DD coil for Surf PI

[6666]

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

The TX and RX can be the same (350uH) and use the same damping resister value on both, if you start to change the values of the coil you will probably have to tweek the damping resister's.


To quote Reg

Hi All,

One of the question often asked is about the inductance of the receive and transmit coils. For simplicity, one can make the transmit coil 300 uh and the receive coil 450 uh. This works very well.

I have made both the transmit and the receive coils the same inductance and the coil worked well also. At that time, I was making both coils 300 uh. However, by making the receive coil 450 uh, one gains a little on sensitivity. It isn't much but every bit helps.

One can vary the transmit and receive inductance a lot and make the PI work well. The main difference will be the minimal delay that you will be able to obtain. The larger the transmit and receive inductance, the longer the delay before sampling will be needed.

A DD design separates the transmit from the receive which makes it easier to get faster or shorter sampling times, so you can get shorter delays with a DD than with a mono with the same inductance. This is one of the advantages of the DD. Also, the ground signal is less on a DD, so there is less ground signal at shorter delays.

As for winding and aligning a DD coil for a PI, just use decent winding techniques for each set of coils. Alignment can be done to minimize the signal out of the receive winding by injecting a signal into the transmit winding and looking at the receive winding with a scope. Adjust the overlap such that there is a minimal of signal out of the receive.

If no scope is available, then one can simply overlap the two with about 1" or more of overlap and try it. Be careful of polarity. Always use a non-ferrous target and set the polarity for a positive signal when the object is passed over the overlap zone. A nickel makes a good target. 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.

[Waikiki_Sweep]

PI detector with DD coil - Garrett Infinum LS has a damping resistor on RX coil inside coil housing - 680 Ohm.



May be it is placed in Infinum because coil is not balanced well.
To be sure it is good idea to check with oscilloscope signal shape after preamp.
Also strong signals from bad ground or big target may create oscillations (ringing) in RX. Why not?
So I vote for using damping resistor on RX.

How about to add preamplifier in coil housing (Minelab style)?

[drbean]

HI REG I HAVE THE SURF PI AS WELL COULD YOU DO A DIAGRAM TO SHOW
THIS MOD A SIMPLE ONE WOULD DO WANT TO FIT COIL TECH DD ON MY UNIT
HAVING A LOT OF ISSUES WITH GROUND NOISE AND KNOW DD LESS SENSITIVE
BUT ARNT LOOKING FOR .3 GRAMMERS LIVE IN BENDIGO VIC AND MY 2100 IS
MUCH QUIETER WITH DD.WANT TO GET MY KID OUT WITH ME BUT UNIT GETTING
TO MUCH GROUND SIGNALS WHICH IS DRIVING HER NUTS.TO MANY UNNEEDED HOLES DUG REGARDS DR

[Reg]

Hi drbean,

I am not sure what you want or which Surf PI you have since there are more than one pc boards to chose from. So, drawing a diagram could get confusing. Instead, I will give you specific instructions which should be clear enough.


Now, regardless of which pc board you have, you have to disconnect the transmit from the receive. In this case, if you look at the preamp (the 5534) you will see a 1k resistor connected to pin 3 of the opamp. The other end of this 1k resistor connects to the transmit transistor. This 1k resistor is the connection between the transmit and the receive.

So, you have to disconnect the end of this 1K where it connects to the transmit transistor by unsoldering it and then bend the resistor in the air, so only the end connected to the 5534 is still connected. You could try to find some open area on the pc board and drill a hole. Then you could bend the resistor such that the open end fits in the new hole. This end that is still free but now in a hole can then be connected to the receive winding of the dd coil.

Now, as mentioned, one wire on the receive winding will connect to the open end of this 1k resistor. If you are going to use any Minelab compatible DD coil, you will need a coil connector compatible which you will mount on the control box. Once that is done, you will connect pin 2 on this connector to this open end of the 1K resistor. Pin 4 of the connector will connect to the hole left when you removed the 1k resistor from the pc board. Pins 1 and 5 of the connector will connect to ground connection on the pc board.

So, when done, pin 2 of the new added connector is connected to the open end of the 1K resistor. This connects the receive winding to the preamp. Pin 4 of the added connecter connects the transmit winding to the detector. Pins 1 and 5 are the grounds needed to complete the coil wiring.

Once this simple change is made you should be able to use a DD coil. If you decide to build or use a mono coil again, you will have to make sure pin 2 and pin 4 are connected on the coil plug. This jumper will reconnect the transmit and the receive. On a ML compatible mono coil, this connection is already made in the plug.

One other change you may want to make is to speed up the autotune feature. This will be done by reducing the value of the autotune resistor. So, you can take a pot like maybe 1 meg pot and parallel it with the 220K that is part of the autotune circuitry. On the White's schematic this is R17. If the pot is connected such the center leg and one end is used, then the pot is adjustable and will adjust the autotune speed. The faster the speed, the less the ground will affect the detector, but some depth is lost doing this.

When using non ground balancing PI in bad ground, always keep the coil a few inches from the ground. The ground signal dies off much faster than a signal from gold, so you can still detect gold but won't have as much ground response to deal with.

If you have access to what is often called a salt coil, you can use that on the non ground balancing detector also. The salt coil is basically a ground canceling coil, so if held level, should do a greater job of reducing the ground response. If you decide to build a salt coil, you will build a mono transmit and then add a figure 8 receive winding. Make sure to add a damping resistor across the receive winding. Usually, a 1K is sufficient.

Let me know if these instructions and ideas make sense.

Reg

[drbean]

thanks reg have sm pro kit which is available on ebay from states works well
very sensative so far looked at grants pi project and he hooked up straight to machine
didnt mention resister so not sure if he is getting the advantage of a dd this board
has sensativity , delay , threshold and volume external pots and is a little easer to build
due to having more area to work with.tryed surf kit from silverdog and for life of me could not
get it to work .gave it to latrobe uni electronic students and they could not find any
problems with the build and parts positions really did my head in so bought this unit.
will give it a crack thank you but can you explain why he didnt have to cut 1 k
or has he missed the point regards dr

[bernte_one]

lorenz x5 has also am dmaping resistor on rx side i think it was 2k
tx around 300µH rx round 1000µH
nulling coil is same like vlf dd measure directly on rx coil and tune for minimum induced voltage smalller than 10mV

[drbean]

OK REG CONNECTED DD AND WORKING THE ONLY THING I DONT UNDERSTAND IS DOES THE
DAMPINING RESISTER GO IN SERIES WITH RX ? HOOKED UP MACHINE AND WORKING WITHOUT BUT
CAN UNDERSTAND THE RESISTER IS FOR REASON.VERY SENSATIVE HAVE COILTECH 5X10 DD ON IT
WILL TRY 14 INCH ROUND NEXT BUT NOT SURE WHERE RESISTER GOES AS YOU SAID ACCROSS
THE RX AND THERE IS ONLY 1 WIRE FROM RX TO PIN 2 THANKS FOR YOUR HELP.IM A BIT OF A NOVICE BUT
FOLLOW INSTRUCTIONS PREETY WELL . IF YOU COULD DRAW A PIC WOULD HELP . THANKS AGAIN NEALLY THERE! DR

[drbean]

ALRIGHT DONE SOME AIR TESTING WHAT THE? PICKS UP BOTTLE TOP AND SCREW GOLD HARDLY A TARGET USING 3 GRAMMER
NUGGET NOW I AM CONFUSED!

[Qiaozhi]

OK REG CONNECTED DD AND WORKING THE ONLY THING I DONT UNDERSTAND IS DOES THE
DAMPINING RESISTER GO IN SERIES WITH RX ? HOOKED UP MACHINE AND WORKING WITHOUT BUT
CAN UNDERSTAND THE RESISTER IS FOR REASON.VERY SENSATIVE HAVE COILTECH 5X10 DD ON IT
WILL TRY 14 INCH ROUND NEXT BUT NOT SURE WHERE RESISTER GOES AS YOU SAID ACCROSS
THE RX AND THERE IS ONLY 1 WIRE FROM RX TO PIN 2 THANKS FOR YOUR HELP.IM A BIT OF A NOVICE BUT
FOLLOW INSTRUCTIONS PREETY WELL . IF YOU COULD DRAW A PIC WOULD HELP . THANKS AGAIN NEALLY THERE! DR

Please switch off your Caps Lock key.
Did you know that typing your posts in capital letters is equivalent to SHOUTING?
Also, it makes your posts more difficult to read.

[Reg]

drbean,

If you are using a factory made DD coil built to be used on a ML PI, then it already has a damping resistor installed inside the coil so there is no need to add one.

If you decide to build your own DD coil then you will need to add a damping resistor across the rx leads inside the coil or connector.

The sensitivity of a DD coil should be a good as a mono coil on small stuff. Make sure the delay is set to minimum when trying to detect smaller gold.

Reg

[drbean]

thanks reg kinda figured as much in the end this thing is very senative with minelab coil will give it a crack
on the weekend will try to get a few gold targets with my 2100 and then run the surf over and see if it picks them up
i beleive the machine will react different to air test in the groun .will keep you posted many thanks for your help your a legend!

[tigershark_0]

Hi Alex,
I bought Surf Pi PRO yesterday, I assume it will arrive in a few days :-)
This DD, do you have any drawing/spec. how to mfg the coil?
I have looked around on internet, but can't find any DD.

I will also search in mineralized soil.


By the way, are some one at this forum using top of a crutch as handle for detector?


BR
Johansa

[drbean]

hi tigershark
if you shoot me a email ill send you a rough scetch of how to do the mod really easy
and works well with a 10 x 5 eliptical coiltech dd. email is [email protected] regards dr bean
oh which kit did you get the one from states or silverdog kit uk?

[6666]

DrBean please post your sketch here tnks

[Reg]

Here are a couple of simplified pics of what needs to be done. One is basically showing R7 has to be disconnected from the transmit circuitry and then the open end of R7 has to be connected to the receive winding.

The other pic shows the layout if a Minelab connector is used so you can use ML compatible DD coils. Keep in mind that there is a damping resistor inside the ML compatible coils across the receive winding.