value must be found, 470r is a theoretical value. for this I use a potentiometer and resistors to visualize the damping curve on an oscilloscope. Do you have an oscilo? last week, our friend Surfdetector made a good video of it on youtube, just to help beginners!

Hi. I'm about to disconnect one end of R1 to bypass with the adjustable resistance rig described in the video. Can I check with you which end of R1 I should disconnect? I think it?s the end closest to PL4? Correct?

[lamoiz]

Hi. I'm about to disconnect one end of R1 to bypass with the adjustable resistance rig described in the video. Can I check with you which end of R1 I should disconnect? I think it?s the end closest to PL4? Correct?

Hi, I don't know, I put the mpp, REV E, and on my circuit it's R4 ... but yes you have to disconnect on the PL connector side, on the other side it's 0v.but it doesn't really matter...
the one on the left in the photo..470r

[Roo2]

Thanks lamoiz. I received the Bourns pot in the mail today so should be able to get this done soon.

[Roo2]

Hi. I spent a couple of hours at my friends house today with his CRO to try to find the optimum damping resistance. We struggled and didnt get a plot anything like that shown in the YouTube video.. See attached. We didnt understand what we were seeing enough to optimise the resistor. The curve moved as the resistance changed but adjusting it down to no bump didnt yield a good detection distance. We just played with the resistance to get the best depth detection and the photo is what the curve looked like at that point. I think the variable resistance setup explained in the first video might not have gone low enough either. Its minimum was about 200R and we swapped to a small pot that went lower and got a better depth result at about 150R. Not sure if was optimal though.

[Qiaozhi]

Hi. I spent a couple of hours at my friends house today with his CRO to try to find the optimum damping resistance. We struggled and didnt get a plot anything like that shown in the YouTube video.. See attached. We didnt understand what we were seeing enough to optimise the resistor. The curve moved as the resistance changed but adjusting it down to no bump didnt yield a good detection distance. We just played with the resistance to get the best depth detection and the photo is what the curve looked like at that point. I think the variable resistance setup explained in the first video might not have gone low enough either. Its minimum was about 200R and we swapped to a small pot that went lower and got a better depth result at about 150R. Not sure if was optimal though.

Leaving the sampling delay unchanged and simply adjusting the value of the damping resistor may lead you to erroneous results if you're using detection distance as a method of determining that you have the best value. Also, the metal target you are using for testing will also have an affect. Forget about checking the detection depth while make the adjustments. The damping resistor value will be optimal when adjusted for critical damping. From my experience the value of 150R is far too low, as I've never had to use any value below 330R.

[Roo2]

Thanks Qiaozhi. I don’t know what critical damping looks like on a CRO... That’s why I resorted to experimenting with distance when the CRO image we got was nothing like that shown in the YouTube video linked above. Can you describe or point me to the process for the MPP revD?

[green]

Thanks Qiaozhi. I don?t know what critical damping looks like on a CRO... That?s why I resorted to experimenting with distance when the CRO image we got was nothing like that shown in the YouTube video linked above. Can you describe or point me to the process for the MPP revD?

My thoughts. I connect scope probe to hot lead insulation not hot lead. Less effect with a fast coil, probe capacitance can effect damping. Don't know if there is a negative effect probing the insulation.

[Qiaozhi]

Thanks Qiaozhi. I don?t know what critical damping looks like on a CRO... That?s why I resorted to experimenting with distance when the CRO image we got was nothing like that shown in the YouTube video linked above. Can you describe or point me to the process for the MPP revD?

If the coil is underdamped you will see a lot of ringing at the preamp output. Although the underdamped signal will reach 0V quickly, it then oscillates above and below the line before eventually settling out after a long period of time. On the other hand the preamp output will move from saturation to 0V in a smooth exponential curve. However in this case you will have to sample later than is really necessary, potentially missing some low conductivity targets and reducing sensitivity. The "Goldilocks" solution is to have the signal move from saturation to 0V as quickly as possible, but with no ringing evident at the preamp output. It has been claimed that slightly underdamping the signal allows sampling to occur a little sooner thann critical damping, but in my experience this often results in some instability.
To find the optimum damping resistor value, connect the damping resistor tool across the coil and adjust it until critical damping is achieved. Obviously you need an oscilloscope to do this. Then disconnect the damping resistor tool, measure its resistance, and replace it with the nearest standard resistor value.

[Roo2]

Thanks. I disconnected the 470R R1 resistor and replaced it with the variable resistance rig (series with coil). I think what I’m reading is to leave the original 470R R1 in place and add the variable resistance in parallel, across the coil.

I understand critical damping now with the help of those plots thanks. I still don’t understand what the CRO plot we got means.. Those long tails going down.. Are they massive under damping?

[6666]

Thanks. I disconnected the 470R R1 resistor and replaced it with the variable resistance rig (series with coil). I think what I?m reading is to leave the original 470R R1 in place and add the variable resistance in parallel, across the coil.

I understand critical damping now with the help of those plots thanks. I still don?t understand what the CRO plot we got means.. Those long tails going down.. Are they massive under damping?

You remove the fixed 470 ohm resistor, replace it with the variable network, adjust the net work for critical damping, remove the net work, replace with fixed resistors to match net work value.

As a side note I have always found it difficult to find the correct damping value for a a coil when useing a single pre amp with a - inverting input , like the mpp always gives a broad mushy response.
What I do now is use a preamp with a + input such as the surf pi, and can get the damping spot on very easily , and I just use that value for that coil, no matter what the preamp.
Dont know how other people are getting exact damping resistors with a - input preamp.

[Roo2]

Thanks 6666.

Hi Qiaozhi - Should I kick off a new thread in the MPP Project area to ask about how to find the R1 value for coil critical damping?

[Roo2]

I just found this posted by Qiaozhi in the Surf Pi project area. Across the coil as described earlier. Is this the same as replacing R1 with it?

[6666]

I just found this posted by Qiaozhi in the Surf Pi project area. Across the coil as described earlier. Is this the same as replacing R1 with it?

Thats the basic network to replace the damping resistor, adjust for critical damping then replace with fixed resistor for the correct value.

[Roo2]

Yep. That?s exactly the variable resistance rig I put into the R1 position but no amount of adjustment gave us a plot remotely resembling a critical damping curve. This is what the CRO looked like when it was detecting a set of pliers well at a distance of about a foot.