inductance of a solenoid with ferrite rod core

If the coil is wound on an iron core the inductance is greatly increased and the magnetic lines of force increase proportionally. There are special winding techniques to to use on occassion to minimise this self capacitance. This self-capacitance combined with the natural inductance will form a resonant circuit (self-resonant frequency) limiting the useful upper frequency of the coil. A simplified formula for inductance has been derived to establish the required number of turns for a given inductance value.Ĭoils wound on a former (with or without a core) may have multilayers of windings which are called solenoid windings.Īll coils also exhibit a degree of self-capacitance caused by minute capacitances building up around and between adjacent windings.ĭepending upon the application this may be of considerable concern. It has been found that the optimum dimensions for a high 'Q' air core inductor is where the length of the coil is the same as the diameter of the coil. Someone asked about a formula which takes into account the spacing bewtween windings, the 10len above automatically takes that into account, if you're confused think about it!. The standard inductance formula for close approximation - imperial and metric is: Having two turns the value would be four units while three turns would produce nine units although the length of the coil also enters into the equation. If a coil was of one turn its value might be one unit. The value of an inductance varies in proportion to the number of turns squared. Typical values of units encountered are milli-henries mH, one thousandth of a henry or the micro-henry uH, one millionth of a henry.Ī small straight piece of wire exhibits inductance (probably a fraction of a uH) although not of any major significance until we reach UHF frequencies. The standard value of inductance is the Henry, a large value which like the Farad for capacitance is rarely encountered in electronics today. We said a piece of wire wound into a coil form has the ability to produce a counter emf (opposing current flow) and therefore has a value of inductance.

Alternatively it could be said 'inductance is the property of a circuit by which energy is stored in the form of an electromagnetic field'. The property of inductance might be described as 'when any piece of wire is wound into a coil form it forms an inductance which is the property of opposing any change in current'.

Inductance of a solenoid with ferrite rod core