Totem-pole gate driver VS Active pull down VS Fast Fet turn off

[Monolith]

Some tests with different gate resistor values turning Tx on and off. Guessing some of my problem is the circuit is hard wired. Different gate resistors effected oscillation across .1 ohm resistor at Tx on. Not much effect at Tx off.
Coil connected to A, avalanches. Should different gate resistors effect oscillation at Tx off. Would a good circuit board layout look a lot different?

I use a coil with different Tx and Rx coils. Noise level at integrator out seems to be the same whether Tx is on or disabled. What negative effect does the oscillation at Tx on or off have? Should I be seeing integrator out noise increase if Tx is enabled?

If your noise level is the same with TX on and TX off, I would say it is as good as it gets.
My take from this discussion, is that it is very difficult to correctly measure the gate oscillation with the tools that we have.

[green]

My take from this discussion, is that it is very difficult to correctly measure the gate oscillation with the tools that we have.

Agree. I had purchased some different Mosfets to try. Tested them to see if I could see a difference. Added 10 ohms in series with coil(coil to B, R=10)to prevent avalanche. 3904, 3906 driver, no added gate resistor.
IRF740 did avalanche, 400V rating. Others 500V rating. Coil 328uH about 1MHz resonance. Rd=1000 ohms, slightly overdamped.

[6666]

Agree. I had purchased some different Mosfets to try. Tested them to see if I could see a difference. Added 10 ohms in series with coil(coil to B, R=10)to prevent avalanche. 3904, 3906 driver, no added gate resistor.
IRF740 did avalanche, 400V rating. Others 500V rating. Coil 328uH about 1MHz resonance. Rd=1000 ohms, slightly overdamped.

Hi Green is it possible to change the 3906 to a Darlington to speed up turn off

[Carl-NC]

Hi Green is it possible to change the 3906 to a Darlington to speed up turn off

That made a fair difference in one of my designs.

[green]

Hi Green is it possible to change the 3906 to a Darlington to speed up turn off

Scope ground is at +12V so - supply on gate is off. Gate goes from zero V(on)to -supply(off)in less than 40ns with the 3906. How much faster do you want it? https://www.geotech1.com/forums/atta...8&d=1615911700

[Ferric Toes]

Scope ground is at +12V so - supply on gate is off. Gate goes from zero V(on)to -supply(off)in less than 40ns with the 3906. How much faster do you want it? https://www.geotech1.com/forums/atta...8&d=1615911700

In a paper by F. B. Johnson in 1956, it states "so long as the time constant of switch-off is less than about one tenth of the decay time of eddy currents in the object, the behavior of the eddy currents is practically the same as if the field had been removed instantaneously". It then goes into a lot of maths. I would say that provided the switch-off is at least a fifth of the object Tau then there is little benefit of going for more. Faster switch-off than necessary will cause higher flyback voltages, and depending on the device, more avalanche situations and more ringing.

Eric.

[dbanner]

I am of the view that flyback voltages can be a function of the soil medium as well as intrinsic circuit characteristics. However small, but it's there.
There were some Japanese fet transistors way back, l would be curious to see how they would perform in a pl circuit.

[green]

In a paper by F. B. Johnson in 1956, it states "so long as the time constant of switch-off is less than about one tenth of the decay time of eddy currents in the object, the behavior of the eddy currents is practically the same as if the field had been removed instantaneously". It then goes into a lot of maths. I would say that provided the switch-off is at least a fifth of the object Tau then there is little benefit of going for more. Faster switch-off than necessary will cause higher flyback voltages, and depending on the device, more avalanche situations and more ringing.

Eric.

Does gate drive have much effect on switch-off time constant?

[Ferric Toes]

Does gate drive have much effect on switch-off time constant?

Hi Green, I have tried all three methods in this thread's title and there is hardly any measureable difference in the switch-off at the coil. As you know, the coil's time constant and damping are the dominant features. The energy stored in the collapsing magnetic field has to be dissipated before receiver sampling can occur. All this takes time and in my tests this takes 1.5 - 2.5uS to complete (depending on the Mosfet used) from the initiation of the switch-off at the gate. This is for 1A pulse current and provided there is no avalanche and the gate is not overdriven.

Eric.

[h9361]

Hello
Dear All
Before everything, thank you for sharing your results.
I need your help about HH1 Rev. C. I have built it without any problem and works with its original coil.
My problem is about using GPX coil over HH1. Now, i have the GPX 11" commander mono coil and tested it over my HH1.
My testing is divided to three parts:
1- without adding any damping resistor. Because GPX coils has internal damping resistor for RX coil. My result was is very bad so that my sensitivity was very very weak and i can only detect big metals.
2- I used Car's method in IMD book, p.155. I used a parallel resistor +Pot in range 181-1670 ohm. My result was a little better but not noticeable.
3- I changed my damping resistor range to 200 ohm to 90K. My result was identical with previous range.
As a result, GPX coil work over HH1 but its sensitivity with every damping resistor is very weak. Off course, Rev. C is using passive pull-down. Do you think i should use Active pull-down?
I hope you accept to help me, thank you for your supporting.

[Qiaozhi]

Hello
Dear All
Before everything, thank you for sharing your results.
I need your help about HH1 Rev. C. I have built it without any problem and works with its original coil.
My problem is about using GPX coil over HH1. Now, i have the GPX 11" commander mono coil and tested it over my HH1.
My testing is divided to three parts:
1- without adding any damping resistor. Because GPX coils has internal damping resistor for RX coil. My result was is very bad so that my sensitivity was very very weak and i can only detect big metals.
2- I used Car's method in IMD book, p.155. I used a parallel resistor +Pot in range 181-1670 ohm. My result was a little better but not noticeable.
3- I changed my damping resistor range to 200 ohm to 90K. My result was identical with previous range.
As a result, GPX coil work over HH1 but its sensitivity with every damping resistor is very weak. Off course, Rev. C is using passive pull-down. Do you think i should use Active pull-down?
I hope you accept to help me, thank you for your supporting.

I have had a GPX coil running with Crossbow, which is an HH derivative, and don't understand what you mean by: "Because GPX coils has internal damping resistor for RX coil".
You stated that the Commander coil is a mono, so there is no separate RX coil.
I found that the damping resistor value required for the GPX coil was 470R. Please see attached document: "Minelab GPX Coil.pdf".
Also the connections required for a Minelab DD coil are in a second document: "Minelab DD Coil - updated.pdf".

By the way, active pulldown will not resolve this issue. You may simply having a wiring problem between HH and the coil.

[h9361]

Thanks Qiaozhi
According to your files, GPX coils has no any damping resistor inside it, there is the resistor only inside of main unit.
So, you believe that the best damping resistor is 470R for both RX and TX in GPX coils ?
See attached, i used the map for my pins. Please see both of your attachment. In first, pin 1 is RX and pin 5 is TX, whereas other is different and Pin 1 is TX and Pin 5 is RX.
In HH1 Rev. D, there is two damping. Do you think i should put 470R for both damping if i want to use DD GPX coil?
Thank you.

[Qiaozhi]

[ATTACH]55023[/ATTACH]
Thanks Qiaozhi
According to your files, GPX coils has no any damping resistor inside it, there is the resistor only inside of main unit.
So, you believe that the best damping resistor is 470R for both RX and TX in GPX coils ?
See attached, i used the map for my pins. Please see both of your attachment. In first, pin 1 is RX and pin 5 is TX, whereas other is different and Pin 1 is TX and Pin 5 is RX.
In HH1 Rev. D, there is two damping. Do you think i should put 470R for both damping if i want to use DD GPX coil?
Thank you.

It looks like it was only necessary to use a 470R damping resistor on the TX loop, as perhaps there was a damping resistor for the RX loop inside the coil shell.
I honestly cannot remember, as it was quite a few years ago.

[IBGold]

The TX coil is internally damped and the RX in DD coils has the damping resistor in the coil mono coils use the internal damping resistors.

Regards, Ian.

[h9361]

Thank you dear Qiaozhi.
I attached TDI PCB, it's pins part. As you told, i think TDI only has used a damping resistor for its TX (680R, I tested 680 for HH1 and mono GPX, its sens is weak ), not RX.
Since TDI also can work over GPX DD and it also hasn't used any damping for RX, so it's proved that GPX coils has used only a damping resistor over RX (only for DD), inside of its shell.