4596	2010-04-28 09:13:49	nm0s_qrp	Adjusting Values of Toroids
Message	Date	From	Subject
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 NM0S
4597	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 capacitance
4602	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