Vallon VMH3CS Mine Detector

[Altra]

Looking at the user manual, there doesn't seem to be an obvious way of successfully triggering the scope without using the TX oscillator as a source. If you had access to a digital storage scope, it would be a simple matter of either using the Single Trigger Mode, or pressing the Run/Stop button to freeze the display.

If your scope has trigger "Holdoff" control you can adjust this to ignore all trigger sources during a time period. On my Tektronix digital scope, it can be set by uS or mS increments. In this case you would set it to just past the opposite polarity tx, or about 600us

[rickb]

Have you tried using the trigger delay functions? The right combination of slope, polarity, level, and delayed triggering can work wonders at times.

Rick

edit: forgot about the hold off. There is always a way out.

[Qiaozhi]

If your scope has trigger "Holdoff" control you can adjust this to ignore all trigger sources during a time period. On my Tektronix digital scope, it can be set by uS or mS increments. In this case you would set it to just past the opposite polarity tx, or about 600us

The Tektronix 2445A does have a trigger hold-off function, so that's a possible solution.

[ivconic]

Again; an inductive coupling is the way to go.
I would make same diameter, same inductance and same resistance copy coil and hook it on scope.
Than place in same plane as original coil and perform measurements.
Most probably it will need copy coil to move relatively to original slightly offset, but it is easy observable on scope display.
That's for measurements when you don't want to mix up into original circuit nor affect origin signal.
...
Ferric.. 77 is just number, meaning nothing.
Keep up the good spirit and you'll easily see many more numbers.

[nickel_n]

I take it easy now and don't look for work where I am under any pressure. When you reach 77, the years go by much quicker and sometimes I will go under the radar for a while doing different things and then something fires up the interest in hobby detectors with slightly different technology and I'll be back, like now.
Eric.

I would like to take the opportunity and say THANK YOU Erick

For all that your work offer to people like me over the years.

Thanks again

[Ferric Toes]

The Tektronix 2445A does have a trigger hold-off function, so that's a possible solution.

Thank you for those suggesting the HOLD OFF control, it works great. Now I can see some meaningful waveforms over positive and negative TX pulses. This is also confirmed in the manual for the scope, which is something that usually gets read as a last resort.
There is no indication so far that it is anything but pure PI. If I hold a chunk of steel close to the coil, I can even see the normal decay. Now to build a diode clipping network to get rid of the flyback transients and see what is going on at lower levels.

[bbsailor]

Eric,

I want to thank you for sharing your technical knowledge on various forums over the years. It was your forum that taught me about PI detectors and motivated me to write my "Making a Fast PI Mono Coil" article with the encouragement of Carl who runs this forum. It is good to see you exploring different PI designs again.

Thanks

Joseph J. Rogowski

[Ferric Toes]

Here are a couple of shots of the coil current on the Vallon detector with 0.1 ohm series resistor.

Bipolar pulses.

Negative pulse expanded.

[mickstv]

Hi Eric, the picture below is what I got when I built my CCPI TX.

Top waveform current pulse measured across 0.1 ohm resistor.

[Davor]

Here are a couple of shots of the coil current on the Vallon detector with 0.1 ohm series resistor.

It appears there are both acceleration and deceleration mechanism involved. There are steep, but near-linear current slopes on both sides. Interesting.

[Ferric Toes]

It appears there are both acceleration and deceleration mechanism involved. There are steep, but near-linear current slopes on both sides. Interesting.

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

[Ferric Toes]

Eric,

I want to thank you for sharing your technical knowledge on various forums over the years. It was your forum that taught me about PI detectors and motivated me to write my "Making a Fast PI Mono Coil" article with the encouragement of Carl who runs this forum. It is good to see you exploring different PI designs again.

Thanks

Joseph J. Rogowski

Thanks Joe, I'm enjoying coming back and it stimulates the mind. Good to see that many past friends are still on this excellent forum.

[Ferric Toes]

Hi Mick, Your waveform looks the cleanest. There are some odd splurges on the Vallon one, so I suspect there are other things going on in the background.

[Davor]

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

My thought exactly. You may observe that current continues rising past the initial kick start. Seem like a perfect way to employ a snubber. In a unipolar machine it would require lots of effort to reverse polarity, but here it fits perfectly. You need to burn some energy stored in a snubber capacitor between cycles for it to run well, so there you have it.

I wonder if there is a difference in timing when ground balance is on. I'd expect to see some if Paltaglou principle is involved.

[Carl-NC]

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

I think that's exactly what they are doing. It's not 100% efficient, ergo the "tilt" during the on-time.