Trying to build a 1st order low pass filter and the calculators I have been trying to use all give me a way low turn count . I have given the info asked for input ,but still get a non-sense answer.... Mild steel core 1" in diameter x 1" long , wire size .031 , mild steel has a relative permeability of 2000 .....,
Shooting for 12.8 mH and another one 4.2 mH ( same core size ect.) Please help me.
Shooting for 12.8 mH and another one 4.2 mH ( same core size ect.) Please help me.
The permeability of iron and steel varies widely with mechanical and thermal history of sample, for instance work-hardening reduces it, annealing increases it.
An inductor for use in a filter has to have low losses at AC, so you need a laminated core, not a solid one, otherwise you pump energy into eddy currents and have an extremely non-ideal inductor. Basically a solid steel core turns a coil into a transformer with a shorted secondary.
For signal handling you'd normally use ferrite core, fine laminations of silicon steel, or an iron-dust core (common for RF).
An inductor for use in a filter has to have low losses at AC, so you need a laminated core, not a solid one, otherwise you pump energy into eddy currents and have an extremely non-ideal inductor. Basically a solid steel core turns a coil into a transformer with a shorted secondary.
For signal handling you'd normally use ferrite core, fine laminations of silicon steel, or an iron-dust core (common for RF).
Using solid mild steel bar as the core of an inductor isn't a good idea as you will get strong eddy currents which lead to core losses (heating of the core).
Commerical inductors and transformers use laminated silicon steel as adding silicon to the steel lowers the electrical conductivity (leading to weaker eddy currents) and constructing the core out of insulated plates rather than a solid bar interrupts the large loop paths that eddy currents like the take within a solid bar.
That said, given an unknown core your best bet is to throw a handful of turns on it (say 10-20t) then measure the inductance with an LCR meter and calculate inductance per turns squared value (Al).
Say you have 20 turns and measure 74uH
Al = 74uH/(20^2) = 0.185uH per turn^2
So if you then put 95 turns on the same core you'd expect
L = 0.185uH*(95^2) = 1669uH = 1.669mH
Commerical inductors and transformers use laminated silicon steel as adding silicon to the steel lowers the electrical conductivity (leading to weaker eddy currents) and constructing the core out of insulated plates rather than a solid bar interrupts the large loop paths that eddy currents like the take within a solid bar.
That said, given an unknown core your best bet is to throw a handful of turns on it (say 10-20t) then measure the inductance with an LCR meter and calculate inductance per turns squared value (Al).
Say you have 20 turns and measure 74uH
Al = 74uH/(20^2) = 0.185uH per turn^2
So if you then put 95 turns on the same core you'd expect
L = 0.185uH*(95^2) = 1669uH = 1.669mH
I didn't think that Nealzy could have used a solid steel bar, but in case he did the calculation is simplified, and doesn't involve the relative permeability or anything else: the inductance will be ~twice that of the coil without a core, because the core ~halves the magnetic path length, even if the effective permeability is extremely low.
It could be 20 or 2,000 and make no practical difference.
There are many online calculators for the inductance of an air-core inductor.
Of course, as has been remarked, the use of a solid core will make a huge difference at high frequencies, not only for inductance but mostly for losses.
In practice, such an inductor would be useless, unless the HF losses are intended
It could be 20 or 2,000 and make no practical difference.
There are many online calculators for the inductance of an air-core inductor.
Of course, as has been remarked, the use of a solid core will make a huge difference at high frequencies, not only for inductance but mostly for losses.
In practice, such an inductor would be useless, unless the HF losses are intended
The magnetic path length isn't the only factor unless the path has a constant area, which is only true when its a toroid or confined to ferromagnetic material in a complete circuit.
Magnetic fields in air are unconstrained, magnetic reluctance is not just a function of length, but of area too - reluctance is proportional to length / area.
A solenoid has a _much_ lower reluctance path outside the coil than inside it, so adding a core can multiply the inductance many-fold, not just doubling it. The longer and thinner the coil the more the difference is as the outside field paths have far more area and lengths not much greater.
In the limit a long thin solenoid behaves as if only the inside of the solenoid has reluctance, just like a toroid.