instability in the wave tail of a PI detector

[pustareka]

BLYAD. SUKA. **** YOU MIND !!!!!!!!!!!!!!!!!!!!

[Orbit]

Pustareko administrator me zamolio da ti kazem da pises na engleskom da te ne bi izbacio sa foruma !
Pustareko, the administrator asked you to write in English !

[Hesamavr]

Wrong. The load should always be connected to the drain.
As waltr says, the source needs to be held at a constant potential.
Look at the Geotech Baracuda for an example (attached), and chapter 12 of ITMD.

Pustareko administrator me zamolio da ti kazem da pises na engleskom da te ne bi izbacio sa foruma !
Pustareko, the administrator asked you to write in English !

Thank you

[Ferric Toes]

Hesamavra,
The Vishay data sheet for the P channel IRF9640 shows the drain at the bottom in it's schematic, so the coil connections are correct. What appears to be wrong is the drive circuit. Go to the 'schematics' subject and scroll to find the Surfmaster Pro by Carl NC. Build on that drive circuit and you should find that it works. Go back to 1K resistor to the diodes and try different damping resistors to minimise ringing. Usually less than 1K but depends on coil inductance and cable capacitance.

Eric.

[Hesamavr]

What are the details of your coil? Inductance and resistance?

I've just noticed that you have your coil connected to the MOSFET source instead of the drain.
Have a look at Geotech Baracuda REV-A as an example. This is no doubt the cause of your problem.

As you see in the schematic, the load is connected to the PIN 2. 2 means Drain and source has connected to +12V which is 3. Therefore the schematic is correct and the load is connected to the Drain. I just misunderstood your tip with something else. The design is correct.

Therefore the problem is not because of this.

[Hesamavr]

Hesamavra,
The Vishay data sheet for the P channel IRF9640 shows the drain at the bottom in it's schematic, so the coil connections are correct. What appears to be wrong is the drive circuit. Go to the 'schematics' subject and scroll to find the Surfmaster Pro by Carl NC. Build on that drive circuit and you should find that it works. Go back to 1K resistor to the diodes and try different damping resistors to minimise ringing. Usually less than 1K but depends on coil inductance and cable capacitance.

Eric.

Yes, you replied sooner. the schematic is correct, but it does the switching, it is nothing more than an NPN transistor to drive the Gate. but you mentioned about damping. in case of instability (I'm not quite sure this is ringing or something else like noise), we should decrease the damping or increase it?

[Qiaozhi]

Hesamavra,
The Vishay data sheet for the P channel IRF9640 shows the drain at the bottom in it's schematic, so the coil connections are correct. What appears to be wrong is the drive circuit. Go to the 'schematics' subject and scroll to find the Surfmaster Pro by Carl NC. Build on that drive circuit and you should find that it works. Go back to 1K resistor to the diodes and try different damping resistors to minimise ringing. Usually less than 1K but depends on coil inductance and cable capacitance.

Eric.

The Vishay datasheet does show the drain connection at the bottom, but note that the internal bulk connection is joined to the source pin at the top. However, in Hesamavr's schematic, the MOSFET symbol shows the internal bulk connection is joined to the bottom pin, which is therefore the source. In this case, the coil is connected to the source and not the drain. Hesamavr also refers to pin numbers that do not exist in the datasheet.

[Hesamavr]

The Vishay datasheet does show the drain connection at the bottom, but note that the internal bulk connection is joined to the source pin at the top. However, in Hesamavr's schematic, the MOSFET symbol shows the internal bulk connection is joined to the bottom pin, which is therefore the source. In this case, the coil is connected to the source and not the drain. Hesamavr also refers to pin numbers that do not exist in the datasheet.

it is correct. G = 1, D = 2, S = 3. Numbers are for the PCB footprint. forget about symbol. I checked it again now for another time.

[Qiaozhi]

it is correct. G = 1, D = 2, S = 3. Numbers are for the PCB footprint. forget about symbol. I checked it again now for another time.

OK - so you're saying that the MOSFET symbol in your schematic is incorrect (i.e. the drain and source pins are swapped over). That's very confusing when you're asking us to help you solve a problem.

Anyway, as Eric says, the drive circuit needs some attention.

[waltr]

The posted schematic is wrong, Source should be to +12V and Drain to coil.

Pin numbers tend to be arbitrarily assigned so be careful referring to these. I have had serious PCB errors due to different pin number assigned by the schematic/PCB library.

If the Drain of the P-ch MOSFET is truly connected to +12V then the circuit should work.

Use the pin diagram in the data sheet (NOT pin numbers):
https://www.vishay.com/docs/91086/sihf9640.pdf

The Drain is the center pin and the heat sink/mounting tab.

What pre-amp circuit are you using?
Can you show us a Scope picture at the Pre-amp output without the scope clipping the signal.
From the little I can see in the video it looks like the coil is possibly un-damped but there could be other issues.

How is the coil connected??? Coax, twisted pair, other?? Pictures.
I have have odd results with haphazard coil connection (wire to coil).
This caused improper inductance, stray pick-up of metals near and interference from AC Mains and other electrical devices.

Circuit on a PCB or bread board? Picture.

[Qiaozhi]

The posted schematic is wrong, Source should be to +12V and Drain to coil.

Pin numbers tend to be arbitrarily assigned so be careful referring to these. I have had serious PCB errors due to different pin number assigned by the schematic/PCB library.

If the Drain of the P-ch MOSFET is truly connected to +12V then the circuit should work.

Use the pin diagram in the data sheet (NOT pin numbers):
https://www.vishay.com/docs/91086/sihf9640.pdf

The Drain is the center pin and the heat sink/mounting tab.

What pre-amp circuit are you using?
Can you show us a Scope picture at the Pre-amp output without the scope clipping the signal.
From the little I can see in the video it looks like the coil is possibly un-damped but there could be other issues.

How is the coil connected??? Coax, twisted pair, other?? Pictures.
I have have odd results with haphazard coil connection (wire to coil).
This caused improper inductance, stray pick-up of metals near and interference from AC Mains and other electrical devices.

Circuit on a PCB or bread board? Picture.

His schematic symbol has the source and drain pins swapped over. I tried simulating the circuit in LTSpice with the MOSFET the correct way round, and it does work. But the MOSFET is turned off passively using a 2k2 resistor in series with a 22R. Hence there's some odd wiggling going on, and the damping resistor has to be set to a low value to achieve critical damping. I expect the MOSFET is also getting quite hot, depending on the TX pulse width.

[kt315]

1.7R and 380uH. Spider coil. 20cmm diameter

give a pic of your coil.

[kt315]

if you don't know English please don't reply.

your problem you use very low inductivity coil. but write us that using 380uH. people certainly try to find a problem in schematic
but it is NOT in the schematic. you are misleading us.

[waltr]

His schematic symbol has the source and drain pins swapped over. I tried simulating the circuit in LTSpice with the MOSFET the correct way round, and it does work. But the MOSFET is turned off passively using a 2k2 resistor in series with a 22R. Hence there's some odd wiggling going on, and the damping resistor has to be set to a low value to achieve critical damping. I expect the MOSFET is also getting quite hot, depending on the TX pulse width.

Ahh yes.
I used a passive resistor to turn on the MOSFET and a bi-polar transistor to turn it off on the modified Hammer Head I built.

In a PI detector it is the MOSFET turn OFF that is important and should be sharp.
MOSFET turn on is not as important since the current raises slower due to the L/R time constant.

When building the prototype for the HH I did find that adding some R in series with the coil, lowers the time constant and helped the output stability.
I discussed this in post #6 of my thread:
http://www.geotech1.com/forums/showt...ake-on-the-HH2

[Hesamavr]

your problem you use very low inductivity coil. but write us that using 380uH. people certainly try to find a problem in schematic
but it is NOT in the schematic. you are misleading us.

Hahaha, why should I do that. I measured it using an RLC meter. It's true. I have connected the coil to the circuit using a coax cable.

My problem is that I can understand the noise, but instability is something else. I don't think I am under damping the coil, even in that case, we should see ringing effect, not instability. The instability annoys me here. besides, Mosfet is not hot at all