I'm used to an air core inductor having a positive tempco of inductance around +120 PPM/k or so, just from the physical expansion of the copper increasing the diameter. But this Coilcraft midi-spring part https://www.dropbox.com/scl/fi/mfplgwzorhsh5sodxz5cu/Coilcraft_150n_Midi.jpg?rlkey=nrhwa9j64swuz7bmo1hiyjr9j&raw=1 is much less, maybe +30 PPM. I wonder if the plastic expansion changes the geometry or something. Lengthens the solenoid? Well, it makes it easier to temperature compensate my oscillator. Out on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours.

# inductor tc

Started by ●November 15, 2023

Reply by ●November 15, 20232023-11-15

On Wednesday, 15 November 2023 at 23:55:30 UTC+1, john larkin wrote:> I'm used to an air core inductor having a positive tempco of > inductance around +120 PPM/k or so, just from the physical expansion > of the copper increasing the diameter. > > But this Coilcraft midi-spring part > > https://www.dropbox.com/scl/fi/mfplgwzorhsh5sodxz5cu/Coilcraft_150n_Midi.jpg?rlkey=nrhwa9j64swuz7bmo1hiyjr9j&raw=1 > > is much less, maybe +30 PPM. I wonder if the plastic expansion changes > the geometry or something. Lengthens the solenoid? > > Well, it makes it easier to temperature compensate my oscillator. Out > on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours.Just a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature? You could heat it above the curie point, if the inductance decreased suddenly...

Reply by ●November 15, 20232023-11-15

On Wed, 15 Nov 2023 17:59:16 -0800 (PST), Klaus Kragelund <klaus.kragelund@gmail.com> wrote:>On Wednesday, 15 November 2023 at 23:55:30 UTC+1, john larkin wrote: >> I'm used to an air core inductor having a positive tempco of >> inductance around +120 PPM/k or so, just from the physical expansion >> of the copper increasing the diameter. >> >> But this Coilcraft midi-spring part >> >> https://www.dropbox.com/scl/fi/mfplgwzorhsh5sodxz5cu/Coilcraft_150n_Midi.jpg?rlkey=nrhwa9j64swuz7bmo1hiyjr9j&raw=1 >> >> is much less, maybe +30 PPM. I wonder if the plastic expansion changes >> the geometry or something. Lengthens the solenoid? >> >> Well, it makes it easier to temperature compensate my oscillator. Out >> on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours. >Just a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?It's plastic so Ur=1. It does have a dielectric constant >1, which has some tempco. But I'm guessing it has a positive mechanical effect, stretching out the solenoid length as temp rises, and that has the opposite tempco as the diameter expanding. The bare-naked spring coil inductors are stretched by the tempco of the PCB. 370HR is 14 ppm/degc. Almost everything in this oscillator contributes a negative frequency TC. The PCB capacitance is especially terrible, around +900 PPM/K. We just cut some big reliefs through several plane layers under the critical nodes, to reduce that capacitance. That will reduce the mag field of the coil from bouncing off the copper planes too.> >You could heat it above the curie point, if the inductance decreased suddenly...There probably isn't a curie point, being basically an air core inductor.

Reply by ●November 16, 20232023-11-16

On 2023-11-15 20:59, Klaus Kragelund wrote:> On Wednesday, 15 November 2023 at 23:55:30 UTC+1, john larkin wrote: >> I'm used to an air core inductor having a positive tempco of >> inductance around +120 PPM/k or so, just from the physical expansion >> of the copper increasing the diameter. >> >> But this Coilcraft midi-spring part >> >> https://www.dropbox.com/scl/fi/mfplgwzorhsh5sodxz5cu/Coilcraft_150n_Midi.jpg?rlkey=nrhwa9j64swuz7bmo1hiyjr9j&raw=1 >> >> is much less, maybe +30 PPM. I wonder if the plastic expansion changes >> the geometry or something. Lengthens the solenoid? >> >> Well, it makes it easier to temperature compensate my oscillator. Out >> on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours. > Just a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature? > > You could heat it above the curie point, if the inductance decreased suddenly... >You can make the longitudinal expansion of the plastic compensate for the circumferential expansion of the copper. To within a couple of percent, L(uH) = (a**2 N**2)/(9a + 10 b), where a is the mean radius of the coil, b is the length of the winding (both in inches), and N is the number of turns. CTE(a) is controlled by the copper, CTE(b) by the plastic. so we get dL/dT = 2 N**2 a da/dT/ (denom) - N**2 a**2 *( 9 da/dT + 10 db/dT) / denom**2 da/dT = a*CTE(Cu) db/dT = b*CTE(plastic) Dividing by L and collecting terms, we get TCL = (dL/dT)/L = (2 CTE(Cu) - (9a CTE(Cu) + 10b CTE(plastic))/(9a + 10b). With CTE(Cu) = 17 ppm/K and CTE(plastic) probably 100 ppm/K, you get zero TC somewhere about a/b = 4.5, i.e. diameter ~ 9 x length. (That is, unless I've made a blunder, which is a definite possibility at 1 AM.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

Reply by ●November 16, 20232023-11-16

On Thu, 16 Nov 2023 01:21:45 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2023-11-15 20:59, Klaus Kragelund wrote: >> On Wednesday, 15 November 2023 at 23:55:30 UTC+1, john larkin wrote: >>> I'm used to an air core inductor having a positive tempco of >>> inductance around +120 PPM/k or so, just from the physical expansion >>> of the copper increasing the diameter. >>> >>> But this Coilcraft midi-spring part >>> >>> https://www.dropbox.com/scl/fi/mfplgwzorhsh5sodxz5cu/Coilcraft_150n_Midi.jpg?rlkey=nrhwa9j64swuz7bmo1hiyjr9j&raw=1 >>> >>> is much less, maybe +30 PPM. I wonder if the plastic expansion changes >>> the geometry or something. Lengthens the solenoid? >>> >>> Well, it makes it easier to temperature compensate my oscillator. Out >>> on my bench, it's measured 49.9944 MHz, exactly the same, for 3 hours. >> Just a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature? >> >> You could heat it above the curie point, if the inductance decreased suddenly... >> > >You can make the longitudinal expansion of the plastic compensate for >the circumferential expansion of the copper. > >To within a couple of percent, L(uH) = (a**2 N**2)/(9a + 10 b), where >a is the mean radius of the coil, >b is the length of the winding (both in inches), and >N is the number of turns.We can measure frequency to PPBs, so a spring as the inductor in an oscillator could be a position/elongation sensor to PPBs. There must be a use for that. (I've been playing with the microinch-sensitive LVDTs that are used for machine shop metrology. They are expensive.)> >CTE(a) is controlled by the copper, CTE(b) by the plastic. > >so we get > >dL/dT = 2 N**2 a da/dT/ (denom) > - N**2 a**2 *( 9 da/dT + 10 db/dT) / denom**2 > >da/dT = a*CTE(Cu) >db/dT = b*CTE(plastic) > >Dividing by L and collecting terms, we get > >TCL = (dL/dT)/L = (2 CTE(Cu) - (9a CTE(Cu) + 10b CTE(plastic))/(9a + 10b). > >With CTE(Cu) = 17 ppm/K and CTE(plastic) probably 100 ppm/K, you get >zero TC somewhere about a/b = 4.5, i.e. diameter ~ 9 x length. (That is, >unless I've made a blunder, which is a definite possibility at 1 AM.)Right, the plastic can reduce the TC. We did once have a traumatic experience with some copper+plastic inductors where the plastic cold-flowed and changed L enough to kick us out of phase-lock after some weeks or months in the field. The fix was to bake them overnight to remove the stresses. I still suffer PTSD from that experience.> >Cheers > >Phil Hobbs