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voltage controller circuit

Discussion in 'Electronic Design' started by [email protected], Jun 29, 2007.

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  1. Guest


    I have 30 volt regulated adapter. I would like to create a circuit
    which will allow me to control the voltage from 0 to 30.

    Can anyone tell me where I can find a diagram for such circuit? I am
    very new to electronic. I am creating this for my ety class -- mostly
    to test series and parallel circuits, and few other lab work.
    However, I may use it for other things too.

    Thanks in advance
  2. Guest


    I built following circuit on a breadboard and voltage regulator (5K
    LM317 regulator) was burned -- sparking were coming out it... <--- this was the c

    This is what I did with LM317

    LM317 has three input (a,b,c)
    a goes to ground,
    b connectedt o resistor 240, -- i was using resistor 220om cause I
    don't have 240ohm
    c connected to LM317?

    what went wrong?
  3. Charles

    Charles Guest

    Sparking? Sounds as if your input voltage is far beyond 30 volts.
  4. On Fri, 29 Jun 2007 22:36:48 -0000, in
    how much current are you using in your load
    check the spec sheet for maximum input to output voltage
    work out the maximum power dissipation for a 317, did it have a

  5. Guest

    I have no idea how to check that. The only formula I think of is i=v/
    I am very new to this stuff, beside parallel and series circuit my
    knowlege is nil :)


    LM317 does have a head sink and it is okay. Sparks, and smoke were
    coming out of 5k potentiometer. Why did I do wrong?

    PS: My regulated adaptor has two output 15v, and 30v. According to my
    DIMM one is 15.11V and second is 31.70V. Adaptor came with my HP
  6. Guest

    Sorry, sparks were coming out of potentiometer. Voltage regulator is
    okay, although it was very hot.
  7. ehsjr

    ehsjr Guest

    The diagram is correct, as far as it goes, except that
    your input voltage is way higher than it needs to be at
    40 volts, which would cause a whole bunch of extra heat.

    For a maximum of 12.71 volts output, you want to keep the
    input voltage to no more than ~16 volts. The 220 ohm
    resistor is not a problem. Your table of resistances
    and output voltage, if those are measurements, suggest
    a high current draw and no heatsink. For example, with
    220 R1 and 2200 R2 and a measured output of 12.71, you
    are over a volt low. Makes me believe you are cooking
    the LM317.

    What to do?
    1) Make _absolutely_ sure you set the 10-40 volt input supply
    to *no more* than 16 volts when you want <13 volts out.
    2) Tell us what the load is - how much current does it draw?
    3) Put the LM317 on a heat sink.
    4) Depending on your answer to 1) you may need to add pass

    Let's get the ~12 volt supply sorted out before going
    to the ~30 volt supply.

  8. Guest

    Tomorrow, I will buy two new potentiometer. In some of my lab
    expirement I need 20-25v output. I still don't understand how my
    potentiometer end up sparking, and smoking.

    I am sure I put it in a wrong way, I just don't know which way would
    be right though...
  9. Guest

    Where do you have the potentiometer connected. I don't see it in your
    schematic. The schematic is correct. Try putting the potentiometer in
    place of the two resistors: put the pot's wiper to the ADJ terminal
    on the LM317, one end of pot to OUT and the other end of pot to grond.
    That should vary the output voltage.
  10. Mike

    Mike Guest

    No, Don't do that! It won't work. The 317 wants the 240/220ohm from out to adj.
    Connect the pot as a rheostat betwee adj and Gnd.

    When truth is absent politics will fill the gap.
  11. ehsjr

    ehsjr Guest

    Your diagram does not show a potientiometer. It
    sounds like you have connected the potentiometer
    to the wrong place. Post a schematic that shows
    the potentiometer.

    You can get 20-25v output. But first, you need
    to get the basic circuit to work properly. Then
    you can add pass transistors.

    You can measure how much current is drawn to
    get a fairly close ball park figure this way:
    put a 1 ohm 5 watt resistor in series between
    the Vout from the 317 and the load. Connect
    your voltmeter across the 1 ohm resistor.
    The reading you get is the current in amperes
    that the load is drawing.

  12. Guest

    I haved replaced the R2 resistor with 25k potientiometer (i will be
    using 5k one later). Right now I am using 25k potientiometer. I have
    also change the input voltage to 4v from a unregulated adaptor (for
    testing). Both the potientiometer and regulator get really hot.

    I think i am putting potioentiometer in a wrong way...

    Mine looks like this

    It has three pin a,b,c
    this is the way i put them.

    a went to ground
    b went to output
    c --- where does this go?

    I have also bought 50k and two 5k potioentiometers.
  13. neon


    Oct 21, 2006
    you cannot input into an LM317 mre then 36 volts and zero volts out is ridiculos just shut the power off then. the output can be 1.2v for minimumn. for the same current outthe lm317 needs to get rid of a lot of heat. heatsink is required.
  14. The pot should go in place of R2, with the center terminal connected
    to the LM317 "ADJ" pin, and one end terminal connected to ground.

    You probably don't want a pot of more than 5K, or the useful range of
    the post will be squeezed down at one end.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  15. MooseFET

    MooseFET Guest

    [... LM317 ...]
    You may also want to include a 0.1U from the ADJ pin of the LM317 to
    circuit ground. The LM317 has useful gain up into the many MHz
    range. This is good because it makes the output voltage very stable
    with changing loads but also can cause oscillations if the leads are
    too long.

    If the LM317 is sharing a source with other loads, you will also want
    to think about what happens when the power switch is turned off. You
    may want to add a diode (1N400X) from the output bact to the supply
    (Cathode to supply). This way the output of the regulator is pulled
    down if the input to it goes down.
  16. neon


    Oct 21, 2006
    lets get to basic the LM317 is a 1.2 v regualtor with capabiliy of 1.5 amps max input is 36 volts. the power dissipation is totaly dependend on power out. this guy will never ever go to 0 volts except on power shut down. want zero then the adj must go below -1.2 volts. all the BS will never change that. it is simple to design provided that power dissipation is considered
  17. zvod32


    Jul 10, 2010
    i had a 14v 500mah wall wart, and i wanted to ajust the speed of some fans i had rigged on i had a 120v pot on it...and the thing sparked and was a 1 mega ohm...any suggestions
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