Minipulse Rev D first attempt

[Impulse]

I believe so but I would not change R3 until the others have chimed in!

Rayjay i will try this once i have time and resources (once i get back from france i will order some resistors and post what happens)

The sun is out, so is my motorbike, coincidence? naaa!

[Old cart]

ok so if i change R3 to i higher value (say 6hom) then i should see a slight 'flat' spot once i reach the correct value? and yes the transmit pulse is 46-48us the build doc says it should be 58us so not a massive difference but i will correct it later because it will be one less thing that could be wrong or causing an issue.

I believe so but I would not change R3 until the others have chimed in!

[Rayjay]

If you change R7 from 1K2 to 1K5, that should give you 56-58us.

[Impulse]

Well, I have never looked at coil current rather than coil voltage but I think what we are looking at here is 48uS transmit pulse followed by a 5uS turn off time. The current does not flat top so the coil is still charging when it turns off. Assuming I am right I think it means the coil has too much inductance for the amount of resistance it has to allow the coil to fully charge.
I have always heard that if the coil is still charging when it turns off then the charging induced field partially cancels out the huge rate of change field when it turns off.
Stated another way the curve must be flat or almost flat to have max transfer of magnetic field into the target. There are several ways to do this:

1. Increase the transmit pulse width substantially.
2. Decrease the coil inductance.
3. Increase the series resistance by either winding a coil with thinner wire or installing a larger value R3, maybe something like 6 ohms.

A little bit of each.

If I am reading this right you have about 3.5 V/3.3 ohms or a little over 1 amp of current flowing at the peak. If I am thinking clearly you could go as high as 10 ohms for the R3 and coil combined resistance and still achieve this same peak current, but the transmit pulse width would have to be 130uS. With the current 390 uH coil and r =3.3 it will take almost 400us for the coil to flat top.

This is all just thinking out loud and I have no idea how to calculate the effects of a possible current and magnetic field flux change with respect to target stimulation so I encourage comments.

ok so if i change R3 to i higher value (say 6hom) then i should see a slight 'flat' spot once i reach the correct value? and yes the transmit pulse is 46-48us the build doc says it should be 58us so not a massive difference but i will correct it later because it will be one less thing that could be wrong or causing an issue.

[Old cart]

Coil L/R transmit pulse optimization

hey guys sorry i took that scope measurement from the wrong place (duh!) so here is R3 scope image

[ATTACH]35835[/ATTACH]

Well, I have never looked at coil current rather than coil voltage but I think what we are looking at here is 48uS transmit pulse followed by a 5uS turn off time. The current does not flat top so the coil is still charging when it turns off. Assuming I am right I think it means the coil has too much inductance for the amount of resistance it has to allow the coil to fully charge.
I have always heard that if the coil is still charging when it turns off then the charging induced field partially cancels out the huge rate of change field when it turns off.
Stated another way the curve must be flat or almost flat to have max transfer of magnetic field into the target. There are several ways to do this:

1. Increase the transmit pulse width substantially.
2. Decrease the coil inductance.
3. Increase the series resistance by either winding a coil with thinner wire or installing a larger value R3, maybe something like 6 ohms.

A little bit of each.

If I am reading this right you have about 3.5 V/3.3 ohms or a little over 1 amp of current flowing at the peak. If I am thinking clearly you could go as high as 10 ohms for the R3 and coil combined resistance and still achieve this same peak current, but the transmit pulse width would have to be 130uS. With the current 390 uH coil and r =3.3 it will take almost 400us for the coil to flat top.

This is all just thinking out loud and I have no idea how to calculate the effects of a possible current and magnetic field flux change with respect to target stimulation so I encourage comments.

[Qiaozhi]

hey guys sorry i took that scope measurement from the wrong place (duh!) so here is R3 scope image

[ATTACH]35835[/ATTACH]

That's better!
In that case the coil current rises up to around 1.05A, and is then falls rapidly once the MOSFET is turned off.

[Impulse]

hey guys sorry i took that scope measurement from the wrong place (duh!) so here is R3 scope image

[Qiaozhi]

ok so i get 0.099vdc across R3 with my multi meter and i put the scope across it too

here [ATTACH]35822[/ATTACH]

Are you sure you have the probe connections in the right place?
You should see the coil charging curve during the TX-on time, followed by the discharge curve during TX-off time.
Or perhaps you're powering the detector from a bench supply? In which case you should check that the scope and power supply grounds are not electrically connected.

[baum7154]

[QUOTE=Impulse;215935

i thought it best to start at the beginning as this is the first thing during the build that i found that is out of 'spec' i cant test my gold ring as i am charging my battery but the board does seem to be slighty better at detecting the gold ring.[/QUOTE]
---------------
You are correct that the gold ring is a stronger/easier target to detect. In fact a closed ring geometry of any metal sets up strong eddy responses that are easily detected by a PI detector.
Good luck,
Dan

[Old cart]

ok so i get 0.099vdc across R3 with my multi meter and i put the scope across it too

here [ATTACH]35822[/ATTACH]

Not sure how to interpret that waveform.

[Impulse]

Q of course. My bad. Impulse, try measuring the voltage across R3. Since Voltage across a resistance is proportional to current this will tell you about the current passing in the coil.
To measure the coil ringing you can monitor the output across the coil, except I recall your scope being limited in the voltage it will tolerate, or the output of the preamp. I think this is TP3. What you are looking for is a smooth return to 0 volts of the flyback pulse ( the one immediately following the transmit pulse). Maybe someone has an example of a correctly damped minipulse coil.

ok so i get 0.099vdc across R3 with my multi meter and i put the scope across it too

here

[Old cart]

TP1 is the TX oscillator output. You won't see any "flattening out" on this signal.
In order to see the behaviour of the coil current during the TX-on time, you need to put the scope across R3 (3R3).

Q of course. My bad. Impulse, try measuring the voltage across R3. Since Voltage across a resistance is proportional to current this will tell you about the current passing in the coil.
To measure the coil ringing you can monitor the output across the coil, except I recall your scope being limited in the voltage it will tolerate, or the output of the preamp. I think this is TP3. What you are looking for is a smooth return to 0 volts of the flyback pulse ( the one immediately following the transmit pulse). Maybe someone has an example of a correctly damped minipulse coil.

[Qiaozhi]

I don't know Minipulse well. If that is the transmit pulse width I don't think that would make much difference, if any at all. The narrow TX pulse width will reduce sensitivity only to long time constant targets like iron or silver coins. Narrow pulses are better for gold jewelry.
Do you see how the voltage curve is starting to flatten near the end of the pulse? That indicates the coil current is no longer rising so the coil has achieved a steady state mode limited by the coil resistance. This is good.
The general shape of the pulse also looks good.

TP1 is the TX oscillator output. You won't see any "flattening out" on this signal.
In order to see the behaviour of the coil current during the TX-on time, you need to put the scope across R3 (3R3).

[Qiaozhi]

ok so the scope is working well heres a pic of TP1 the pulse width is 46us how can i adjust this to be 58us?

[ATTACH]35821[/ATTACH]

You can adjust the value of R7 (1k2). I expect the difference is due to capacitor tolerance.

[Impulse]

I don't know Minipulse well. If that is the transmit pulse width I don't think that would make much difference, if any at all. The narrow TX pulse width will reduce sensitivity only to long time constant targets like iron or silver coins. Narrow pulses are better for gold jewelry.
Do you see how the voltage curve is starting to flatten near the end of the pulse? That indicates the coil current is no longer rising so the coil has achieved a steady state mode limited by the coil resistance. This is good.
The general shape of the pulse also looks good.

hey oldcart yes i see the 'flat' spot on the pulse its good to know that the basic coil i have is doing ok how do i check that there is no ringing? ,

i thought it best to start at the beginning as this is the first thing during the build that i found that is out of 'spec' i cant test my gold ring as i am charging my battery but the board does seem to be slighty better at detecting the gold ring.