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

[SaltyDog]

Hi Guys,

My kit arrived yesterday (Rev-E) and put it together today and all working.

I can detect my gold wedding band at 7" with minimum gain and delay(around 12us) ..
On the coil I have, (450uH 3E) I seem to only have about 80v p-p flyback pulse (unless my 20Mhz scope is not showing all ) ..
I suspect damping is too much ...

What is the procedure to tune damping ...?

Cheers

[surfdetector]

You should build this circuit and attach it temporarily in place of R4 to determine the best damping resistor value for your coil
Cheers

[waltr]

[ATTACH=CONFIG]49688[/ATTACH]

You should build this circuit and attach it temporarily in place of R4 to determine the best damping resistor value for your coil
Cheers

Yes, best way to find correct damping resistor value.

[kt315]

Yes, best way to find correct damping resistor value.

145 page of ITMD.

[SaltyDog]

145 page of ITMD.

You are speaking in code .. what is ITMD?

[kt315]

nice... there is someone, happy man, who still does not know what is ITMD LOL!

https://www.amazon.com/Inside-Metal-...DDMSEQ5SG7TWCJ

congrats, you are on a site of ITMD authors. a net catched you fisher man.
ahaha!

[SaltyDog]

nice... there is someone, happy man, who still does not know what is ITMD LOL!

https://www.amazon.com/Inside-Metal-...DDMSEQ5SG7TWCJ

congrats, you are on a site of ITMD authors. a net catched you fisher man.
ahaha!

Ha ha ... very good ... I will buy it ...

Anyone have a picture of the optimum critically damped wave?

[green]

Ha ha ... very good ... I will buy it ...

Anyone have a picture of the optimum critically damped wave?

scope pictures, middle picture slightly underdamped

[SaltyDog]

Here is what I have across R4 ..

[Carl-NC]

It looks like the damping resistor can be increase a little to get closer to a slight underdamping. The flyback is being limited by the Mosfet avalanche, so to get a higher flyback you will need a higher voltage Mosfet.
If I read correctly, you are turning on the coil for 400us which seems excessive, unless you are hunting for big silver bars. Excessively long turn-on times will push out the minimum delay.

[SaltyDog]

It looks like the damping resistor can be increase a little to get closer to a slight underdamping. The flyback is being limited by the Mosfet avalanche, so to get a higher flyback you will need a higher voltage Mosfet.
If I read correctly, you are turning on the coil for 400us which seems excessive, unless you are hunting for big silver bars. Excessively long turn-on times will push out the minimum delay.

Hi Carl,

I am doing exactly as instructed in the documentation, is states to set pulse width to 400us ... at 125hz rate ..I am also using the FET that came with the kit .. are you saying this info is incorrect?
(Doesn't seem right that the FET would avalanche at 60v .. EDIT: IRF840 is rated min 500V)

Cheer

EDIT: It seems my scope only has input = 5V, so with a 10:1 probe was clipping at 50v .. I have measured with a 100:1 probe and I am getting slight clipping at 500v(might be scope or avalanche), so all good on that front ..
I have also pushed back the FET on time to 200us .. my pulse delay is now approx 12us, I think that is the best I can do with this cheap coil ... I will build a better basket-weave one .

[Qiaozhi]

Hi Carl,

I am doing exactly as instructed in the documentation, is states to set pulse width to 400us ... at 125hz rate ..I am also using the FET that came with the kit .. are you saying this info is incorrect?
(Doesn't seem right that the FET would avalanche at 60v .. EDIT: IRF840 is rated min 500V)

Cheer

From the scope image, it looks like the amplitude of the flyback is 50V.
Perhaps your scope probe is set to the x10 setting?
Also, for your coil you should be using 100us TX pulse width and 1000pps pulse rate.

From the Build Document (Technical Description):
Initially a simple 400uH 9” diameter mono coil, wound with 0.56mm enamelled wire, was
connected. The TX pulse width was set to 100us, with a pulse period of 1ms (1000pps). Due to this
being a “slow” coil, the earliest sample delay possible was 30us. The sample pulse width was set to
60us. A Victoria penny was detectable at 12” in an electrically quiet environment.

The DD coil was a Garrett Infinium LS, which an elliptical coil of 14” x 10”. The initial (more
aggressive) settings defined in step 3 were used. i.e. a pulse width of 400us, with a pulse period of
8ms (125pps). Since the Garrett coil is a much “faster” coil, the preamp was coming out of
saturation at 8.4us. In this case the minimum sample delay was set to 10us, with a sample pulse
width of 65us. A Victorian penny was then detectable at 13 to 14”.

At the end of Step 3 it states:
The final pulse width and period can be adjusted again later to suit
the particular coil you plan to use.

[Carl-NC]

EDIT: It seems my scope only has input = 5V, so with a 10:1 probe was clipping at 50v .. I have measured with a 100:1 probe and I am getting slight clipping at 500v(might be scope or avalanche), so all good on that front ..
I have also pushed back the FET on time to 200us .. my pulse delay is now approx 12us, I think that is the best I can do with this cheap coil ... I will build a better basket-weave one .

OK, that explains the early clipping. 450uH is a bit on the high side for fast sampling, try 300uH. You may not need to basket-weave.

[SaltyDog]

OK, that explains the early clipping. 450uH is a bit on the high side for fast sampling, try 300uH. You may not need to basket-weave.

I have tried 100us pulse and 1000pps, but no better results than 200us 500pps ..
I can't rewind coil to 300us, this is a potted coil .. so might as well create a basket weave coil (currently, pre-amp coming out of saturation in 12us)

[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?).