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LM317T Adjustable Voltage Regulator HELP ?

Discussion in 'General Electronics Discussion' started by pete1986, Nov 19, 2013.

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  1. pete1986

    pete1986

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    Oct 30, 2013
    [​IMG]
    input voltage is 12v from a pc power supply

    for some reason the schematic above is not working.i get an output of 1.29v when i should be getting 7.50v

    however if i replace resister (R1) 240K WITH A 150K resister it works and i get a voltage of 10.35v (this was to test the lm317t to see if its in working condition)

    i used a resister calculator and it said
    1200k resister and 240k resister i should get a voltage of 7.50v
    however i get a voltage of 1.29v...if i remove just the 240k resister i still get a output voltage of 1.29v

    so i dont understand why this is not working and im not getting an output voltage of 7.50

    can anyone help ? i used the resister calculator from this web page
    http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    The resistors are way too large. Take the K off the end. I.e. use 240 and 1200.

    Did you see the note in red that R1 must be between 100 and 1000 Ohms?

    Bob
     
  3. pete1986

    pete1986

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    Oct 30, 2013
    that be why. cant believe i didn't see it lol thank you for pointing it out. just wondering can i replace R1 resister with a 0.15w Variable Resistor 1M ?
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Use 240 for R1 and 5k or 10k variable for R2.
    There should also be capacitors across input and output to ensure stability.
     
  5. pete1986

    pete1986

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    Oct 30, 2013
    thank you i have some Tantalum SMD Capacitor 10uf 16v 3216 A
    would these be ok to use ? the input wont be anymore than 12v

    and ill see if i can get some 10k pots online :)
     
  6. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    If you want to adjust it around 7.5V I would use a 1K fixed resistor and a 500 Ohm pot. This would allow adjustment from about 6 to 9 Volts. With a 10K pot, most of the range would be wasted.

    Bob
     
  7. pete1986

    pete1986

    10
    0
    Oct 30, 2013
    just wondering. can i do this the other way round >

    Use 240 for R1 and 10k variable for R2.

    change it to

    240 for R2 and 10k variable for R1 ?
     
  8. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    no,
    its R2 that must be the variable .... R1 sets the reference

    Dave
     
  9. pete1986

    pete1986

    10
    0
    Oct 30, 2013
    thankyou :) i purchased a 4290 variable resistor 10k and a 240 ohms resistor.

    after wiring it all up it works great. i can alter voltage low as 1.23v and a max 10.43v
    im powering it with 12v

    just wondering how i could get it to be a max of 11v

    also on the variable resister there was a leg not connected to anything i believe this is the other side of the resistor. because you only use the wiper leg and one side of the resisteor. anyway i connected the leg 3 to ground and it didn't change anything.so i leave it disconnected of is it best to connect to ground ?

    this is how i have it connected>
    [​IMG]
     
    Last edited by a moderator: Nov 21, 2013
  10. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    The extra terminal should be connected to the wiper if anything.

    Bob
     
  11. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The 317T needs about 2V across it (the droput voltage) in order to work so you will not get above 10V with 12V going in.

    If there is ripple on the input voltage, then it is the voltage trough which is critical.

    To get more output voltage you need more input voltage.
     
  12. pete1986

    pete1986

    10
    0
    Oct 30, 2013
    perfect thank you to everyone thats helped me :) that makes complete scene now and after looking at the ic info you are indeed correct. im running a pc fan at 9v to make is silent because at 12v it was loud. iv run the fan at 3v and there is no heat from the lm317 which i think is amazing
     
  13. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    You will need more than 12V going into the regulator

    Dave
     
  14. pete1986

    pete1986

    10
    0
    Oct 30, 2013
    [​IMG]

    just wondering if anyone can tell me why some people recommend putting a 1N4002 Diode
    is it a reverse polarity protection ?

    iv only got 1N4007 diodes so just wondering if its worth buying the 1N4002
     
  15. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The 1N4007 will do all that the low voltage 1N4002 can do and then some !

    The diode is there since when the power supply is turned off, the input voltage may fall faster than the output voltage. The reverse voltage on the 317 is not welcome.
     
  16. pete1986

    pete1986

    10
    0
    Oct 30, 2013
    to protect it from reverse voltage could i just connect a 1N4002 positive side od the diode to the positive side of the input voltage.
     
    Last edited by a moderator: Nov 21, 2013
  17. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    connect it as shown in the circuit you posted

    Dave
     
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