What actually is Impedance/Reactance?

Discussion in 'General Electronics Chat' started by Barnaby Walters, Mar 3, 2011.

  1. Barnaby Walters

    Thread Starter Member

    Mar 2, 2011
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    Hi there,

    Just getting back into electronics after trying to do too much too soon without understanding anything a few years ago (When I was 14). I've just been reading through some posts about using decoupling caps, and I realised that I've never understood inductance — but it is clearly well known for causing problems!

    So — long wire and breadboard strips cause it, what actually is it?

    Thanks a lot,
    Barnaby

    P.S. Just realised I started off saying Inductance, then switched to Impedance! Confused now :)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. Papabravo

    Expert

    Feb 24, 2006
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    Inductance is is the opposition to a changing current. Even a wire has some inductance. If you coil the wire around a cylinder then the coil has more inductance than the wire alone.

    Impedance, expressed as a complex number in Cartesian or polar form, combines resistance, inductive reactance and capacitive reactance. The resistance makes up the real part, and the two reactances make up the imaginary part.
     
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  4. Barnaby Walters

    Thread Starter Member

    Mar 2, 2011
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  5. JMac3108

    Active Member

    Aug 16, 2010
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    Barnaby,

    Impedance is the general term for the opposition to current flow, all three kinds RESISTANCE, Capacitive reactance, and Inductive reactance.

    Your question about inductance: as PapaBrove said, its the opposition to a changing current. It has to do with the magnetic field that builds up around a conductor when current flows through the conductor. Coils are wound to generate inductance, and these are used for various purposes in electronics. Often though, inductance is unwanted as in the case of long wires or breadboard strips as you said. In really high speed circuits you even have to be concerned about the impedance of traces and vias on a PCB (printed circuit board).

    Hope this helps.
     
  6. Barnaby Walters

    Thread Starter Member

    Mar 2, 2011
    103
    4
    Thanks, that's a great explanation.

    So… Inductors (and indirectly anything with trace inductance) opposes a change in the amount of current flowing. It's quite hard to visualise how that effects a circuit as a whole — are there any fairly simple circuits I can make to demonstrate this? I have a whole box of coils (Which was partially why I asked the question!) of varying sizes.

    Thanks,
    Barnaby
     
  7. JMac3108

    Active Member

    Aug 16, 2010
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    Barnaby,
    Here is how PCB trace inductance can affect a circuit ... In a high speed digital circuit the PCB has both inductance (traces and vias) and capacitance (from traces to ground plane). The result is an RC circuit and ringing and overshoot and undershoot on digital signals with fast rise or fal times (fast edges). These problems are commonly called "signal integrity" and can cause system failures, and in the wors case damage to components. You will not see this kind of effect in a slow speed circuit.

    If you want to experiment with inductance, try generating a square wave and running it through a coil and looking at the result on an oscilloscope. Keep increasing the frequency and see what happens.
     
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