What happens when you change the AWG of the coil?

[ChrisHansen2Legit2Quit]

My schematic calls for 10 turns of .25 mm (30 AWG) at 15 mm across, 3 mm height. This is the search coil.

My schematic calls for 120 turns of .25 mm (30 AWG) at 12 mm across, 50 mm height. This is the reference coil.

It's a BFO detector.

I see magnet wire ranging from 18 AWG up to 40 AWG. At increments of 2 (18, 20, 22, 24, etc...) & at $20 each, I'd go broke getting all the values.

I do have on hand 24 AWG. I assume I can substitute, but is it worth to change my winding turns?

*Search coil will be oscillating at about 104khz, with an amplitude of about .5v p to p.

Thanks.

 

[Darktrax]

There are online calculators for coil inductance, but when there are as many as 120 turns, then honestly, provided you do not care that the final physical size of the coil made with the thicker wire will be a bit fatter, I think it should still work.

Thicker wire gives lower resistance, so higher Q-Factor.

Inductance is mainly decided by number of turns, and the radius of the coil assembly.

Also, how long is the coil, and whether it is multilayer. If you used AWG#24, but stayed with the number of turns, you probably made only a more efficient coil.

Where the units are inches, here is one formula..
L = (Radius^2)*(Turns^2)/(9*Radius + 10*Length)

There are so many links to get the various more accurate formulae. The Wikipedia site has a LOT of formulae for all kinds of coil shapes. There are several online calculators with pictures too!

https://en.wikipedia.org/wiki/Inductor
https://www.electronics-tutorials.ws/inductor/inductance.html
**broken link removed**
https://www.stroobandt.com/antennas/inductance.html

https://www.66pacific.com/calculators/coil_calc.aspx

Just Google "coil inductance" and be overwhelmed.

 

[ChrisHansen2Legit2Quit]

Also, how long is the coil, and whether it is multilayer. If you used AWG#24, but stayed with the number of turns, you probably made only a more efficient coil.

Darktrax,

Thanks for the response. I will go ahead & use AWG #24

AWG #24 is .5 mm in diameter. Reference coil calls for 120 turns with physical dimensions of 50 mm x 12mm

120 turn x .5 mm wire diameter = 60 mm total

Now I know why the engineer used AWG #30. AWG #24 will not fit! I am left with 10 mm over hang.

Question is,

What happens when you start over lapping? It's a whopping 20 turns & I assume it won't matter how far up or down I wind these "last few" turns, but just checking if there's any "scientific reasoning" behind it.

I assume there is. Perhaps when coils are manufactured, it's a "no-no" to engineer a coil that has multiple windings, but not equal in turns.

Thanks again

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EDIT: As noted, this is for the reference coil. This coil actually fits inside a water fitting tube. This tube is threaded around the outside & then you fasten a nut on it. This nut of course can be moved up & down the coil (via threads) to change the frequency.

^Thought I'd make note of that.

 

[Darktrax]

BFO detector - as in "Beat Frequency Oscillator" traditionally used to re-supply the missing carrier when mixing to resolve single-sideband signals?

If you have the circuit, and know how it works, you have have lot of freedom to mess with it. Losing 20 turns out of 120 can easily be made up my re-tuning by changing the capacitance involved. I feel unsure about a nut around the outside of the coil. It may well vary the frequency, but it also counts as a "shorted turn", which can sap enerygy and lower the Q. May that is intentional, but it sounds a bit "cut and try" to me.

You don't even have to have that physical construction at all! You can make a coil of any sort you like. 50mm x 12mm ? seems huge. Maybe there is a reason. You could even use a small former with a tuneable ferrite slug inside.

What happens when you start over lapping?

If you have to overwind the spare 20 turns, then do it. The result is slightly different to having a longer single layer, but hard to notice given the way this coil is tuned.

There are times when coil designers get fussy about inter-coil self-capacitance. There are "random-wound" coils, and physically stable (very pretty) multilayer overwinds that look like textile reels, and use of "Litz wire" (many strands of hyper-fine wire in a covering to make a thicker structure). There is no special rule that says you have to stay with one layer unless there is a mechanical or tuning constraint. The only thing to aim for is to make your (changed) coil end up with near the same inductance, or tweak something else, like the capacitance around it, to compensate. Maybe the fast way is to build it, try it, and then add to or subtract from the capacitance involved to get it to the right frequency.

 

[ChrisHansen2Legit2Quit]

Yes, beat frequency oscillator.

I am unaware of what a "BFO" is traditionally used for. I do know there are 3 types of metal detectors; a BFO being one of them.

Schematic,

Reference coil,

AWG #24 only makes 90 turns at 2" (50mm); not 100 turns. The hot glue is temporary. I will most likely thread the bottom through the hole, & work my way back another 30 winds.

This is the water fitting that fits over the reference coil. It is threaded, therefore the nut you see can be moved up & down to change the frequency (to match the search loop coil's)

*I have a mediocre understanding of how this device works: There are two oscillators (these are the coils or no?) that send out the same frequency. I'm probably wrong here, but you hear something as these oscillators "speak" to one another & that's called a "beat note." When you pass the device over a conductor, eddy currents are induced into the conductor, thus creating another magnetic field, which in turn interfere's with the device's magnetic field or "beat note" & you hear this as a change in sound ...or whatever your means of detection.

Why would you need dual oscillators? Why couldn't a single oscillator function just as well? It'd be like me taking a single coil & connecting it to an ammeter & running it over metal objects & watching the needled jump.


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[Darktrax]

OK - this one is far less critical than the type you need in SSB receivers.

There are 2 oscillators. They work at a much higher frequency than you could ever hear, into several hundred kHz probably. One or both may have their coil inductance modified by being brought near metal.I am guessing that one, called the "reference frequency" is probably intended to keep a relatively constant frequency, and not be involved in being brought near the metal, but it need not be so.

When two different frequency signals F1 and F2 are put into a linear amplifier, they may have gain, but they do not distort or affect each other. If instead the device is non-linear, like a diode - or in this case, a transistor biased into non-linearity, near cut-off, or clipping, then you get some multiplied products of the originals as well.

There will be the original F1 and F2 in there, but also there will be some at frequency (F1+F2), the sum, and also (F1-F2), the difference. In this circuit, it is the difference that is used.

Known as the "beat frequency", it is the same principle as you would hear if you tweak a string on a guitar, while the same note is played on another instrument close by, and one is tuned a little. As the notes get closer to being the same, you hear a throbbing, or beat which gets slower and slower as the notes match. The beat frequency is the difference, and it gets very slow until it disappears, and the notes are the same..

The oscillators are a high frequency version of the same phenomenon, and the whole idea is great sensitivity. If both oscillators A and B are going at hundreds of kHz, then it only takes a tiny relative change in one to produce a very noticeable (audio) difference frequency.

Q1 and Q2 are the oscillators. Q3 with Q4 are the non-linear mixer-audio combination. It looks as if Q3 is the mixer, then the capacitors filter out the sum component, leaving only the difference. Q4, Q5 and Q6 amplify the difference audio frequency enough to drive the speaker. There will be some part of the procedure where the oscillator is "set zero beat" at the start condition, and from then on, any metal nearby that would affect the inductance, and pull the frequency, which should result in a very audible note, getting higher in pitch as you get closer. You might also hear variants, such as the note dropping to zero beat, then rising in pitch again.

In all of this, it is no big deal to just wind the coil to a physical size that suits you. Provided both have the same inductance to get to zero beat, thats OK. Even if one coil is different dimensions, it can have turns added or removed to get the oscillation frequency to match. Here is where I think the nut being moved on the pipe .. is the way the initial oscillation frequency is set.