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

[h9361]

Dear Qiaozhi
During last 10 years, has anyone been able to add GB to HH1 like TDI ?

[Qiaozhi]

Yep RevD r55 1K pin7 IC9

Sorry - I must have been looking at the REV-C version.

[h9361]

Yep RevD r55 1K pin7 IC9

Hello
Dear 6666
Have you tested this, Non-VCO? Do you think we have such tone (exp. 600HZ) by changing R55?
In Rev. C, Non-VCO is very strident. It seem that sound is very very sharp and bad. For exp, i set my TX over 900Hz and compared my output sound with 900Hz in the link tone generator. I think the two sounds are very very different, especially my HH1 very very strident. In addition, my sound level in Rev. C is very weak so that its volume dose not increase, do you think it's normal?

[h9361]

My mean is HH1 Rev. D, Pin7 Ic9. Do you think we can such tone by changing R55?

[6666]

As far as I can see there is no R55 in the HH1 schematic.
The non-VCO option uses the TX oscillator as the audio tone, and this can be adjusted with R2.

Yep RevD r55 1K pin7 IC9

[Qiaozhi]

Please forgive me for my simple questions.
I'm going to build HH1 Rev. D because it has an independent audio part. Actually, i want to use a Non-VCO mode with a fixed sound like TDI.
Please see the link, it's an online tone generator. Please set frequency for 600Hz and hear it, square type. Do you think i can generate such tone by changing R55?!
Thank you for your helps. I need to thank you for All, especially Mr. Qiaozhi and Mr. Carl because i learned many things from PI and etc. in thins forum.

As far as I can see there is no R55 in the HH1 schematic.
The non-VCO option uses the TX oscillator as the audio tone, and this can be adjusted with R2.

[h9361]

Please forgive me for my simple questions.
I'm going to build HH1 Rev. D because it has an independent audio part. Actually, i want to use a Non-VCO mode with a fixed sound like TDI.
Please see the link, it's an online tone generator. Please set frequency for 600Hz and hear it, square type. Do you think i can generate such tone by changing R55?!
Thank you for your helps. I need to thank you for All, especially Mr. Qiaozhi and Mr. Carl because i learned many things from PI and etc. in thins forum.

[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.

[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.

[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.

[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]

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]

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.

[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.

[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?