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Understanding impedance

Discussion in 'General Electronics Discussion' started by Rajinder, Sep 3, 2017.

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  1. Rajinder

    Rajinder

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    Jan 30, 2016
    Hi,
    I am a relative novice in Electronics. I can never truly understand the meaning of impedance. I understand it is a measure of resistance at AC (which includes inductance and capacitor reactance.
    What I can't understand is when I here for example
    'This circuit has an input impedance of 10K'
    Does that mean that anything connecting to this must have a lower impedance to not load the input?
    Similarly, I hear stuff like 'the PIC microcontroller A/D has a input source impedance of 10K'
    Again does that mean what I am connecting must have a lower resistance?

    Similarly I have a problem of understanding output impedance and how it interacts with input impedance. Can someone help me understand preferably with a worked example?
    Thanks in advance
    Raj
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The first thing is that the non-resistive portion of impedance is frequency dependent, so the impedance is given at the operating frequency.

    If you have some device with an unloaded output of x volts and an impedance of y ohms driving an input with an impedance of z ohms, the voltage seen by the input is the same as that if you have a voltage of x volts applied to a voltage divider composed of two resistances y and z ohms (the voltage would be measured across the resistor z.

    Some outputs will have a level defined as if they are driving an assumed load. For example an RF signal generator might be designed with a 50 ohm impedance to drive 50 ohm coax into a 50 ohm load. An output level of 1 volt will measure 2 volts if there is no load present.
     
  3. Rajinder

    Rajinder

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    Jan 30, 2016
    So if I have a voltage divider with say 2 x20K resistors, the output taken from the bottom resistor. Then the input impedance of the other circuit must be greater than 10 X the 20K resistor do the output voltage is not dropped or loaded. This is my basic understanding.
    But I still don't understand.
     
  4. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Reasearch 'maximum power transfer'.

    In any circuit, if you don't match the source and load you get losses and the 'idea' is to maximise efficiency at all times. Of course there are occasions where this is simply not possible or practical but the maximum power transfer concept explains how and why this occurs.
     
  5. duke37

    duke37

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    Jan 9, 2011
    If you have a voltage divider of 20k and 20k, driven by a low resistance then the output centre tap will appear as a voltage source and a series resistance of 10k. Any load even 10 times the 10k will drop the voltage. 10k will drop the voltage by 50%. 100k will drop the voltage by about 5%, you can work out what this is exactly by considering as a voltage divider.

    An AVO8 will have a resistance of 20kΩ/V thus on a 100V range will have an input resistance of 2MΩ.
    Some digital meters have a 10MΩ input resistance (on all ranges) but some cheap ones have a lower input resistance of only 1MΩ. If these are used to measure 1000V, they will fail as there is nothing inside to take the power.

    Maximum efficiency is obtained when the source and load are mismatched. Consider a 1kW electric fire, this will have a resistance of about 60Ω, it is connected to a supply of very low resistance for high efficiency.
     
    Last edited: Sep 3, 2017
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The voltage divider formed by the output and input impedances allow you to calculate the voltage seen at the input.

    For signal applications it is often desirable that the input impedance is high compared to the source impedance in order not to load down the source.
     
  7. BobK

    BobK

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    Jan 5, 2010
    Actually it is more complicated than that. Impedances are complex numbers and they cannot be represented by a scalar like a resistance can. Case in point: If you put and inductor and capacitor in series with equal scalar impedances, the resultant impedance across the two is zero. If you put them in parallel the resultant impedance is infinite. This is what allows us to make circuits tuned to a specific frequency.

    Bob
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    It is impossible to understand impedance and reactance without understanding the math used to describe these two phenomena. If you aren't familiar and comfortable working with the arithmetic of complex numbers you will never "get it" when it comes to understanding complex impedance and its relationship to reactance and resistance. As @BobK mentioned, there are two kinds of reactance, inductive reactance and capacitive reactance that can combine at a particular frequency (the resonance frequency) to produce virtually zero total reactance (series connection of inductor and capacitor) or almost infinite total reactance (parallel connection of inductor and capacitor).

    It is perhaps unfortunate that reactance is measured in the same units as resistance because the two are completely different, both in principle and in practice. A reactance stores electrical energy and does not dissipate real power. A resistance consumes electrical energy and dissipates real power (as heat) in doing so.

    When considering a sinusoidal voltage across, and a current through, a component exhibiting reactance, the current and voltage are always ninety degrees out of phase with each other. This is effect is modeled mathematically by considering reactance to be represented as an imaginary number, j = sq.rt. (-1), multiplied by a real number, X, positive for inductive reactance and negative for capacitive reactance. It gets a lot more complicated after that, especially when trying to analyze circuits with resistive and reactive components. But fortunately there are math programs as well as short-cuts that can take most of the pain away.

    Learn the basics of complex arithmetic first and then analyze a few circuits involving resistors, capacitors, and inductors. By analyze, I mean apply specified voltage or current sources between various nodes and then determine the resulting voltages and currents between other nodes.

    Check your understanding by modeling circuits with any one of several free online simulation programs. Follow up by bread boarding real circuits and making oscilloscope measurements of the voltages and their phase relationships. You will need a variable-frequency sinusoidal audio signal generator to use with your oscilloscope, which can be an el-cheapo digital o-scope kit widely available on eBay. A constant-amplitude sine wave generator is preferable, but even a simple triangle wave generator can produce a "gud enuf" sine wave with diode break-point shaping of the triangular peaks. Google for examples on the Internet.

    Note that reactance and impedance are functions of a sinusoidal frequency. While the electrical property of having a reactance or impedance value exists at any frequency, the actual value of the reactance or impedance is only defined at a specific sinusoidal frequency of your choosing.

    Good luck in finding your "ah ha!" moment with regard to this subject. I struggled with it for many years as a teenager (sometime near the middle of the last century) because I didn't know, or understand, complex arithmetic. All I had to work with were the scalar "formulas" for inductive and capacitive reactance. I didn't have a clue, for many years, as to how to include resistance in the equation to obtain impedance.
     
    bushtech likes this.
  9. Cannonball

    Cannonball

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    May 6, 2017
    Hi.
    Impedance is the resistance of a capacitor and or a coil to an ac voltage. The higher the frequency the higher the impedance of the coil to ac and the lower impedance to the capacitor. The cap and the coil are 180 degrees apart. At resonance they will cancel each other.
     
  10. Rajinder

    Rajinder

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    Jan 30, 2016
    Wow, so much to learn.
    Thanks for the advice.
    Another question. I have always read that a circuit should have high input impedance and low output impedance. Why is this. Can someone explain. I thought a high impedance doesn't draw much current and low impedance can. Is this correct?
     
  11. duke37

    duke37

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    Jan 9, 2011
    The output circuit should provide whatever current is needed so it should have a low impedance. Thus current draw will not change the voltage much.
    The input circuit should have a high impedance so that it draws little current and does not affect the voltage greatly.
    For maximum power transfer, the input and output should be matched but this is a different requirement. An interesting exercise would be to understand the output of a valve with say 100kΩ output impedance coupled to a speaker of 3Ω impedance.
     
    Arouse1973 likes this.
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