Control of Voltage Regulator

Discussion in 'General Electronics Chat' started by electrocub, Sep 2, 2009.

  1. electrocub

    Thread Starter New Member

    Nov 9, 2008
    8
    0
    I built the following voltage regulator and do not have as fine as control as I would like. First I used a regular 1 turn pot, then a 10 turn pot but when I turn the pot, the numbers jump 2 or 3 hundredths of a volt at a time. The 10 turn pot is a 10K pot instead of the 5k pot that the circuit called for but I tried it a couple of ways;first with a 10K ohm in parallel which made it a 5k pot like the circuit called for, then at the 10k rating. Still no change. It still bounces around. Am I asking too much to have the numbers change with more control?

    Thanks

    CJ
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    Do you mean that the ten turn potentiometer is making "steps" of 0.03 - 0.04 volts?
    If you want to adjust "fine" use a second potentiometer of 100 Ohms in series with the original potentiometer.
    The original potentiometer will be "course" adjust and the 100 Ohms "fine" adjust.

    Greetings,
    Bertus
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Does the final setting also jump by .01 volt at a time? Or is the control noise only evident as the setting is changed? Is it a new or a junk box 10 turn pot? What happens if you use a 22 turn trimmer?

    At any rate, you can try a .1 uF capacitor to ground from the pot's wiper.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your value for R1 is 240 ohms which is for the more expensive LM117. The cheaper LM317 needs to have a value that is 120 ohms max and then the value of the pot also must be half.
    Or the output voltage will rise when there is no load.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I think you need to remove the 10uFd capacitor off of the ADJ terminal. That can cause the LM317 to oscillate.

    hgmjr
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    The datasheet recommends the capacitor to improve ripple rejection by 15dB and to improve the transient response.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, both the LM117 and LM317 load regulation specifications are given with load on the output that is 10mA <= load <= Imax.

    I believe you're thinking of the LM317L (100mA version), which requires a minimum load of 5mA for guaranteed regulation.

    For the benefit of our OP, since the regulator requires a minimum 10mA load for guaranteed regulation, and Vref (measured between ADJ and OUT) may be anywhere from 1.2v to 1.3v, a 120 Ohm resistor between ADJ and OUT causes the necessary minimum current to flow from the OUT terminal.
     
  8. mkbutan

    Senior Member

    Sep 30, 2008
    270
    16
    I think you need to remove the 10uFd capacitor off of the ADJ terminal. That can cause the LM317 to oscillate.

    hgmjr.


    no i think it is for filtering the output voltage

    and D-2 should be in between output of 317 & C-3 to the J-2(output)
    and D-1 may not be required i think so


    Do you mean that the ten turn potentiometer is making "steps" of 0.03 - 0.04 volts?
    If you want to adjust "fine" use a second potentiometer of 100 Ohms in series with the original potentiometer.
    The original potentiometer will be "course" adjust and the 100 Ohms "fine" adjust.

    Greetings,
    Bertus



    and yes it is possible to have the fine tuning in the ref. ckt.
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Location is everything in electronics. The adjust point is a control point, you don't need the capactor there. The diodes are only needed if you are worried about current back flowing, such as what a large capacitor on the output of the regulator might create.
     
  10. mkbutan

    Senior Member

    Sep 30, 2008
    270
    16


    yes you are right
    the location is everything in the electronics ckt's.
    the capacitor should be in between ground (0)and supply(+) no need of D2 in between
    The adjust point is a control point, you don't need the capacitor there.
    yes the adj. point is a control point you don't required any capacitor there the C-2 should be in the place of C-3
    sorry i can't draw the ckt. as i don't have the s/w for the same
    pl.ref. any free s/w for the ckt. drowning
    thanks
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I found the circuit that the OP is using in the LM317 in the National Semiconductor datasheet. I see that the use of a capacitor is recommended. I missed that circuit in previous datasheet reads. Very interesting.

    hgmjr
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    A 10k pot with 5mA passing through it will give you 5V per turn for a 10 turn pot. That's about 14mV per degree of rotation. That's why you are having problems.
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You absolutely need the cap! Especially when using a something like a pot or a switch to control the voltage.

    My favorite bench supply was modified, I changed the main voltage adjust pot to a 10-turn pot (full sized, not a trimpot). It's nice, I can tweak the output voltage to within a couple mV.
     
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