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HI, I fancied building a metal detector, so I bought a book and by good fortune I bought Inside the Metal Detector, and after reading it, 4 times, and reading on this forum I want to build the Crossbow Classic. An advance build so I decided to start with the Mini pulse Plus from Silverdog. A couple of evenings after receiving it I had it working and detecting metal with a home made coil made from degaussing wire from an old TV. I decided to I needed to know more about the damping resistor to tune the coil, if that is the correct terminology. I thought I had an idea that this resistor was to remove the ringing, as described in the book. Reading the signal on the damping resistor R1 as per step 4 in the build instructions I only had 250v. Reducing the value of R1 reduces the fly back voltage and even as low as 100 ohms no ringing. Taking the value up to 2k ohms increases the fly back voltage up to 400v increasing the value higher has no additional effect. Could anyone let me know if I have the wrong end of the stick. Is this what you would expect, or point me in the direction of the answer. If I have put this post in the wrong place could an administrator please put it in the correct place.
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Step 4 in the Build Document describes how to check the flyback voltage by measuring across the damping resistor with a scope and an x10 probe. This is not the way to adjust the damping resistor value. Of course the flyback voltage will be reduced when you reduce the value of R1, as it is across the coil.Originally posted by wayne,inspain View Post
What you need to do is connect channel one of the scope to TP1 (with reference to TP10), and channel two to TP3. Then use a damping resistor tool (described on page 155 of ITMD Edition 2) in place of the damping resistor, and adjust it to provide critical damping of the signal at TP3 (i.e. neither underdamped nor overdamped). Once you've achieved critical damping, measure the final value of the damping resistor tool, and replace it with a fixed resistor of the closest value. -
Thank you Qiaozhi, I will get right on that.Comment
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Yep just re-read page 155, there is a lot to take in.Comment
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I tried that last night, it worked just as described, I had spent hours looking for that information, it was right under my nose and sorted in 17 minutes. I will make some more coils and test them as well.Comment
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Hi, I have another question that I would be grateful for any help with. Still working with the damping resistor, I would like to get this area clear in my head before I move on. I have made some mono coils and a basket weave, I think that is what they are called, and got what I believe to be a reasonable form on the scope.
I tested the response of the detector with my wife's very thin gold ring and it picked it up at 150mm. 6ins.
I then under damped the same coil and the picked the same ring up at 250mm 10ins.
Have I got this right or is it another stick I have the wrong end of.Comment
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I am not sure why those pictures have turned out on their sides.Comment
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1- slightly under damped does work and can see from the scope pictures sampling could be done a little sooner.
2- there may be an increase of response, even if sampling delay is the same. In this case the damping R is no longer part of the waveform.
I have also found slight under damping is a bit more sensitive. What is important is that there is NO ringing so the second scope picture looks ok.Comment
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Hi Waltr, thank you for you reply, could you explain what you mean when you say that the damping R is no longer part of the wave form.Comment
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Once the Coil Voltage reaches ZERO there is no potential across the R and no current through the R.Comment
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