Friday, April 5, 2013

Making the air core plastic toroids.

Making the air core plastic toroids.

when i started the homebrewing, in the local market we had penty of the ferrite cores available as used in the 2 band radio receivers. I used extensively in my projects. Once i got the empirical vales of it everythig was piece of cake. But times chages. From the market all ferrite cores vanished suddenly.

In my TX-RX i used the 4 mhz as IF and Oscillators. There was no little pot variable capacitor in local market. My gosh what is happenig to the electronics things ! I have worked Valves, Transitors, IC, chips but i like making things with few components in old ways.

For my 80 meter receivers, i needed band pass filters, tuned tanks and everythig else was Xtal controlled. I thought why not make it instead of frustrating or buying expensive thing from DX. You will require frequency couter and Multimeter and also few tools.

Okay lets begin.

Choosing the PVC pipe:-

I choosed the PVC water pipe. It was coffee white. Water plumbig pvc pipes are thick and electrical pipes are thin. Pipes outer dameter was __ inch. I wanted a standard material which can be available in the local market and with time don't vanish like water tap washers or ferrites !

Making Plastic rings:-

In the press i put the pipe and cut with the hack-saw. Smaller the hack-saw finer the cuttigs. You will require less polishing. Now, mark the tubes lengh say abot 4 mm. after marking cut the tube gently,slowly also rotate it. If you work slow and carefully here better it will be output. Also check that hack saw is cutting in a line. Now, your plastic toroid is almost readdy. Polish it with emery-paper. don't use emery stone or metal-file, our plastic will damage them. See that there is no rough edges feel it with your hands. Using blunt metal road polish the edges. Atleast make more than 5 rings of exact size. Now, Choose your rings of equal sizes. Don't be disappointed, this little mechanical work is the tricky part and once it is over you will like it and will be expert.

Copper Wire:-
I used 30 swg wire of10 meter lengh for 4 rings, 1 meter was left. i.e each one requires about 3 meter of wire.

Making shuttle:-
For a sigle turn imagine you need to pass 3 meter wire each time across the toroids. how horrible ! First time i made this way and got tired. but, there is a cool wayout. Take a soft wodden spoon used in ice-candy. smooth it with emery-paper and make the both ends square. Now carefully cut the figure of V in both ends. You can also use 10 cm long 1 cm wide thin paperboard/cardboard. Spool all the wire say 10 meter on this shuttle.

winding the toroid :-
What is the problem ! Count the number and don;t forget it, don't twist the wire, see that wire is not knotted during the shuttle pass, leave 2 cm wire tail at start or put 3 extra turns and unwind it later.

Take toroid in left hand,take shuttle in the right hand and pass the shuttle, count the turn, slightly pull the wire (not the shuttle) once again move it. Slowly work it. You wil reqire about atleast half an hour to carefully wind a toroid.

Inner diameter and outer diameter are different so the coils turns will touch each other in inner side and there will be gap in outerside. so I wound 10 turns and gave the little space. See the pictures. This also makes the counting easy.

When finished you ought to put atleast 1 cm gap between both ends of coils. Well as per my empirical calculations this coil resonates about 4 MHZ with 330 pf.

When coils are ready you got to scratch the wires warnish with help of emery papers. sharp / blunt blade accidentally cuts the wire !

Testig the coils:-

DC test-Test for continuity.
Visual test- Test that there are no knots, turns overlapped.

On the finished toroids you can put warnish or put plastic adhesive tap over the ends turns so they don't slip. Fresh wire don't generally slip.

Now, you don't know the coils resonance and there is no way to tune it. For this we will use the  oscillator. Put this coil and 330pf as indicated in the oscillator circuit. Measure the frequency with freq couter. Now determine the approximate values of C for your desired freq and fine tune it. With permenant ink Mark the toroid # serial number and write down all the values on papers. Say for 4 mhz your toroid needs 330pf plus 22 pf, another needs only 330 pf and another needs 270pf plus 47 pf wow ! each toroid has different personality.

Now with values you found out, use it in your circuits. These toroids are good for band pass filter where exact tuning is not required and they are not noticeably influenced by shields, metals or other circuits.

i.e at preset i don't know how to wind secondary and take output from the secondary. so, i use taps. At the windig time at the tape point twist the wire on itself 1 inch and wind the turns like nothig happened. Then remove the enamel with emery-papers.

Please note that you will require emery-paper, hack-saw, bench-, file as mechanical tools. Also the frequency counter, multimeter absolutely. I have published this paper with the intention that others can also use this. this vales are for 80 meters use and hope to derive vales for 20 meter with smaller tube or larger gauge wire ! 73 de VU3INJ.


No comments:

Post a Comment