1st ever DD coil.

[hoadlies]

Bind the turns tooth floss is good. or the waxed whipping twine sailing guys use.)

Add a few drops of cyano to stiffen it up!. Dont flood as you may need to bend the shape later without fracturing the enamel covering the wire!!!


The L and R does not alter with the cyano.


The L goes up a few % when you bind it - as the turns get closer - but dont stress as the final frequency is not a big issue. Plus you can tweak the freq, with the cap value later if you feel the need to get 6.57688976kHz ( I run mine higher to see smaller items 12.5KHz Tx with no issue )



Wrap the coil post glue, in pvc insul tape, then foil with a fine drain wire then pvc tape again - Dont forget, foil needs a gap or the Mag H field wont get out. Make sure you have a shell that will take your coils when you got all these wraps on them.



Dont pot the coils in the shell untill you know it all works - as wasteful. I would say if you just put coils in a shell and potted you would have a 5% chance of success.!!!


Tack Tx coil in the shell with, say 4 small spots of hotmelt glue -- then place Rx - dont glue this one.


Location in room is critical - dont sit your coils/shell on anything with metal in it -- trust me



Put scope on Tx coil and trigger scope on that ch - you will have approx 16v peak to peak +/-4v and about 6.5 kHz not that critical 6.5 to 7kHz is fine
( I use 12.5kHz Tx no issue)

Dont worry if slight kink in the sine wave as all have this minor non linearity - (if you view the AC mag field with a clamp on, its much cleaner)

Put Ch2 on the Rx preamp output. You will probably see 1-2volts pp initially. Scale both traces so you see both plots as similar amplitudes on screen

Adjust the timebase so you see only 1 or 2 cycles on sreen - so you can see the phase offset between the 2 traces..


You need to do a few things together here

1)Move the free coil about in the shell, while viewing the scope

You will learn quickly the how the amplitude (and phase) of the coupled signal varies with its location in the shell.


You need to priortise the phase relationship over amplitude

Builders who have working kit say that the Tx and Rx need to be 180 deg out of phase. Ive seen a few, and this phase relationship is common to all of them. You have approx a 20 degree slip either way.


So you need this phase offset AND a low amplitude Rx signal - again fortunately the Rx signal level is not super critical. ( I got lots of small coins with a 2voltpp null one time)


Rule of thumb is 200mV to 300mV of preamp output - if its lower, good.


If you get your 180 degree shift and a reasonble null - progress to testing

If you cant get the Rx sig phse anywhere near, desolder the Rx coil and swap the coil connections over.
Re adust in shell for 180

If your good,

Make sure not in pin point mode..

try to reject ferrite - use geb pot to find cut off.

try to reject clean small iron/ steel ( not plated , or ring, or large) using the DISC pot (cut the head of a small bolt perhaps, dont use iron that may be magnetised!)


If reject ferrite and iron then try on gold bronze silver etc.

you should get all alloys other than ferrite and iron.


If sens to mid size coin is about 40cm, and struggling to reject ferrite - add 2n2 to Rx coil/cap combo - to move Rx tank down away from Tx freq.


these are the starting points I use. It can be a real pain - some of us like the fight. S

HI G/NUT ,, THANKYOU for the comprehensive post above ,,,,,,,That will be really most helpfull ,The only Down-side is I only have an old Single CH -Scope !!!!

A LOT of good tip's you give on ''NOT POTTING COIL'',,untill testing properly [I may have overdone the Super-glue [BUGGA] I will see ,If so,,its Back to the Drawing board,,, HA.Haar..

I appreciate your input ''Most helpfull to a NEWBIE ,But an OLDIE as I am ..

CHEERS ,,,,,,,,BARRY in a Hot AUSTRALIA ..

[golfnut]

Im the wrong side of 45 so dont beat yourself up !! Keep bathing your feet in Fosters...


For the single channel scope, You may have external trigger.

Select Ext trig

If you do put that the trace will stop - as No trigger...


Now hook up the Tx signal to the Ext Trig input - make sure you dont exceed the recommended input volts for the Ext trig IN.


Shouuld be OK, now when you put the ext trig from your Tx in the trace should start up again as a green line

Yes?

Now temporarily put CH1 input to the Tx Signal also - fiddle about with timebase and amplitude and you should be able to get 1 cycly of Tx on screen (still triggered by ext trig)

Now remember what the trace looks like - and swap CH1 to the preamp out. You will need to crank the Amplitude on the volts/div - dont move the timebase or Tig in.

The diplayed wave will be different to the Tx referenc you viewed a minute ago.


If its 180 out - then your cooking make sure the level of this is 300mV or less approx - try waving ferrit iron then real alloys.



Do this with the coil away from metal

S

[hoadlies]

HI G/NUT ,, GREAT INFO I WILL GIVE IT A GO ,,tomorrow ..

OH Not yet as I still await my kit from the UK should be here soon ..
YES the scope has an exturnal trigger ..

OK I said the last post I would Bind-not glue---- the RX coil & test mH ,,I did that & I have36mH & 76..Ohms resistance ,,ITS approx 270-280 turns of 0.24mm wire ??????SHOULD I remove some windings ????or test as above ,if Ok do no more ..

YES MATE I MUST say the Heat is getting to me here [scrambles the old Brain-matter..]

CHEERS ,,MATE ,,,,,,,,BARRY..

[golfnut]

The coil you made will be fine Barry. Slow and steady mate.



btw its 18.4C in my living room and cold, grey and wet outside. Even the birds look pissed off.


We have pay for our heat !!! £40 gas AND £40 electricity - PER MONTH here its crap £40 (tv phone and internet / mnth) £100 /week food etc

plus we dont have gold all over the place. If we did you would not get any permission either!!!!!




S

[Valters]

Thanks Golfnut for explanation, but i have 2 more question. (IDX PRO DD 27,5 coil)

1. In the start point RX frequency are similar to TX frequency? Only if i have problem with ferrite rejection or metal disc I add a extra capacitors.

2. When i moving one of coil i see on my PC oscioloscope changes of signal amplitude, but phases ar changing only in 0-90 degrees. I swap Rx connectors on PCB and get the same result (0-90degrees). Is problem in the coil?
I will made a new coil, but as now in Latvia are -18C (-0.4F), I need a stong motivation to go to garage...