Kit arrived, all working ... request help with damping ..

[green]

reply #11(Doesn't seem right that the FET would avalanche at 60v .. EDIT: IRF840 is rated min 500V) https://www.geotech1.com/forums/atta...3&d=1584387794

Anyone have a thought why the avalanche volts is so low? Appears avalanche volts about 6 times turn on volts(P.S. volts?).

Read reply #11 again. My error, avalanche is close to 500v.

[SaltyDog]

Hi All,

I have tried with a very fast coil. It is 160uH 1.6E Spider weave, photo attached ..

I am getting TX discharge curve in 2uS

I have set my min delay to 5uS ... I can't seem to get lower with the current parts ..

However air testing is quite good.

My gold ring on it's side is 200mm and with the ring in the same orientation as the coil 300mm .. not too bad.

Be interesting if field testing is as good ..

I know you are mean't to be in the 300uH..500uH range, but not possible with the former I used, unless you fill it completely .... and that would be counter productive I feel ..

The outside dimension of the coil is approx 190mm ..

[6666]

Hi All,

I have tried with a very fast coil. It is 160uH 1.6E Spider weave, photo attached .. [ATTACH]49710[/ATTACH]

I am getting TX discharge curve in 2uS

I have set my min delay to 5uS ... I can't seem to get lower with the current parts ..

However air testing is quite good.

My gold ring on it's side is 200mm and with the ring in the same orientation as the coil 300mm .. not too bad.

Be interesting if field testing is as good ..

I know you are mean't to be in the 300uH..500uH range, but not possible with the former I used, unless you fill it completely .... and that would be counter productive I feel ..

The outside dimension of the coil is approx 190mm ..

Thats very fast, will be interesting to see what happens with in ground testing, good luck.

[SaltyDog]

That might be a little while, need to get it into a case etc

By the way, the wire used is cotton covered copper ..

[surfdetector]

That might be a little while, need to get it into a case etc

By the way, the wire used is cotton covered copper ..

What size wire are you using for the coil?
Cheers

[Qiaozhi]

Be interesting if field testing is as good ..

Depending on your soil conditions, you'll probably find that it falses a lot due to ground signals. I expect you will have to increase the main sample delay to 10us. If you go to the beach, then 15us is more suitable for searching at the surf line.

[SaltyDog]

Wire size is 0.5mm .. cotton covering makes overall size about 1.3mm ... one of the reasons I am getting such low inter-winding Capacitances ..

[SaltyDog]

I have noticed the following waveform on the pre-amp output ...



it seems the current op-amp is not fast enough to follow the slew rate as it overshoots quite a bit. I will have to experiment with a faster one ..

[SaltyDog]

Depending on your soil conditions, you'll probably find that it falses a lot due to ground signals. I expect you will have to increase the main sample delay to 10us. If you go to the beach, then 15us is more suitable for searching at the surf line.

I thought the whole idea of the 2nd sample was to cancel out ground effect ..

[SaltyDog]

Update: It looks like it is not the pre-amp after all. The input to the preamp (pin2 of U2) has an under-shoot, so looks like
I am back to playing with the damping resistor ..

See ->

[Carl-NC]

I thought the whole idea of the 2nd sample was to cancel out ground effect ..

It's for Earth field cancelation. Ground cancelation requires a second channel.

[waltr]

I thought the whole idea of the 2nd sample was to cancel out ground effect ..

No, second sample will not cancel ground.
Second sample is to cancel EFE (Earth Field Effect).

[Qiaozhi]

I thought the whole idea of the 2nd sample was to cancel out ground effect ..

The second sample is to cancel the Earth field effect, not ground.
Read the Build Document, at the top of page 19.

[SaltyDog]

I think I got the message load & clear .......... 3 times .......