complete project Circuit diagram

[eclipse]

Changed it to 100 ohms and it looks better now, not too sure.. the voltage peak is 250V now.

[Tinkerer]

Changed it to 100 ohms and it looks better now, not too sure.. the voltage peak is 250V now.

It is OK to regulate the Flyback voltage with the resistor. The important thing is to avoid avalanching the Mosfet or overvoltage of the parts, capacitor and resistors. Avoid Wirewound resistors when possible.

What is the timing schedule you are using?

[eclipse]

The mosfet is rated 650V and unrestricted went all the way up to 800V. I did not expect to see high heat build up (probably arround 50C) on the flyback control resistor (let's call it that).
Right now I've connected it to a regular mono coil 550uH as was impatient to turn it on and building a IB coil will take me a while.
I'm using the default timings 600PPS with 100uS TX provided by you. Some parts are kept tall as I want to be able to desolder them to be re-used as this build is more of a proof of concept test board where things will be swapped/desoldered and possibly destroyed
I took note and added copper pour can be connected to GND with J1 so I can actually see if/what improvement it gives. Also you will notice audio connector which can be connected to 2nd board with alternative audio solution possibly AD654 based.

[Tinkerer]

I like your board. Also the test point loops.
I am a bit surprised with the low PPs rate. Usually my preference is high rate 2k and up to 20k. This design is for an induction balanced coil. The bucking coil in series with the TX coil is wound in the opposite direction of the TX coil, around the RX coil. The number of turns (stated as 1 on the schematic) depend on the diameter of the RX coil.

[Tinkerer]

Tinkerer Hex file

found a hex file. Not sure if good, but worth a try.

[eclipse]

I hooked up test DD coil and here's few shots from the preamp out and TX. Signal looks quite different from the references I've seen.
I got no response change for before TX reaches 50uS (~50%TX ON) and goes to about 20uS-40uS after TX OFF.
Response to small targets like gold ring and silver coin is very low.
Scope is set to 50mV/DIV for both channels (TX is 1:100, preamp is 1:10) and 50uS/DIV.

Default signal (no traget)



Pair of pliers



Large tray

[waltr]

I hooked up test DD coil and here's few shots from the preamp out and TX. Signal looks quite different from the references I've seen.

Did you adjust the coil coupling?

[eclipse]

I've done adjustments by trial-and-error with the scope and this is currently the only way I've found it actually (somewhat) works, don't know if there's a better way.
I don't use the bucking coil and the wire is plain 0.56 enamelled wire because I didn't want to waste the litz wire before I know little more.

[Tinkerer]

I've done adjustments by trial-and-error with the scope and this is currently the only way I've found it actually (somewhat) works, don't know if there's a better way.
I don't use the bucking coil and the wire is plain 0.56 enamelled wire because I didn't want to waste the litz wire before I know little more.

It is OK to use any kind of wire, but you must use the bucking coil for this design, otherwise your sensitivity will be very low.
the output of the preamp looks different, because this is not a traditional PI design. At the time I was designing and testing hundreds of different ways of making an induction balanced pulse induction metal detector. Most of the designs were innovative in some aspect or other.

[eclipse]

Okay I will add the bucking coil and start readjusting the coil, maybe even try to build the concentric coil meanwhile.

[waltr]

It is OK to use any kind of wire, but you must use the bucking coil for this design, otherwise your sensitivity will be very low.
the output of the preamp looks different, because this is not a traditional PI design. At the time I was designing and testing hundreds of different ways of making an induction balanced pulse induction metal detector. Most of the designs were innovative in some aspect or other.

Tinker, can you please post an updated scope capture of the pre-amp output (or a link to this). I have read all of your posts and know I've seen your experiments but not easy to put all the pieces together.

I build a modified Hammer Head PI but use IB coils which I null. If not properly nulled I have seen scope trace similar to what eclipse got.
Here is link to my post on IB for PI.
http://www.geotech1.com/forums/showt...237#post238237

[Tinkerer]

Tinker, can you please post an updated scope capture of the pre-amp output (or a link to this). I have read all of your posts and know I've seen your experiments but not easy to put all the pieces together.

I build a modified Hammer Head PI but use IB coils which I null. If not properly nulled I have seen scope trace similar to what eclipse got.
Here is link to my post on IB for PI.
http://www.geotech1.com/forums/showt...237#post238237

You are correct, nulling is the most important part of IB coils. The smaller the residual of the nulling, the more gain you can apply.
Sorry I can not show you anything recent on these designs. I lost nearly all my files some time back, when I moved. The computer crashed and the backup hard drive with 240Gb of files stopped working when I moved. All I have is a few old backup DVD's with no index. It takes many hours of searching to find anything and I just can not find the time for that at present.
I posted a few things on the Forum, but, as you say, scattered and hard to put together. Probably the best way is to try to search for stuff of similar date, but a lot went lost when the Geotech forum reorganized.
If you can show me stuff I posted, usually I remember something.
Right now, I seem to remember, that the TX schematic posted earlier, with a single turn Bucking coil, was meant for a DD coil. After balancing the RX and TX coils, a single loop, stretching from about center coil, towards the rim, so that the 2 wires were lying about parallel 10mm apart, could then be moved like the finger of a clock, searching for the near perfect null.
Also on this TX design, I used a Mosfet as variable resistor to dampen the Flyback.
The RX circuit can be used for various TX. At some other time I can explain the several special features.

Thanks for your link, I will try to find some time to read it.

[green]

Tinker, can you please post an updated scope capture of the pre-amp output (or a link to this). I have read all of your posts and know I've seen your experiments but not easy to put all the pieces together.

I build a modified Hammer Head PI but use IB coils which I null. If not properly nulled I have seen scope trace similar to what eclipse got.
Here is link to my post on IB for PI.
http://www.geotech1.com/forums/showt...237#post238237

Hi waltr, was looking at your scope traces and was confused until I noticed the scope trace goes right to left instead of left to right.

[mschmahl]

Here are some pictures of my experiments. But it it some time ago. These are with a big (50 cm) IB coil. Center taped RX.

[mschmahl]

Here are some pictures of the coil...