| Inductors | |||
|
Definition Technical Explanation Value Identification Passive Filters Formulas
I have very little practical experience with inductors. This article is based almost
entirely on theory. Please contact me if this page is incorrect
or misleading.
Definition: An inductor is, to some degree, the opposite of a capacitor. Where a capacitor stores energy in an electric field, an inductor stores energy in a magnetic field. The result is a component which passes DC with relative ease, (inductors act like a resistor in DC circuits) while blocking AC with an attenuation proportional to the inductor's value. Inductance is measured in Henry's. (H) The name "inductor" comes from the devices ability to induce a magnetic field from electric current. Technical Explanation: Most people have at some stage in their life encountered electromagnetism, where a magnetic field forms around an electrical conductor. Most people will also have encountered the reverse effect, where a magnetic field induces a current in a conductor. Inductors are usually constructed in a coil of some fashion because the close proximity of the conductor to itself reinforces this magnetic field, resulting in a considerable increase in efficiency compared to a straight conductor. So when an electrical current flows through an inductor a magnetic field is created. A cross-section of an inductor and its magnetic field is shown below.  X represents current flowing "into the page", ° represents current flowing "out of the page". Also, when a magnet is moved into an inductor, (or near any conductor) an electrical current flows. The important point here is that the electric current is only created while the magnet moves. (When the magnetic field changes) So.... current creates magnetic fields and changing magnetic fields creates current. When an AC current flows through the inductor a changing magnetic field is created. Because this field is changing it creates an electrical current in the inductor itself which opposes the current creating the field, thus AC signals are attenuated. (The AC current induced by the field is out of phase with the current inducing the field.) If a DC current flows through the inductor a static magnetic field is created, but because the magnetic field is not changing it creates no current in the inductor and the DC current flows through the inductor as if it were a resistor. Therefore, the inductor blocks AC signals while passing DC signals. Value Identification: In many cases inductors are just insulated wire wrapped around a former, (something to give them shape and enhance the magnetic field) and have no markings to represent their value in Henry's. However, inductors of a radial construction with a colored band code are reasonably common. This type of inductor looks similar to a resistor except it is a cylindrical shape with flat ends, rather than rounded ends. The color code is the same as the IEC color code found on resistors. The value printed on radial inductors is in µH, not H. Passive Filters: If capacitors block DC signals and pass AC signals at frequencies relative to their capacitance and inductors pass DC signals while blocking AC signals relative to their inductance, combinations of these two components can be used to pass or block selected frequencies. Passive filters are called "passive" for two reasons; they are made from passive components, (resistors, capacitors and inductors only) and they therefore require no external power source to operate. It should be noted that passive filters usually do not perform as well as active filters, (using opamps or transistors) when it comes to linear response, cut-off rates.... etc. The main use of passive filters is for crossovers in multiple driver speaker systems where external power is unavailable. Here's four very basic passive filters:  Low-Pass filter The inductor will pass higher frequencies less efficiently, and the capacitor will pass any remaining high frequencies to ground. DC will be passed almost completely by the inductor and ignored by the capacitor.  High-Pass filter The capacitor will pass lower frequencies less efficiently and the inductor will short remaining low frequencies to ground. DC will be blocked entirely by the capacitor, which must come before the inductor (as shown) to avoid a short circuit for DC current.  Band-Pass filter This is a combination of the above high-pass and low-pass filters. The overlapping response curves give a frequency range which will be passed by the filter. The dotted parts of the curves are irrelevant, but have been included to represent the combination of the high-pass and low-pass filters.  Band-Reject filter The band-reject filter (or "notch filter") removes a band of frequencies from the signal. How does it work? Don't ask me.... For all filters, adjusting the capacitance and/or inductance will result in different frequency thresholds. The relationship between frequency and capacitance or inductance is different for each filter design, but a knowledge of how capacitance effects frequency is useful; Smaller capacitors will pass higher frequencies while attenuating lower frequencies, and larger capacitors will pass lower frequencies as well as higher frequencies. (this is why inductors are required) A 10µF capacitor will pass most of the human hearing range with reasonable efficiency. Formulas: Like other passives, inductors can be combined to create non-standard values. The calculations are the same as for resistor combinations. 
The result for parallel inductance is a reciprocal and must be reciprocated to obtain the correct result. If you're making you own inductors, here's simple formula for determining a close approximation of inductance in terms of coil radius, (r) coil length, (l) and number of coils. (n)  Where r and l are in cm and L is in µH. This formula is for inductors without a former (ie. "air core"). I've never used this but I assume the constants are probably influenced by the properties of the conductor itself. If you have any comments or questions please don't hesitate to contact me. |
|||
| Return To Top | Last Updated: 26/02/2007 | Home Page | |