Voltage drops at sudden current drawing.

Discussion in 'The Projects Forum' started by electr, May 24, 2009.

  1. electr

    Thread Starter Active Member

    May 23, 2009
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    Hello guys.

    Why when the current suddenly araises its level, for example from 10mA to 100mA, then the voltage suddenly drops for a breif moment, for example drops briefly from 3.3V to 1V?

    I'm not talking about current spikes, but having the current change from one steady level (10mA) to a higher steady level (100mA).

    I could understand it if the dropout was constant and not breif, since more current means that more voltage is waisted on the wires (which have resistance), but why the voltage drops out only for a breif moment?

    Thank you very much.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you tell us more about the circuit?
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
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    How are you developing your 3.3V? Is it a voltage regulator? I ask this because I've seen people place far to much capacitance on the output pin of regulators, which causes what you describe to happen.
     
  4. steinar96

    Active Member

    Apr 18, 2009
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    Shouldnt placement of capacitors between the ouput and ground prevent this from happening.

    Unless they run dry ofcourse before the supply can respond to the increased current need.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    The capacitance on the output pin of the Vregulator should be about 10uF max. I usually use about 5uF. The bulk of the filtering should be on the input of the regulator.

    When the output cap becomes to large the regulating response time of the regulator is compromised. How much capacitance do you have on the output pin?
     
  6. electr

    Thread Starter Active Member

    May 23, 2009
    49
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    Thanks guys.
    Currently I wanna try to understand electrically why there is a brief voltage drop at sudden high level of current drawing.

    CDRIVE, you said that when the output capacitance of the regulator is too large, the response time of the regulator is too slow.
    I dont understand why is it a problem?
    Why does it cause the regulator's output voltage to drop when the regulator supplies the current?
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    You still haven't posted what value cap you have on the output.
     
  8. electr

    Thread Starter Active Member

    May 23, 2009
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    The output capacitor is 10uF, 25V electrolyte.

    I'd really like to understand the theory behind this phenomenon.
    I dont manage to see why theoretically there will be a momentary voltage drop in the voltage line, and dont manage to connect the line's capacitance to this phenomenon.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    10uF is not a problem and it should work fine, as it's typical of what most data sheets spec. You have something else going on there. Post your schematic.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I donno what or how you have the circuit wired and which components you are using but I can tell you the sudden drop of voltage :p

    There are couple of reasons that voltage fluctuate when current is drawn.
    1. The source cannot handle the current demand
    2. poor filtering
    3. poor linear regulator circuit design with no feed back
    4.thin wires causes a voltage drop from source to the load
    5. faulty regulator or capacitor.
    and so on.:D
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    I ruled out (1) & (4) because his output comes back up while the 100mA load is still connected. The rest are plausible though.
     
  12. electr

    Thread Starter Active Member

    May 23, 2009
    49
    0
    Thank you for the help guys.

    R!F@@,
    Could you please detail about the filtering?
    What could be wrong in filtering that would make this momentary voltage drop?
    Thank you.

    CDRIVE,
    I want to get into it.
    I'm not interested in getting rules of thumb, since one day someone is going to ask me 'why' and i'd like to be able to explain it to him.
    I have a good knowledge about capacitors, time constant, energy and so on, but i'm just lack of experience to connect all the dot together to be able to understand this voltage drop as general, not specifically in my case.
     
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hmmm!.. piece of cake buddy.
    But first, give me a schema or a pic of your supply and load details.
    then I can explain all the thingy's
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    I'll make you a deal.. You post your circuit, as requested, and I'll do my best to explain why a large cap on the output of a regulator is counter productive. ;)
     
  15. electr

    Thread Starter Active Member

    May 23, 2009
    49
    0
    I dont expereince it with my circuit.
    Its a phenomenon that I observed once with a circuit someone I know has planned and I wanted to understand about this.

    CDRIVE, could you explain why would large response time cause a breif voltage drop?
    I really try to understand it.
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Most, if not all, Vreg chips employ current limiting. A large cap can momentarily draw enormous current causing the chip to fold back.

    By the way, since this turned out to not be a project at all it should have been posted in a different section of the forum.
     
  17. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    True
    Only if you place a large uncharged capacitor across the output. If the capacitor is charged up to the supply voltage then it will reduce the effects of transient current load on the regulator rather than increase it.

    I would think it more likely that the voltage regulator is going into oscillation when the load current is increased. The types of capacitors that you use and their ESR can have a greater influence on oscillating voltage regs that the capacitor values. Take a look at this article http://www.national.com/nationaledge/jul02/article2.html
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    No argument there. I had to go back more than 12 years to find the original schematic for the device that experienced this phenomenon. It was a control circuit that a colleague (who is also a forum member) designed... Yo, Ron! Anyway, he stuck a 1N4002 in series with the output pin of a 7812. Connected between the cathode and GND was 2000uF cap. The purpose for the cap was to keep the circuit live during brief outages. The purpose of the diode was to prevent back feed to the 7812. The dip in output voltage of the 7812 was corrected by adding a small 10uF cap on the output pin of the 7812. FYI, the voltage dip occurred when relay coils (driven by a uln2004) were energized.

    To summarize, it was the diode that caused the dip, not the cap alone, if at all.

    Since that time the circuit was further modified where the diode and the 2000uF cap was abandoned completely. I stand by my statement that a large cap has no place on the output pin of a regulator because it will destroy the response time of the regulator. Also, I rarely second guess the data sheets. ;)
     
  19. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    CDRIVE Quote
    I firmly believe that this is true :)

    Rifaa
     
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