PI pulse timing frequency and pulse width question.

[DanZ]

I'm sure this has been answered many times but haven't found it on here.
Is there an advantage using a specific freq/pulse width setting vs. others? If I set the freq. setting higher and width wider I can get up around 500 mA current draw on the batteries but set the freq. at it's lowest and width down as well then current draw drops to around 130 mA. Big difference in hunting time before batteries need recharging. I realize you can tweak the settings to get around nearby interference but just curious if there are any other advantages of using specific settings.
Thanks in advance and regards.

[porkluvr]

It is a good idea to adjust your PW to have a period long enough to allow your coil current to very nearly stabilize so that rate of change, is low, before transmit pulse ends.

Letting PW go for any longer than that will amount to wasted power.

On the other hand, ending the transmit pulse before coil current has time to closely approach its peak, will also cause some transmitted power to be wasted because the charging current will be in a direction that opposes the flyback current, and it is only the flyback current which serves to illuminate the target. There are some exceptions but I cannot elaborate on that.

To expand on what you already know about PPS, higher PPS should usually equate to deeper (and faster) response.

I imagine there is somebody who can give a much better explanation on this.

[bbsailor]

It is a good idea to adjust your PW to have a period long enough to allow your coil current to very nearly stabilize so that rate of change, is low, before transmit pulse ends.

Letting PW go for any longer than that will amount to wasted power.

On the other hand, ending the transmit pulse before coil current has time to closely approach its peak, will also cause some transmitted power to be wasted because the charging current will be in a direction that opposes the flyback current, the and it is only the flyback current which serves to illuminate the target. There are some exceptions but I cannot elaborate on that.

To expand on what you already know about PPS, higher PPS should usually equate to deeper (and faster) response.

I imagine there is somebody who can give a much better explanation on this.

Porkluvr, and anyone interested

The flyback voltage is simply an artifact of the fast turn-off of the TX pulse. See Eric Foster's explaination of this commonly misunderstood issue. http://www.findmall.com/read.php?34,...51#msg-1119751

I hope this helps clear things up.

bbsailor

[bbsailor]

Here is another interesting discussion about the flyback voltage from back in 2002. http://www.findmall.com/read.php?34,...567#msg-129567 It seems that coil size, target TC, TX pulse current, and coil discharge TC (closely related to damping resistor value and coil circuit capacitance) all relate to optimizing your PI design for specific targets. Also note that coil sweep speed will determine how many pulse signals from a target may be integrated while the target is in the TX/RX field.

bbsailor

[porkluvr]

Thanks you for the informative links, BB. Eric's the Mac Daddy of PI technology so it is always good to read what he has to say.

I have been trying to absorb Eric's teachings and in my own mind I think I have it straight. I did already know that a sudden voltage rise and then its subsequently falling to zero, is only an artifact of the coil's magnetic field suddenly collapsing. It is a visual clue that something is happening, but is not the meat of the matter.

I've had to step back and re-consider the words of some of my recent rantings. After a little thought, I have realized that my choice of words "flyback pulse", while it's been around a while and easy to say, does not accurately describe the process taking place and does nothing to dispel a common misunderstanding (and one that I held as a view in the not too distant past).

Come to think of it, "Flyback pulse", brings an image of the voltage waveform to mind. A better description can be worded as "sudden collapse of the magnetic field".

"Sudden collapse of the magnetic field" is unwieldy, but I do want my mumblings to be more accurate, and I had better get used to it or find some other words that don't drum up misunderstanding. Or maybe it would be better to post links like you have done, so that information is available directly from the source.

Let me try and put this in a nutshell: (Please correct me if I am wrong).

After >3tc (coil circuit time constants), coil current has reached its peak value and any targets within the magnetic field are saturated as much as possible.
Shutting off the transmitter (hopefully in a brick wall fashion) causes the magnetic field to suddenly collapse, and it is the strength and suddenness of the collapsing field which causes eddy currents to be generated in the target. Those eddy currents are what the coil picks up and sends to the receiver.

Simple, but the wording is critical.

[green]

[Thanks you for the informative links, BB. Eric's the Mac Daddy of PI technology so it is always good to read what he has to say.\

Thanks, I'll second that. Some circuits I think I understand, with coils and coupling my brain is made of rock. Maybe if I read enough it'll sink in.

[surfdetector]

Porkluvr, and anyone interested

The flyback voltage is simply an artifact of the fast turn-off of the TX pulse. See Eric Foster's explaination of this commonly misunderstood issue. http://www.findmall.com/read.php?34,...51#msg-1119751

I hope this helps clear things up.

bbsailor

This link appears to have been lost on the Findmall site does anyone have another link to this content as I would like to read the explanation. Thanks.

[6666]

This link appears to have been lost on the Findmall site does anyone have another link to this content as I would like to read the explanation. Thanks.

https://www.findmall.com/threads/201...lyback.150754/
The owners of findall have changed things around and I had to rejoin, I did a quick search and it may be this

[surfdetector]

https://www.findmall.com/threads/201...lyback.150754/
The owners of findall have changed things around and I had to rejoin, I did a quick search and it may be this

Many thanks. That does appear to be the correct information.

[dbanner]

Eddy currents only appear in the target as a function of the rate of change of current in the coil, therefore, an expanding or collapsing magnetic field( lines of magnetic flux moving through /across the target)
A steady state current produces no eddy currents in the target.
The collapsing magnetic field at ramp down must also stimulate the target, but in opposite polarity. It is my understanding that the eddy currents generated during this phase is what is used for detection of the target.
But I have a feeling that there is much debate over the issue.