EMRFD Message Archive 4596
Message Date From Subject 4596 2010-04-28 09:13:49 nm0s_qrp Adjusting Values of Toroids Hi all.
I performed an interesting experiment this morning, and thought I'd share the results with the EMRFD group.
I had constructed a toroidal inductor, and wanted to see how much the value would change by changing the spacing of the turns.
The inductor consisted of 21 turns of 22AWG, evenly spread around a T50-2 core. The value of the choke measured 2.2 uH, compared to the ideal value of 2.1uH expected from the AL value of 47 for that part.
I then squeezed all the turns so that they occupied about 2/3 of the toroid. The inductance then measured 2.5 uH, a difference of 0.3 uH.
This was an unexpectedly large change, considering that 21 turns on a T50-0 core (phenolic, u=1) produces an inductance of only 0.28 uH.
This gives the experimenter the capability to adjust the value of an inductor. Toroids are much easier to come by than those variable inductors in shielded cans.
73 Dave NM0S4597 2010-04-28 09:49:47 Leon Heller Re: Adjusting Values of Toroids 4598 2010-04-28 09:56:35 Tim Re: Adjusting Values of Toroids Tweaking for resonance by spreading/compressing the turns certainly has a long tradition, not just for toroids but for air-wound solenoids too.
There's also a tradition of tweaking "gimmick" capacitors to change resonant frequency or coupling slightly.
I wonder, though: how much of the change in resonant frequency that we typically see when squeezing turns on a troid, is due to inductance change, and how much is due to changes in self-capacitance?
A quick websearch turns up this very nice writeup
http://www.g3ynh.info/zdocs/magnetics/appendix/Toroid_selfC.html
which leads me to a conclusion that self-capacitance, after you get above a couple of turns, is not a very big factor at least for the usual L to C ratios we use at HF. Although he does point out that the stray capacitance4602 2010-04-28 11:39:20 Art Re: Adjusting Values of Toroids Tim et al,
Be aware that adjusting the turns distribution on the core also
degrades the self shielding capability of the toroid, allowing it to
feed rf into nearby circuits (especially other inductors). The self
shielding ability of the toroid isn't often discussed in technical
forums, but the practice is widespread and commonly accepted.
Air based toroidal inductors also lose a great deal of their self
shielding capability if the turns are compressed.
The self shielding capability also allows us to mount the toroid
close to the PCB-which is a nice side benefit. But, when the magnetic
flux is skewed, losses into nearby pcb traces and the ground plane on
the PCB increase too.
There is no free ride.
Does anyone in the group have information or web based technical
references that detail the uniformity of the flux distribution when
the turns are compressed or expanded on a toroidal inductor?
Regards,
Art
>Tweaking for resonance by spreading/compressing the turns certainly
>has a long tradition, not just for toroids but for air-wound solenoids too.
>
>There's also a tradition of tweaking "gimmick" capacitors to change
>resonant frequency or coupling slightly.
>
>I wonder, though: how much of the change in resonant frequency that
>we typically see when squeezing turns on a troid, is due to
>inductance change, and how much is due to changes in self-capacitance?
>
>A quick websearch turns up this very nice writeup
>
>http://www.g3ynh.info/zdocs/magnetics/appendix/Toroid_selfC.html
>
>which leads me to a conclusion that self-capacitance, after you get
>above a couple of turns, is not a very big factor at least for the
>usual L to C ratios we use at HF. Although he does point out that
>the stray capacitance