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input resistance measurement

Discussion in 'Renewable Energy' started by shawnmk, Dec 7, 2014.

  1. shawnmk

    shawnmk Member

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    Hello,

    I'm using a TEG and LTC3109 circuit. I want to do a maximum power measurement calculation for both TEG separately and then TEG+LTC3109. But I need to match the input resistance for this.

    I don't know to measure the input resistance of these components.

    How to measure the Ri's of these?
    when they are on?
    should I connect + to +?

    When I do this measurement of Rin of a TEG using a multimeter, the ohms readings just keeps increasing.
     
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Why? It's not RF or a transmission line. It's DC.

    In an audio amplifier, the output Z could easily be 0.8 ohms, yet the speaker is 8 ohms. You would actually loose power driving an 8 ohm speaker with an amplifier with an 8 ohm output Z.
     
  3. shawnmk

    shawnmk Member

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    I'm not sure why but that's what the sources online says. Help me to understand when not to follow the maximum power transfer rule.
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    This http://en.wikipedia.org/wiki/Impedance_matching wikipedia article is of interest.

    I need to caution you in reading the article though. This quote:

    [quote-wukipedia]Devices intended to present an apparent source resistance as close to zero as possible, or presenting an apparent source voltage as high as possible. This is the only way to maximize energy efficiency, and so it is used at the beginning of electrical power lines. Such an impedance bridging connection also minimizes distortion and electromagnetic interference; it is also used in modern audio amplifiers and signal-processing devices.[/quote]

    I think, is much more relevant. An ideal voltage source has an output Z of zero and an ideal current source has an output Z of infinity.

    An indirect way of measuring a DC impedance is to place a variable resistor in series with the output and adjust that variable resistance until the voltage is 1/2. Take the resistor out of circuit and measure it. That will be the DC output resistance. That's not practical for DC power.

    If you have a few I-V pairs, you can fit a line through those pairs. The slope will be the resistance.

    An electronic load can operate at constant voltage, current power or resistance.

    In AC sinusoidal signals, the cosine of the angle of the phase difference between voltage and current is the power factor. For pure resistive loads, it is 1.
    You have three powers: Real, reactive and apparent power. See: http://en.wikipedia.org/wiki/AC_power

    Max power point tracking in solar cells is an entirely different animal. A solar cell has a max power point and that's where you want to operate.

    I'm not sure where the max power point of a thermal electric generator (TEG) is. Highest temperature difference or some particular temperature difference.

    I could be missing something too.
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You may want to match the TEC impedance for maximum power transfer (not efficiency) but I'm not sure.
    Perhaps one of these articles will help.
     

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