wierd voltage divider issue

Discussion in 'The Projects Forum' started by aruna1, Jul 8, 2011.

  1. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    Hi
    I'm working on a power supply based on a smps and a linear regulator. idea is to use buck converter to drop high voltage to a value 3 volts above the required value, and use a linear regulator to get the output. I'm using LM2596 as buck converter and LM317 as linear regulator.
    I wanted to measure the output voltage using a PC16F877A and display on a LCD. so I used a voltage divider at LM317's out put to reduce output voltage ito safe value.

    now comes the question. power supply is designed to give variable voltage and 21.24V is the maximum. circuit is fine and voltages are regulating correctly. I used a dummy load to test power supplies ability to with stand current requirements. so far I tried up to 1A and regulator works fine. how ever there is a problem at voltage divider. I'm using 10k trimpot as divide to set divide point voltage. so I have tuned it so that when power supply give 21.25v divide point output is 5V.
    under no load condition divide points voltage is correct and varies as i change the output voltage. but when I load the circuit and begin to draw current voltage at divide point increases. actually when I load the regulator, regulator's output drops slightly. I can see this on multimeter. but voltage of dividing point increases rather than decreasing. its funny, just like ohms law got inversed. that is as voltage across trimpot drops voltage on divide point increases. (it should be decreasing right?). Huh

    got any idea what the heck is going on?
    i have attached the linear regulator circuit here.

    PS. I tried changing the trimot but results were same
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The return path from the load has some resistance. Is it a really small-gauge wire?
     
  3. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    ya its a thin wire. but how does that make the effect on voltage divider?
    thank you
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If there is resistance in the return path, and your voltage divider is on the other end of the wire from the regulator, the voltage drop across the wire will make it look like the voltage on the divider is increasing. It's like you're adding resistance on the bottom of the pot.
     
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  5. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    I see I'll solder a thick wire and post the results here.
    thank you
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Not related but-

    The 220Ω resistor between OUT and Iadj is too big. 317's need to put out as much as 10ma to work. Normally 220Ω or 240Ω is used with 5ma LM117's. 120Ω for 10ma 317's.
     
  7. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    oh, but data sheet is for both lm117 and lm317. only one circuit diagram with 240 ohms in that data sheet
     
  8. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    I soldered a thick wire, but problem is still there :(
     
  9. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    @sgtwookie.
    it seems you are right.it seems problem is with the resistance of the current paths. I added parallel paths to circuit also and seems problem is gone.
    thank you very much
     
  10. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    funny, problem is back, I have added parallel paths to all current carrying paths, but problem is still there
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Did you build this circuit on a breadboard?
    If so, that may be your problem.
     
  12. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    no circuit is on vero board(strip board). soldered
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    OK, the problem was solved, and then it came back. Did the solder crack on one or more connections?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    I think we're getting to the point where we not only need a complete schematic, but also the board layout, and photos of the board.
     
  15. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    no solder cracks;

    cant draw complete diagram, mutism doesnt have required symbols :(, and for the lay out, now its a mess,with all those parallel paths(wires)
     
  16. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The specs are different between the 117 and 317. There is a minimum current spec for both, the max value (needed to guarantee operation) for the 117 is 5ma, 317 is 10ma. The National Semi datasheet shows this. Their applications at the end of the datasheet also show this.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    220 Ohms vs 120 Ohms for R1 is a relatively minimal problem right now.
    If Vref (the voltage from OUT to ADJ) is the nominal 1.25v, there is ~5.68mA current flow through R1. As long as there is an additional load of 4.82mA on the output, the voltage will be within the guaranteed regulation limits.

    If the total output current from the OUT terminal is less than 10mA, the output voltage may rise; the less the current output below 10mA, the worse the regulation becomes.

    aruna1,
    Without a complete schematic and layout, it's going to be rather difficult for us to help you remotely. Schematics and layouts with photos remove most of the guesswork. Guesswork takes a LOT of time.

    However:
    • Did you measure the actual voltage on the wiper to GND and compare it to the output of the PIC? (if they are different, that is a problem)
    • Is there a voltage drop from the low side of the pot to the power supply return? (if so, that is a problem.)
    • Is the PIC power supply getting loaded down when you draw higher current on the load you're monitoring, thus throwing off the PIC reference voltage (decreasing the PIC's ADC reference voltage, so the PIC thinks the input voltage is higher)?
    • Is there sufficient filtering on the input of the regulator, so that the output voltage does not sag during higher current output?
     
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