Output Resistance of DC Supplies

Discussion in 'General Electronics Chat' started by Roam, May 7, 2011.

  1. Roam

    Thread Starter New Member

    May 5, 2011
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    Hi guys!

    I'm a bit confused. I have a question regarding the effective dc output resistance of dc power supplies. For a "good" power supply, should this internal resistance be large or small?

    Well, I used to think that good power supplies should have a small internal resistance, so they would maintain a constant voltage until exhausted before droping to 0. However, I did an experiment and found that regulated power supplies (which give a more steady outoput with less ripples), tend to have a higher output resistance than the unregulated power supplies. So does this mean the dc power supplies with larger resistance are more desirable?

    I greatly appreciate it if anyone could confirm this, and also explain briefly why higher internal resistance reduces the output ripples.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Most regulated power supplies are designed to have a constant voltage output, with a low output resistance. Generally, regulation of such power supplies uses voltage feedback, which reduces the output resistance.

    Some power supplies are made to have a constant-current characteristic. In this case a different form of feedback control using current sensing may be used, increasing the output resistance. This may be an overload protection feature applying at high currents only to an otherwise constant voltage supply. Specialised supplies are also made with a basically constant current output, e.g. to power LEDs.

    I have not heard of regulation aimed at high output resistance to minimise ripple, and wonder if you have misunderstood something. Can you explain further?
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    An ideal dc supply would have zero ohms of resistance. Constructing them out of real components, however, does not allow this.

    Looking at it very simply, The components used to regulate the supply eliminate most of the ripple. They are able to take a range of voltages in and output a somewhat stable voltage.
     
  4. Roam

    Thread Starter New Member

    May 5, 2011
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    I also did an experiment on one unregulated power supply, and I varied a resistive load which was connected across the supply output. With the heavy load (1k ohm) I got a more smooth output, and for the lighter load (220 ohm) I got ripples with larger amplitude (looking at the oscilloscope). So, my question now is that the heavier this resistance, the more linear/steady output will be?
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I am afraid that your ideas about this have become a bit mixed up. In the context of a voltage supply, the larger value of resistance is a lighter load. The larger resistance draws less current (current = voltage/resistance).

    The lower resistance is a heavier load, since it draws more current. This results in larger changes in the voltage on the smoothing capacitor in the power supply, hence more ripple.

    Note however that the resistance you have been varying is the external or load resistance. The internal resistance is a parameter which describes the reduction in the supply output voltage when the load current increases, which is another matter entirely. See link: http://http://en.wikibooks.org/wiki/A-level_Physics_%28Advancing_Physics%29/Internal_Resistance
     
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