potential problem

[gef12]

Hi All
Have borrowed a scope and for the first time am looking at my HH opt 2.
I have the mono coil (coiltek 10" eliptical) working fine. But seem to be having probs with my Minelab 11" dd coil. I have been varing the Damp resistor and not seem to be making much difference can see it swing both ways thou can someone tell me whats happening here.
It seems a little low on the fly back also the fly back on the mono is approx 170 v but is much less with the dd.
cheers all

[gef12]

potential problem

Here are the pics
gef

[bbsailor]

DD Coil Damping

gef12

With a DD coil configurationj, each coils need a damping resistor. In the DD configuration, make sure that you add R14, the damping resistor for the RX coil. Make R14 is around 820 ohms and show the results on the forum.

bbsailor

[gef12]

potential problem

HI BBsailor
Ye when i said that i had varied the damping resistor I meant to say that i had played with both rx and tx.
Anyways i have done as suggested and inserted 820 ohm parallel combo on the rx side.. I have heard that the minelab dd has damp resistor in side the coil shell .. ok but have also paralleled that with 6k ohm .. this was giving me max sensitivity ..
gef

[gef12]

potential problem

Hi
ALso have added 20 ohm resistor in series with FET to reduce coil current and help limit ground noise.
cheers for now
gef

[bbsailor]

gef12,

The damping resistor can be located
1. on the circuit board
2. inside the coil
3. in the connector
4. a combination of on the circuit board and with a smaller .25w located in the connector to trim the damping for a particular coil when many coils are being used.

Attach the oscilloscope to the output pin of the first high gain amplifier stage. There should be no ringing visible on the oscilloscope. Place a 5K pot in parallel with the damping resistors and adjust for no ringing. Then substitute with a fixed value. You need to adjust both the TX and RX damping resistors.

The benefits of a DD coil compared to a mono coil are
1. smaller active window in the overlap region is less sensitive to mineralized ground;
2. RX coil can have about 1.5 X the inductance of the TX coil to make it a little more sensitive;
3. When the DD coil is balanced or close to balanced there will be less voltage on the input of the amplifier. This means that DD coils allow a little faster sampling;
4. The RX damping resistor does not see the capacitance of the TX MOSFET so its value is typically a little higher than what would be found in a mono coil.

What type coil are you using in the picture below?
Diameter?
Inductance? Each coil
Resistance? Each coil
Cable length from coil to PI control box?
Operating Frequency (PPS)?
Pulse Width range?
Delay range?

The 20 ohm series resistor will reduce the current in the TX coil and keep the flyback voltage around 100 V. This will also help you sample a little faster. The final PI and coil design depends on what type, size and depth of metal that you are seeking. Low PPS ranges, large coils and large coil currents are good for relic seekers. Higher PPS ranges, smaller coil in the 10 to 12" range and smaller coil currents typically in the one amp or less range are good for coin hunting and gold hunting.

What is your detection range on a nickel (US 5 cents) and a one inch square piece of household aluminum foil?

bbsailor

[Carl-NC]

Is this with a RevD board?

[gef12]

Hi BB n Carl
Firstly to relax your mind Carl no is not Rev D, sorry to alarm you.
The wave form seems to have cleared up. Yes BB am familiar in the fact that tx and rx have to have damp resistors. I was under the impression that the ML DD coils had damping resistor in the coil shell so was working around that before i could use the scope I was only doing sensitivity tests and adusting tx damp resistor that aways so I kinda worked out that aprox 6k in parrallel with what ever was in the coil housing was good for it ( I was wrong).
Puting the 5k pots on tx and rx side and adjusting both was the key. Was able to get the ringing (now that I know what it looks like on a scope) minimized. I ended up puting an 850 ohm as the damp resistor , so I am now thinking that perhaps there is no damp resistor in the coil housing. The answer to your other questions is unsure of inductance the tx is around 1.5 and rx is 17 ohms , length of cable after I shortened it is 42 inch , coil is 11 inch, op freq is 1400hz, I have got the delay down close to 8us on the DD and is 10 on the mono. Detection distance for nickel 8" and 7" for alluminium 1" square foil.
I have circled an area of concern on the uploaded pic ... is this normal for a DD coil ????
Thanks again BB for your help Great learning tool this forum.
Cheers all
gef

[gef12]

Problem fixed

Now I can put the 20 ohm resistor back in circuit cause I talk it out to alleviate possible problem. So those measurements taken were without 20 ohm resistor.
thanks again
gef