Threshold Pot doesn't work

[h9361]

yes, i put a gap.
No, i did not re-nulling coil with shield.
For re-nulling, do you think i should change number turn of buck coil?

[MartinB]

yes, i put a gap.
No, i did not re-nulling coil with shield.
For re-nulling, do you think i should change number turn of buck coil?

Might be a silly question, but did you connect the shield to ground?

[ionut_mtb]

Hello
Dear Walter
As you know, i have built a concentric coil.
Today, i tried to add a shield (graphite or foil) to both inner and outer coil but i had a problem.
As soon as connecting shield to ground (+B), my TP4 will be broken.
This is TP4 without shield
[ATTACH]58226[/ATTACH]

And this is with shield connection
[ATTACH]58227[/ATTACH]
Do you think it's normal for concentric coil?
Thank you.

when you connect the shield you need to modify dumping resistor.

[h9361]

Might be a silly question, but did you connect the shield to ground?

Yes, i connected.

[h9361]

I'm trying to not change dumping resistor

[waltr]

when you connect the shield you need to modify dumping resistor.

Yes, I forgot about changing the Damping resistor value.

What I have done to have multiple coils for the same detector circuit is put a larger value resistor on the PCB.
Than add another resistor one the coils connect that tune the coil to the circuit.
The two resistor are in parallel.

Also, with balanced coils for a PI, both the TX and RX coil have a damping resistor.

[h9361]

Hello
Dear Walter
I tried so hard to null with shield.
My best TP4 form is follow:

There is a lot of noise at my home.
Do you think it's OK?

[waltr]

Yes, looks ok and probably noise in your house.
You need to get it outside and see how it works on real targets.
Remember that the first sample is a few microseconds after TX off. There is always some flyback so the stuff close the TX off will not be sampled.

[h9361]

Thanks dear Walter.
Please see attached, i measured its us TP4 and TP6.
Which point of measurement is correct?

[waltr]

Hard to tell.
Are you just looking to see what the 'sampling delay time' is? Maybe it is the 10usec measurement which is pretty good for a PI.

Maybe better to measure the Control signals. Compare the TX MOSFET gate signal to the Sampling switch signal.

Regardless, the first sample needs to be after the flyback settles. On my HH I have an external delay adjustment which I set between 10 & 25us depending on which coil, ground conditions and desired targets.

[h9361]

Hello
Dear Carl
Since you have a lot of information and experience about metal detectors, do you know minimum time delay of GPX 5000 (Coin/relic time)?
For exp, can you guess time delay of the Normal? (i think it's 12-15us)

GPX timing chart

[Carl-NC]

Sorry, I don't know.

[h9361]

Hard to tell.
Are you just looking to see what the 'sampling delay time' is? Maybe it is the 10usec measurement which is pretty good for a PI.

Maybe better to measure the Control signals. Compare the TX MOSFET gate signal to the Sampling switch signal.

Regardless, the first sample needs to be after the flyback settles. On my HH I have an external delay adjustment which I set between 10 & 25us depending on which coil, ground conditions and desired targets.

Hello
Dear Walter
I have a tecknical question about PI concentric coil.
As you know, such coil has two coils inner and outer. My question is that why is tried to use a thinner wire for inner coil?
Do you think wire thickness affect over Delay Timing?
Thanks

[h9361]

Sorry, I don't know.

Hello
Dear Carl
Please forgive me for my questions. According to your advise, i built several coils with shield without any problem.
Now, i start to build a big coil 55cm for my HHD. Before shielding, i checked TP4 form and it was very nice.
But as soon as adding any shield such as foil or graphite, my TP4 form is changed (attached). Do you have any comment for this problem?
Thank you.

[waltr]

What was the BEFORE shielding waveform look like.
If I see a comparison then maybe I could answer.