- What is electricity?
- Resistance, Conductance & Ohms Law
- Practical Resistors
- Power and Joules Law
- Maximum Power Transfer Theorem
- Series Resistors and Voltage Dividers
- Kirchhoff’s Voltage Law (KVL)
- Parallel Resistors and Current Dividers
- Kirchhoff’s Current Law (KCL)
- Δ to Y Network Conversion
- Y to Δ Network Conversion
- Voltage and Current Sources
- Thevenin’s Theorem
- Norton’s Theorem
- Millman’s Theorem
- Superposition Theorem
- Mesh Current Analysis
- Nodal Analysis
- Capacitance
- Series & Parallel Capacitors
- Practical Capacitors
- Inductors
- Series & Parallel Inductors
- Practical Inductors
Practical Inductors
Before looking at a few of the many types of inductors a few properties of real world inductors should be kept in mind:
- The primary characteristic of inductors is the fact that they resist any attempt to change the current passing through them, smoothing out those changes by a function of some time constant.
- The secondary characteristic of inductors is the back e.m.f. generated by a collapsing magnetic field around it. The point to bear in mind here is that it is of the opposite polarity to the current that generated the magnetic field in the first place. This voltage can be extremely high, in the order of hundreds (even thousands) of volts even in a 12 V circuit. Protective measures MUST be taken to prevent damage to surrounding semiconductor components.
- Inductors are created from a length of conducting material of some kind. Therefore all real inductors have a resistance integral to their design, and this can amount to a significant value. We don’t often consider it, but even copper wire has a resistance, and since an inductor is basically just a long wire that resistance soon adds up.
- Inductors also have a very small amount of capacitance between the individual turns of the coil. Often this is small enough to be ignored, but at very high frequencies can still be quite a significant factor that has to be accounted for.
Inductor Types
Air Core
Arguably the simplest of all deliberately made inductors. For R.F. applications may be as simple as a single turn wound around a former, such as a pencil or drill bit, then the former removed.
The core is air, with a very low permeability – and hence low inductance. Often used to construct R.F. tuning coils and snubber circuits.
Ferrite Core
Ferrite is a material that is made up from a mixture of iron oxide with other select metal oxides (such as manganese, magnesium or zinc oxides). Which are then heated to to somewhere between 100℃ and 1300℃.
Ferrite has a high level of magnetic permeability coupled with high electrical resistivity and low eddy current losses. When used as a core for an inductor it concentrates the magnetic field which allows greater inductance values to be achieved.
This combination of properties makes it very useful for high frequency applications.
Bobbin Inductor
A coil of wire wound on to a ferrite core, then covered by a shrink-wrap tube for durability.
Similar properties to ordinary ferrite core inductors. Their small size and ruggedness makes them very suitable for use in wall and in-lead power adaptors.
Ferrite Beads or Sleeves
Often used as simple noise filters on external DC power leads for electrical equipment. Slipped on to the lead with a single turn around the ferrite sleeve helps to suppress high frequency noise. The sleeves are often hinged to allow them to be added without dismantling the equipment.
Axial Inductors
Yet another variation on the ferrite core based inductor.
A very thin copper wire is tightly wound on to a dumbbell shaped ferrite core to which leads are attached via end caps. The whole thing is then encased in epoxy and nominal value, in microhenrys (µH), is printed as coloured bands around the body (same code as resistors).
Toroidal Core
A length of wire is wound around a doughnut shaped core (usually made of ferrite). The design of the core, being a closed loop, contains the magnetic field very well resulting in higher inductance for the size.
Due to the higher magnetic field and inductance less turns of the coil are needed for a given inductor value – thus resulting in a shorter length of wire and hence lower impedance.
By winding more than one coil on to the former they can be paralleled up to improve current carrying, or left separate and used as a transformer.
They are often used in switching power supplies and high current applications.
Laminated Core
A laminated core inductor is made by winding a wire around a bobbin and then inserting tightly packed sheets of steel with a high silicon that has been heat treated to increase the permeability and to lower the hysteresis and eddy current losses.
Shielded Variable inductor
Made by wrapping wire around a hollow threaded bobbin. A threaded core made out of either ferromagnetic material, or brass is then screwed in to the core. By adjusting how far the core is screwed in to the bobbin, the inductance of the coil can be fine tuned. Winding the core in will increase inductance, while winding it out will decrease it.
A metal can is then added to reduce E.M.I. and noise from the inductor.
Shielded SMD
Made by winding a thin coil of wire around a small bobbin and then securing it in ferrite housing which forms the shield. The whole thing is designed to be surface mounted directly on to a PCB.
Multilayer Chip Inductor
Made from thin plates of ferrite on to which a coil pattern is printed using a metallic paste. Precise alignment and stacking of these layers forms a coil.
Wireless Charging Coil
Not strictly speaking an inductor as such, but works using principle of induction.
Made from a flat coil of special multi-stranded wire (called Litz wire) laid atop a flat ferrite base. The wire is multi-stranded to reduce the skin effect (where ac current flows on the surface of a conductor and not inside the volume of the wire, thus increasing resistance and self heating losses).
The ferrite base is used to improve the inductance and focus the magnetic field.