I'm used to an air core inductor having a positive tempco ofJust a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?
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.
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 ofJust a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?
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.
You could heat it above the curie point, if the inductance decreased suddenly...
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 ofJust a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?
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.
You could heat it above the curie point, if the inductance decreased suddenly...
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 ofJust a question, what about the permeability of the enclosing material? Could it be that it would have minute permeability that is affected from temperature?
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.
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
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