Minipulse Plus Rev My time has come!

[mushaba]

Hello everybody,as mentioned above I started my Mpp Beach Rev E.
Im stuck to step 4 because I don't have a coil yet
Posting some measurements..

[waltr]

Looking good so far.

I would just wind a simple temporary coil for testing. Get the inductance close (GeoTech's coil calculator works fine), typically 300 to 400uH.
Any diameter is good from 4 inch to 10 inch. A smaller coil is easier to handle during testing.
Leave the wires off the coil long and twist them together then connect to PCB. The twisted lead provide pretty good shielding.

[mushaba]

Made an 8" pattern to wound the coil



Made the coil from 0.50mm double enameled wire...Final specs 382 uH 2.5 Ohms unshielded.. Im using 390Ohm damping resistor.



Steps 1 to 5 looking good although at step 4 i measure spikes at 500V..

Step 6
adjusted the minimum sample delay of 10us.



Probe to TP8 the EFE delay pulse width i measure 114us not 100us as the documents says,is that right?
Probe to TP9 after PL6 shorted (i turned the pot to one end) the pulses are 15,6us not 10us...With gain pot i adjust from 15,6us to 30us....

Am i doing anything wrong??

[mushaba]

Probe 1 TP3 and probe 2 at TP9

There is a lot of noise or ringing,is that because my coil isnt damped as it should?

[godigit1]

There is a lot of noise or ringing,is that because my coil isnt damped as it should?


Try turning off any florescent lights anywhere near or other electrical equipment maybe.

[mushaba]

What is the correct way to damp the coil.I saw a schematic with 1,2Kohms in parallel with 1,2K and 4,7k in series,this should make it flat?
It detects a can of soda at 40cm,the weird is that it can't detect anything gold..

[mushaba]

There is a lot of noise or ringing,is that because my coil isnt damped as it should?


Try turning off any florescent lights anywhere near or other electrical equipment maybe.

Yeap,the coil is not properly damped because i dont know how it looks the output waveform on TP3.I'm gonna try put instead of the R4 the damping resistance setup i found on "Making a Fast Pulse Induction Mono Coil" by Joseph J. Rogowski.



Also the specs of my coil wire are:

.

Before i do the damping i tryed turning off the florescent lights as godigit1 suggested and the noise became smaller



Also the detecting lenght of a soda can increaed from 40cm aprox. to 60cm..

A short video presenting the build so far (with lights on)

https://youtu.be/fPpfm3D6Jqo

A video after i turned the lights off

https://youtu.be/_hjVybY9ZmU

Im pretty sure im doing something wrong with the timmings

Any help will be appreciated

[Qiaozhi]

Try rotating your coil to see if the noise on the pre-amp output signal changes. The noise is usually lower when the coil is in a horizontal (rather than vertical) position.

You need to adjust the damping resistor to critically damp the signal. The noise you can see on the scope is not the "ringing" you are trying to eliminate. After the pre-amp comes out of saturation, the signal will exponentially decay towards the 0V line. If the damping resistor is too small, this signal will overshoot the 0V line before settling down. This is what's called ringing.

To reduce the noise pickup, you also need to shield your coil.

[mushaba]

Try rotating your coil to see if the noise on the pre-amp output signal changes. The noise is usually lower when the coil is in a horizontal (rather than vertical) position.

Ok

You need to adjust the damping resistor to critically damp the signal.

What do we mean by "damp"????Not so good with this term

The noise you can see on the scope is not the "ringing" you are trying to eliminate. After the pre-amp comes out of saturation, the signal will exponentially decay towards the 0V line. If the damping resistor is too small, this signal will overshoot the 0V line before settling down. This is what's called ringing.

In my case i do not see this happens,am i right?

To reduce the noise pickup, you also need to shield your coil.

Im waiting foe a cooper tape i ordered,i only have a shinny type and i read that its not suitable for the faraday shield.


Taking measurements again and following the steps i've noticed two things:

1) At Step 6 TP2 pulse level is -12V and pulse goes to 0 ... TP6 level is at -5V and pulse goes to 0,is this correct?
2) At Step 7 TP4 the DC level goes to 0 when a target is near the coil , this is clearly wrong..

[Qiaozhi]

What do we mean by "damp"????Not so good with this term

If you remove the damping resistor and look at the pre-amp output, you will see lots of oscillations that will gradually die away. The damping resistor "damps" these oscillations so that the pre-amp output transitions smoothly from saturation to 0V. The key thing here is to achieve "critical damping". If the signal is slightly under-damped, it will overshoot the 0V line before settling back down. If the signal is over-damped, the exponential curve will move to the right. What you are trying to do is remove the huge signal induced by the flyback when the MOSFET opens in the most efficient way, leaving enough time to sample any target signals before they disappear.

In my case i do not see this happens,am i right?

Try fitting a higher value damping resistor, such as 2k. You should see it then.

Taking measurements again and following the steps i've noticed two things:

1) At Step 6 TP2 pulse level is -12V and pulse goes to 0 ... TP6 level is at -5V and pulse goes to 0,is this correct?

Yes, that's correct.

2) At Step 7 TP4 the DC level goes to 0 when a target is near the coil , this is clearly wrong..

That's not correct.
I would suggest comparing the pulse widths at TP9 and TP10. They should be the same width, but occur at different times.
Do you have Q1 and Q3 inserted correctly?

[mushaba]

Think i found the root of my problems..

At first i dumped my coil near 990 Ohms

Now,at Step 6 says 1ch at TP2 and 2ch at TP6.According to Fig.10 there should be a delay (Main Sample Delay) of 10us.As you see from the photo below the two curves have no delay between them...Thats probably why i dont sample gold right?

[mushaba]

Also found at Step 5

[godigit1]

Your coil Is under damped a bit. One reasone you are not seeing gold beacuse you are using a small chain. Even a good VLF has a hard time with this target. A metal detector only sees one link in the chain and the one closest to the coil. Your coil is 20 guage wire thats a bit thick in my opinion
Ive found chains but always by the clasp the chain porblematic.
Use a ring to set up.
This video from another member Tibuck19 shows how minute the voltage change is at the integrator for gold. its hard to see.
Your can is huge that is why you see a full response and the gold is invisible.
https://www.youtube.com/watch?v=OyS8sfv0XJ0&t=302s

[godigit1]

Also you should hook your coil up directly you test cable is way too long.
Good Luck

[homefire]

Yep, something about the Links in chain. It's like a small circular Transformer Coil Cancels out the Eddy Currents . If the chain has any type of Fob or Charm they show up way better then just a small chain alone.