Minipulse Plus Rev My time has come!

[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

[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]

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]

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.

[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).

[green]

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.

Not saying if constant rate or constant current at Tx off is better or worse. I get a higher Rx signal with constant rate if average coil current is the same, and a higher Rx signal with constant current if peak current is the same. With the same average current, constant current peak is half the constant rate peak. Should stabilize sooner but I think I still get little more detection distance with constant rate if average current is the same. Just saying what I get, use what works best for you.

I have coil current in a control loop so when I compare different coils the Tx profile is always the same. Haven't tried comparing charging to 3 time constants with cc or cr.

[waltr]

Not saying if constant rate or constant current at Tx off is better or worse. I get a higher Rx signal with constant rate if average coil current is the same, and a higher Rx signal with constant current if peak current is the same. With the same average current, constant current peak is half the constant rate peak. Should stabilize sooner but I think I still get little more detection distance with constant rate if average current is the same. Just saying what I get, use what works best for you.

I have coil current in a control loop so when I compare different coils the Tx profile is always the same. Haven't tried comparing charging to 3 time constants with cc or cr.

All True Green but I was keeping explanations simpler to help mushaba get up to speed on the basics of PI design and so he can get this detector usable.

[mushaba]

All True Green but I was keeping explanations simpler to help mushaba get up to speed on the basics of PI design and so he can get this detector usable.

The think I have in mind after your and greens intell is at first we should have a coil with correct specs so it can "charge" fully.At it's discharge(I think "collapse" is the right word) the rapid discharge is creating the eddys we re looking for..constant (tx pulse)rate or constant (coil) current are the things to be investigated by me in the short future.
What "kind" of opamp setup is the receiver of the MPP?

[Qiaozhi]

Here's a thread where we discussed flat-topping the coil current, and a simulation showing the benefits of allowing the coil current to top out -> https://www.geotech1.com/forums/show...top#post221739

However, this is not the whole story, as it's not always possible to flat-top the coil current due to the TX pulse rate being used. In order to top out the current, you will most likely have to lower the TX rate to allow enough time for this to occur. If you must use a higher pulse rate, then flat-topping may not even be possible. As usual, it's a bit of a trade-off, depending largely on what you're trying to achieve.

[waltr]

Here's a thread where we discussed flat-topping the coil current, and a simulation showing the benefits of allowing the coil current to top out -> https://www.geotech1.com/forums/show...top#post221739

However, this is not the whole story, as it's not always possible to flat-top the coil current due to the TX pulse rate being used. In order to top out the current, you will most likely have to lower the TX rate to allow enough time for this to occur. If you must use a higher pulse rate, then flat-topping may not even be possible. As usual, it's a bit of a trade-off, depending largely on what you're trying to achieve.

Thanks for the link to that discussion George. Good review.
I was trying to keep some basics simple for mushaba so he can get his detector actually working first.
I do believe he had the MOSFET going into Avalanche and causes issues. A series R helped there by reducing the total magnetic field in the coil.
He now has a lot of additional stuff to think about and try.
I try to keep with first order effects in simple explanations. I know there are many more orders that increases the complexity and can make the mind spin.

You say TX Rate but do you actually mean TX pulse width (with TX rate a side issue of having enough time between TX pulses, duty cycle, to measure decay)?

mushaba,
This is part of why I suggested measuring coil current and trying different series R values.
Also check response to various targets with different series R's.

If you like experimenting I suggest get and build a second board.
Get the first one housed and one a full working detector to use to find great things. Then the second board one for experimenting.
I have one full working detector and three other boards I use for experimenting. When I find a circuit, or code (I use a processor for all timing), change that makes a good improvement I then mod the working detector.

[godigit1]

Hi Mushaba,
I had a little time to mess with my new coil, and indeed i was able to recreate the flat topped coil current. I wasnt saying Green or anyone was wrong I just had not seen that on my fet before.
But with a Mpp rev d (which only has the 740 fet) at stock settings 1000pps i had to push my coil voltage above 460v before it became that pronounced.
Im goint to have read the thread posted by Qiaozhi and thank you Green for posting this as well it would have just flown by me without seeing your waveform.
Thank you for posting this thread Mushaba its also helped me put a couple of the puzzle peices together or at least i think I starting to get something through my thick skull.

Hi waltr,
What you have done with the HH is awesome I think im going to have to learn enough code to adjust a pi, I have a HH just sitting there done and waiting.
Be Ready for lots of questions. LOL
Anyway on coil tc and charging, So if I have a 450 uh coil at 1.9 ohm =a TC of 236.84 for this coil and we want on time of three to five times that.
Which would be 710.52 us which is huge so we take a resistor with a value of 13.5 ohm add it in line and now the on time would be 99.48 us.??
Is that correct that would be the way to get a 450 uh coil to peak with a lower on time?
Thanks

[Qiaozhi]

You say TX Rate but do you actually mean TX pulse width (with TX rate a side issue of having enough time between TX pulses, duty cycle, to measure decay)?

No, I meant the TX pulse rate. If the pulse rate is too high, then there may not be enough time to allow the current to flat-top, and still leave enough time to do everything else.

[waltr]

Hi waltr,
What you have done with the HH is awesome I think im going to have to learn enough code to adjust a pi, I have a HH just sitting there done and waiting.
Be Ready for lots of questions. LOL
Anyway on coil tc and charging, So if I have a 450 uh coil at 1.9 ohm =a TC of 236.84 for this coil and we want on time of three to five times that.
Which would be 710.52 us which is huge so we take a resistor with a value of 13.5 ohm add it in line and now the on time would be 99.48 us.??
Is that correct that would be the way to get a 450 uh coil to peak with a lower on time?
Thanks

Thanks and please ask questions. My HH2 thread is open for anyone to ask and discuss, that is how we learn.
I and others have some threads in the Programming sub-forum in hopes of help people move to using micro-controller/processor instead of timing chips.
Ask there on those topics.

In general, yes, adding series R to the coil is a way to decrease the coil TC and get the current to 'flat top' before turning off.
This has been under lots of discussion and with some disagreement. George (Qiaozhi) posted a link above to one of these discussions.
What is more important is experimenting to see what works best.

No, I meant the TX pulse rate. If the pulse rate is too high, then there may not be enough time to allow the current to flat-top, and still leave enough time to do everything else.

Ok but I see this as duty cycle: TX Pulse time verse TX pulse rate.
If you keep the pulse rate constant and increase pulse time then TX ON duty cycle increases leaving less time to sample signals.

This is one of the design trade-offs in PI detectors. We want a high pulse rate for better response and sensitivity but this then requires a short pulse time with limits the coil TC if we want a 'flat-top' coil current. Or with a longer pulse time we must decrease the pulse rate to be able to do the sampling.

This then gets back to the do we need a 'flat-top' current? I have studied the SD2000 schematic and timing. They do two TX pulse widths with the same coil series resistance.
Then have short and long sampling times. I do not see how they get a 'flat-top' coil current with the short TX on times so maybe it isn't required.
But seems that matching the pulse time to Target's TC is more important.

[Qiaozhi]

This is one of the design trade-offs in PI detectors. We want a high pulse rate for better response and sensitivity but this then requires a short pulse time with limits the coil TC if we want a 'flat-top' coil current. Or with a longer pulse time we must decrease the pulse rate to be able to do the sampling.

This then gets back to the do we need a 'flat-top' current? I have studied the SD2000 schematic and timing. They do two TX pulse widths with the same coil series resistance.
Then have short and long sampling times. I do not see how they get a 'flat-top' coil current with the short TX on times so maybe it isn't required.
But seems that matching the pulse time to Target's TC is more important.

As you said, everything is a trade-off. A lot depends on what you're searching for. If it's small gold nuggets, then you may want to increase the TX pulse rate, and use a series resistor to limit the maximum current in order to achieve flat-topping, but (as a consequence) lose a lot of depth. In that case it's probably unavoidable with the standard PI setup. For coin searching, cutting off the current before it flat-tops may be the best solution if you don't want to lower the TX pulse rate. For instance, try 1000pps with a 100us pulse width and a 3R3 series resistor. Then do the same test (same pps and pulse width) but with a suitable series resistor to allow the current to flat-top. The former test will have the best result on coins, as the magnetic field will be much greater, and it more than compensates for the opposing field from any eddy currents in the target.

[mushaba]

Thanks a lot guys the amount of information is huge and precious,please give me some time as a learning curve..My MPP does work but I want to get the most out of it,it's already has enough detection depth even with my erroneous coils..I see the hole build more as a new field lesson for me and at the end of this course I will have a working detector that I understand it's working at it's most part.I know I won't get it all at once but I'm a good learner,still in a field I've never been involved with..I work with Atmel mcus,I have my own old stk500 but it does it's job perfectly for my needs so far,I work most project of mine with the mcu assembly and for more complex designs I use Bascom.Happy to help if I can..I must admit when I started involving with mds the hammerhead (5 version) caught my eye because it was a complete project with lot of exlpanation and lot of flexibility...A friend guided me to Svens Mirage which I think that is also a great project,loved George's Crossbow and ended up with MPP due it's two stage receiver..Also have a Delta Pulse Legend that I don't think I will work with because no I structions are available.The gift for me is knowledge,and I thank you all responding or no for all the usefull things you post here in geotech..Now days there are lots of good MD with a price range affiortable but as Steven Tayler signs in Amazing "life is a journey not a destination"...Just today I received my ptfe silverplatted wire and I'm anxious to go to my homeshop and get down to it...Stay tuned and sorry for writing too many of my thoughts and also sorry for my bad English,I think hundrends of braincells are destroyed when you are reading my posts...