June 21, 2012

Let's talk about inductors

I have a lot of crossovers coming together at the moment. My coil winder:

Why wind  your own? Basically, a good inductor for woofers especially has a lot of copper. In a sensible passive crossover design, the inductors are expected to be the most expensive part. Winding your own cuts down on cost if you have a good source of copper. It's also useful if you need a value that is not available.

Inductors, capacitors and resistors

All conductors have an inductance, but in inductor is a component that stores energy in its magnetic field where a capacitor stores energy in its electric field. A resistor doesn't store energy, but converts it into heat.

Resistors tend to have a small amount of inductance - wire wound high value resistors are a lesser choice. Mox resistors are used in crossovers for their lower inductance. Resistors are the least problemmatic parts in a crossover and they are also the cheapest parts.

Capacitors are more expensive again, but their faults are also relatively benign. Electro caps should of course not be used for serious long term use and they are known to lose their value over time.

Inductors are more problemmatic in a crossover. The most desirable is air cored. Cheap inductors use a ferrite core to increase inductance whilst keeping resistance down.

Unfortunately a ferrite core introduces distortion and is not a good choice for audio.

A good inductor for a crossover looks like this:

A good large value inductor might use AWG 14 copper to achieve reasonably low DC resistance.

How an inductor works

An inductor is essentially an electromagnet and it tends to resist any change to the current flowing through it. In other words, it has inertia as if it does not like the way that a music signal changes rapidly!

Inductance is based on four things:
  • The number of turns (more turns = higher value)
  • Core material
  • Cross-sectional area of the coil (more area = higher value)
  • Coil length 

Power rating

Due to resistive losses, all inductors have a power rating. Ferrite cores tend to saturate as power goes up, which means that the core can't support any more magnetism. Performance is seriously degraded.

Inductors can also exhibit microphonic effects where mechanical vibration causes the inductance to change.

No comments:

Post a Comment

All comments are moderated.