DD coil for Surf PI

[xlev]

I've seen people having DD coils on their Surf PI's, but no one showed how they did the modification of the board to be able to connect
it to the board.

Has anyone here done it, and would be kind enough to share the info?

The reason I am interested, its because I think it would help with the high mineralization found here in Australia.

Any help would be greately appreciated

Regards

Alex

[Reg]

Hi Alex,

Changing the Surf PI so it can use a DD coil is extremely simple. The key is to separate the TX from the RX. On a regular Surf PI, the input to pin 3 on the preamp is connected to the transmit coil through a 1K resistor. On many schematics this is R7.

When wanting to use a DD coil, this resistor has to be disconnected from the transmit circuitry. In other words, one end of the 1K resistor still connects to pin 3 of the preamp but the other end will now connect only to the receive winding of the DD coil. Be careful because the polarity of the receive winding has to be correct. Get it wrong and the audio will decrease on a target instead of increasing. Use a non ferrous conductor target when checking.

So, depending upon the layout of the pc board, the free end of the input resistor (the end opposite the preamp) may simply stick up in the air, or the pc board may be cut if the free end through a pc board connection that only goes to the tx winding and is not a junction point or have other connections. In other words, only separate the end of R7 from the TX circuitry.

Finally, the receive winding will require its own damping resistor across the receive windings A typical value of 1K to 1.5K will usually work.

Many years ago, I posted a schematic of how this can be done on a different simple PI here on the geotech forum.

Reg

PS: Look at the Hammerhead schematics and you will see on Rev D1 where a DD coil setup is drawn near the preamp. So, comparing an earlier version of the HH detector and the D1 version, the addition of a DD coil can be determined. My discussion of adding a DD coil was for adding a DD coil to the HH.

[kt315]

The reason I am interested, its because I think it would help with the high mineralization found here in Australia.

look 174 page in Inside The Metal Detector.

[6666]

A bit like this.

[maikl]

A bit like this.

Hi 6666,
give me the link of the scheme...
Best regards.

[kt315]

there Reg told about DD in HH. you have to correct/to adaptive for/to fix his shematic according P-channel MOS-FET in Surf.

http://www.geotech1.com/forums/showthread.php?10729-on-DD-coil-on-the-Hammerhead&highlight=Hammerhead%26amp%3BDD%26amp%3 Bcoil

[6666]

Thanks for finding that thread Kt315

[6666]

MaikL
I just modified the original whites schematic based on Reg's information that he posted above, for Surf PI with two coils.

[xlev]

Hi 6666 could you tell us exactly how you modidify the Surf?Do you have a picture of the mod?

[Reg]

One thing I noticed on my basic schematic change to allow the use of a DD coil is I didn't include one important part and that is the damping resistor for the receive coil. Yes, both the TX and the RX require damping resistors. One can install the receive coil damping resistor in the coil, or as I have done, install the resistor in the connector.

Now, if you use the common 5 pin mic connector like those used on Minelab PI or the TDI then you can use coils that fit these two PI's also. If you decide to use this connector, then I recommend you also use the same pinout as what is found on the TDI or ML coils and that is, pins 1 and 5 are ground connections. Pin 4 is the TX and Pin 2 is the RX connector. Pin 3 is unused and can be left open for the time being. So, pin 1 and 2 are used for the coax for the receive and pins 4 and 5 are used for the transmit.

Yes, at some future time I will make recommendations for pin 3.

Now, if you want to use a mono coil, then all you have to do is to connect pin 2 to pin 4 and you are back to the original. This is why I recommend placing the receive damping resistor inside the coil houisng or connector. Because the signal is so small in the receive circuitry on a DD coil, a 1/4 watt resistor can be used.

BTW, if you already made a DD coil based upon my original drawing and didn't place the resistor in the coil, remember, it works as well installed in the connector housing and will fit fine.

Reg

[6666]

Hi 6666 could you tell us exactly how you modidify the Surf?Do you have a picture of the mod?

All I have done is provide the moded surf schematic,
I will try and draw the mod for you, if required.

r7 on silverdog PCB is connected at one end to the coil, the other end goes to the diodes,
All you do is unsolder and lift the coil end of r7 (1K) out of the pcb and solder one end of the new RX coil and damping resistor to the free end of r7 and of course the other end of the new RX coil and damping resistor is connected to GND.
Do you still need a drawing ?


Reg's plug

http://seeq.com.au/PIC/jaycar/PS2018_JC.jpg

[maikl]

MaikL
I just modified the original whites schematic based on Reg's information that he posted above, for Surf PI with two coils.

OK 6666

[xlev]

Thank you so much 6666. Could you tell me the value of the dampening resistor you used? Also with the coil,I know people say to keep the inductance of the coil around 350uH.With the DD coil,would you make the Tx an Rx each 175uH,or would each coil be 350uH?

Thanks everyone for all the help,great forum

[dimovnis]

hello. these are my coil. I tried with all possible cables and protection but he still reacts to the ground, I made sure more than 10 probes and only I have left to do DD coil. The depth of my surf is very good but what has that problem with the earth.

[kt315]

Yes, both the TX and the RX require damping resistors.

I think you misleading. RX is not needed in damping resistor. what aim this resistor does in RX if there is not self resonanse?? or it has be but in very small accidental form.
I offer to use a capacitor on RX side like you see in VLF coil construction, this is more effective against different noise, some kinda RC filter. a low ohms resistor in series, 24> ohm, and the capacitor. anyway, people, try it and report about your expirience you see.