I am getting interested in designing my own version of a digitally controlled metal detector. I know this is a daunting task, so I wish to start out with a very simple analog metal detector from days gone bye. I have this old reliable T/R (I/B) metal detector and I am in the process of reverse-engineering the circuit. I have drawn out the schematic and the only thing left is to determine how the search coil is wound. I am hoping that there is someone with a lot more experience on how manufacturers wire their search coils since I am hitting a snag.
If you know why I am getting the anomaly during my tests please chime in and assist me while I am building my replicant version.
There are 4 wires coming from the search coil, and after checking each one, I discovered that they are all connected to each. There is also a 5th wire present which is attached to the RF metal shield and it is open to all other wire connections. This coil’s 4 active wires consist of a red, green, black and white wire. After a comprehensive test of the coil’s resistance and inductance measurements, I concluded that it probably consists of two large coils and they are likely are wound in the “Double-D” configuration. I have added an insert showing how I visualize the various sections of the search coil are connected on the schematic diagram and I have added a chart showing the measured resistance and inductance on every wire (the search coil was out-of-circuit).
In the schematic, I show that the transmit coil section is made up of L2 and L3 and the receive coil is L1. I realize that measuring inductors with a DMM in resistance mode is not ideal, but during my tests, I found when testing L3 it was acting differently than what I would expect. I figure that the total resistance across all the coils would decrease a little if the wires of L3 were shorted together. When I measure the resistance between the white and the red wires, I get 9.6 Ω even if the green and black wires are shorted together or not. I checked every possible connection and read their resistance while shorting the other two wires together and the measurement never changed at all. This makes me question if I have the configuration correct.
I figured that perhaps L3 was wired in a delta or wye configuration. So I broke out the inductor tester and it shows that the total inductance drops like expected when I short out L3. The total inductance drops 40 µH from 900 µH to 860 µH. So now I have one meter showing that these coils are in series and the other disagreeing. I have assembled a simple graph showing the resistance and inductance between the various connections.
The two outside wires (red and white) naturally, have the highest resistance and inductance. I realize that if I add the sum of all the coils the total will not match. I also know that depending upon the direction on the winding it will either add or subtract to the inductance.
As I mentioned before. If I measure the resistance across the red and white wires, I get the 9.6 Ω reading. If I short the green and black wires together, the resistance remains the same.
If I measure the inductance across the red and white wires, I get the 900 µH reading. If I short the green and black wires together, the inductance drops to 860 µH.
The operating frequency is 100KHz pulsed every 3.540ms (282Hz)
Three pulses (282Hz)
One pulse expanded (100KHz)
If you know why I am getting the anomaly during my tests please chime in and assist me while I am building my replicant version.
There are 4 wires coming from the search coil, and after checking each one, I discovered that they are all connected to each. There is also a 5th wire present which is attached to the RF metal shield and it is open to all other wire connections. This coil’s 4 active wires consist of a red, green, black and white wire. After a comprehensive test of the coil’s resistance and inductance measurements, I concluded that it probably consists of two large coils and they are likely are wound in the “Double-D” configuration. I have added an insert showing how I visualize the various sections of the search coil are connected on the schematic diagram and I have added a chart showing the measured resistance and inductance on every wire (the search coil was out-of-circuit).
In the schematic, I show that the transmit coil section is made up of L2 and L3 and the receive coil is L1. I realize that measuring inductors with a DMM in resistance mode is not ideal, but during my tests, I found when testing L3 it was acting differently than what I would expect. I figure that the total resistance across all the coils would decrease a little if the wires of L3 were shorted together. When I measure the resistance between the white and the red wires, I get 9.6 Ω even if the green and black wires are shorted together or not. I checked every possible connection and read their resistance while shorting the other two wires together and the measurement never changed at all. This makes me question if I have the configuration correct.
I figured that perhaps L3 was wired in a delta or wye configuration. So I broke out the inductor tester and it shows that the total inductance drops like expected when I short out L3. The total inductance drops 40 µH from 900 µH to 860 µH. So now I have one meter showing that these coils are in series and the other disagreeing. I have assembled a simple graph showing the resistance and inductance between the various connections.
The two outside wires (red and white) naturally, have the highest resistance and inductance. I realize that if I add the sum of all the coils the total will not match. I also know that depending upon the direction on the winding it will either add or subtract to the inductance.
As I mentioned before. If I measure the resistance across the red and white wires, I get the 9.6 Ω reading. If I short the green and black wires together, the resistance remains the same.
If I measure the inductance across the red and white wires, I get the 900 µH reading. If I short the green and black wires together, the inductance drops to 860 µH.
The operating frequency is 100KHz pulsed every 3.540ms (282Hz)
Three pulses (282Hz)
One pulse expanded (100KHz)
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