Minipulse Plus Rev My time has come!

[green]

No because i destoyed the coil from post #64..

Practicaly you say to operate my MPP with the 3dss and connect ch1 to TP3.Then take another coil,cdd parallel to it a 10nF capacitor and face it to the working coil and measure voltage across capacitor?


Done the above precedure with another spider coil i made,connected parallel a 10nF ceramic and faced the working coil,the measurment was 89.21 KHz on the scope

That is the procedure I use to calculate coil inductance. Calculates 318uH with a 10nf cap. You measured 274uH with your meter. Did you measure the capacitor with a multimeter? What kind of ceramic is it, I use a npo ceramic or polypropelene. Don't know if an X7R would effect reading. I'll try a X7R latter to see if it effect my reading. Seems like the inductance readings should be closer. Could be my procedure, not a good 10nf capacitor, frequency reading, your inductance meter or something else? Don't add the capacitor across the coil when measuring coil resonance(coil open circuit).

[waltr]

So it like a rule that Transmit pulse should be 3 time TC?

Yes, what we want is the coil current be be steady before turning off coil.
This then has the target exposed to a steady magnetic field and eddy current from increasing magnetic field have also settled to zero.
Now when TX is switched Off the magnetic field collapses very quick inducing the target to produce strong repulsing eddy currents.
It is the target's repulsing eddy current that affect the coil decay curve and are detected.

3 times is a minimum, some literature says 5 times but 3 to 5 times is not much change so 3 works well.

Try different series resistor values when you get to measuring TX on coil current with your scope.
With small series R the current will still be increasing when TX switches off but will level out with higher R.

[mushaba]

That looks like the MOSFET switching the coil ON.


Don't be embarrassed to ask if you don't know.
It means Time Constant and is equal to the inductance divided by the total series resistance (TC = L/R).
So for a 300uH coil of 2 Ohm, the TC = 300e-6/2 = 150usec. Ideally the tx pulse should then be at least 450usec.
With an added 7.5 Ohm, TC = 300e-6/9.5 = 31.5usec. Times 3 = 95usec so a 100usec pulse is good.

Thanks a lot waltr this lesson was the best I've ever had in this forum..I'm trying to catch up but most people here in geotech are in advance stage regarding coils and op-amps,I never had the opportunity to work with them

So it like a rule that Transmit pulse should be 3 time TC?

Fly-back looks much better. Note that it does not have that spike then flat top (like the other pics) indicating the MOSFET is not avalancing.

I think this design with slower coils and higher resistance had a lower TC so fly-back did not peak as high.
A better coil, lower C and R caused the issue on fly-back. Adding series R will not make the coil slower, only C does that.

Thanks again for a valuable lesson!!!

[waltr]

What is that negative voltage,what is happening there?
[ATTACH]46019[/ATTACH]

That looks like the MOSFET switching the coil ON.

Hello waltr thanks for your reply and help! What TC acronym stands for (embarrassed to ask but i dont know)

Don't be embarrassed to ask if you don't know.
It means Time Constant and is equal to the inductance divided by the total series resistance (TC = L/R).
So for a 300uH coil of 2 Ohm, the TC = 300e-6/2 = 150usec. Ideally the tx pulse should then be at least 450usec.
With an added 7.5 Ohm, TC = 300e-6/9.5 = 31.5usec. Times 3 = 95usec so a 100usec pulse is good.

I dont have a current probe for my scope...I will connect an 1 Ohm resistor from power to source of irf840..

I always use a small value resistor to measure current. Vdrop/R = current. safer than meter and easier then current probe.

[ATTACH]46020[/ATTACH]

Works smoothly,the sound of the threshold became more stable but very nervous to turn..

Fly-back looks much better. Note that it does not have that spike then flat top (like the other pics) indicating the MOSFET is not avalancing.

I think this design with slower coils and higher resistance had a lower TC so fly-back did not peak as high.
A better coil, lower C and R caused the issue on fly-back. Adding series R will not make the coil slower, only C does that.

[mushaba]

Thanks

Have you tried looking at amplifier out on your MPP while positioning either your 3DSS or basket coil near the operating coil to see what resonant frequency the have?

No because i destoyed the coil from post #64..

Practicaly you say to operate my MPP with the 3dss and connect ch1 to TP3.Then take another coil,cdd parallel to it a 10nF capacitor and face it to the working coil and measure voltage across capacitor?


Done the above precedure with another spider coil i made,connected parallel a 10nF ceramic and faced the working coil,the measurment was 89.21 KHz on the scope

[green]

Here sir https://www.geotech1.com/forums/show...475#post241475

Thanks

Have you tried looking at amplifier out on your MPP while positioning either your 3DSS or basket coil near the operating coil to see what resonant frequency the have?

[mushaba]

Works smoothly,the sound of the threshold became more stable but very nervous to turn..

[mushaba]

Where do I find a schematic for REV-E?

Here sir https://www.geotech1.com/forums/show...475#post241475

[green]

Where do I find a schematic for REV-E?

[mushaba]

Hi Mushaba,
From the Build doc:
"Monitor the signal across the damping resistor using an oscilloscope with x10 probe, where you should see a flyback signal of about 350V or higher."
I dont know if your still on the Hot of the coil or not? But look at the damping resistor at . R4 pl2 side .
Does your signal there still flat top at the end of the spike?

Hi godigit1,Thans for your reply and help.yes



What is that negative voltage,what is happening there?

I just got over being sick, and just got two MPP running again so tonight I can check mine at the hot side see if that is there as well. Ive never tested that way but Im always learning.
To me your coil looks slightly over damped. Back things off to 5us and take a shot turn off averageing if you have it on.
What is your final Resistor value?.

Hope you get well soon!!I must do something wrong at the damping process,i connect a pot instead of R4,i didnt see differences if i connect (the resistor parallel reistor+pot)..When the output (TP3) has the smallest (negative) duration i disconnect the pot measure the resistance and put a resistor to place...The above pictures are with 3300 Ohms R4..

1- best when tx pulse width is at least 3 time coil TC (L/R).

Hello waltr thanks for your reply and help! What TC acronym stands for (embarrassed to ask but i dont know)

2- best stability is with less coil current.
3- adding 10 Ohms is series with the coil, dropped peak coil current from 1.6 Amp to 0.6 Amp, but did not decrease detection distance. In fact detection distance was increased due to circuit being more stable so Gain in integrator and Sat could be increased.
4- With 10 Ohm in series the fly-back Voltage is greatly reduced.
In the design I also have a 1 Ohm resistor from MOSFET to power which allows easy coil current measuring with a scope.
Also allowed a resistor to be inserted in series with coil to MOSFET to adjust the total coil TC.

Try measuring with O'scope the coil current. Best is when coil current almost level off before turning off the coil. Then try adding some R in series with the coil.

I dont have a current probe for my scope...I will connect an 1 Ohm resistor from power to source of irf840..

Increasing coil current does not increase detection distance by much. To double distance coil current needs to increase by 8 times.
Increasing distance is better done in the receiving side.

Thank once again for the understanding and great help!

It depends on the coil inductance and resistance, and the TX pulse width. You should avoid letting the flyback voltage exceed the breakdown voltage of the MOSFET as this will waste energy, make the device get hot, and also slow the decay. The value of 350V is just a guide as what to expect for a typical 300uH coil with a TX pulse rate of 1000pps and a pulse width of 100us.

(from reply#37) coil inductance, resistance and pulse width effect peak current at Tx off. Power supply volts,D10 and R3(3R3) effect peak Tx current also. With a fast coil(low capacitance) peak volts at Tx off equals peak Tx current*Rd[(R1*R12)/(R1+R12)]. With R1=470 Rd=320. Would take 2A peak current at Tx off to get 600V. For damping with a fast coil R1 would increase taking less peak current at Tx off to get 600V.

Hello green thanks for your reply and for the help!Im building the Rev-E "D10 and R3(3R3) effect peak Tx current also"

Also I just got to thinking, you and I have the same inductance meter. I have found mine to be unaccurate by about +-30% down in the uH ranges its good higher.
I found this out when I sent Green some coils for testing a couple years ago and he measured different values than I did.

This inductance meter sucks but its the only thing i have right now...I ordered an LCR meter Atmel based from ebay,i heard they work pretty decent..

[godigit1]

Also I just got to thinking, you and I have the same inductance meter. I have found mine to be unaccurate by about +-30% down in the uH ranges its good higher.
I found this out when I sent Green some coils for testing a couple years ago and he measured different values than I did.

[waltr]

All correct Green.

When I was building my HH2 I experimented with coil current and TX pulse time verse detection distance and circuit stability.
What I found is:
1- best when tx pulse width is at least 3 time coil TC (L/R).
2- best stability is with less coil current.
3- adding 10 Ohms is series with the coil, dropped peak coil current from 1.6 Amp to 0.6 Amp, but did not decrease detection distance. In fact detection distance was increased due to circuit being more stable so Gain in integrator and Sat could be increased.
4- With 10 Ohm in series the fly-back Voltage is greatly reduced.
In the design I also have a 1 Ohm resistor from MOSFET to power which allows easy coil current measuring with a scope.
Also allowed a resistor to be inserted in series with coil to MOSFET to adjust the total coil TC.

Try measuring with O'scope the coil current. Best is when coil current almost level off before turning off the coil. Then try adding some R in series with the coil.

Increasing coil current does not increase detection distance by much. To double distance coil current needs to increase by 8 times.
Increasing distance is better done in the receiving side.

[green]

Thanks green i will try that as soon as i can..
Detecting and air tests doing great doth on mono and 3dss (without shielding),still cant understand why i have that big flyback spikes at 600-700 volts,something is wrong and i cant figure what,some help would be nice!!

It depends on the coil inductance and resistance, and the TX pulse width. You should avoid letting the flyback voltage exceed the breakdown voltage of the MOSFET as this will waste energy, make the device get hot, and also slow the decay. The value of 350V is just a guide as what to expect for a typical 300uH coil with a TX pulse rate of 1000pps and a pulse width of 100us.

(from reply#37) coil inductance, resistance and pulse width effect peak current at Tx off. Power supply volts,D10 and R3(3R3) effect peak Tx current also. With a fast coil(low capacitance) peak volts at Tx off equals peak Tx current*Rd[(R1*R12)/(R1+R12)]. With R1=470 Rd=320. Would take 2A peak current at Tx off to get 600V. For damping with a fast coil R1 would increase taking less peak current at Tx off to get 600V.

[godigit1]

Hi Mushaba,

From the Build doc:
"Monitor the signal across the damping resistor using an oscilloscope with x10 probe, where you should see a flyback signal of about 350V or higher."
I dont know if your still on the Hot of the coil or not? But look at the damping resistor at . R4 pl2 side .
Does your signal there still flat top at the end of the spike?
I just got over being sick, and just got two MPP running again so tonight I can check mine at the hot side see if that is there as well. Ive never tested that way but Im always learning.
To me your coil looks slightly over damped. Back things off to 5us and take a shot turn off averageing if you have it on.
What is your final Resistor value?.

[mushaba]

Thanks green i will try that as soon as i can..
Detecting and air tests doing great doth on mono and 3dss (without shielding),still cant understand why i have that big flyback spikes at 600-700 volts,something is wrong and i cant figure what,some help would be nice!!