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I tried several times but without success to get the trimmer changing the output voltage of the LM317T . I tired to link the Pin 1 and 2 then all goes to ADJ pin of the LM317T and pin 3 to ground or 2 and 3 to ADJ and pin 1 to ground or using only 2 pins as shown below but nothing worked . Nothing wokred means the voltage remains constant .

Note that the trimmer is working fine when it is not combined with the voltage regulator ( 1 to + , 2 to Vout, 3 to - ) Could you advise which pins numbers to use and how?

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Last update :

The input voltage is 5V of an arduino uno . The output voltage required is 3.6 V

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  • \$\begingroup\$ The LM317 is in the www.badbeetles.com website .I would check disipation in the pot ,Adress bypass caps and use 120R instead of 240R. \$\endgroup\$
    – Autistic
    Nov 7, 2015 at 10:44
  • \$\begingroup\$ I used a 200R and the regulator seems to be working but with some unprecision (due to no capacitors were used until now) . I dont see the necessity for checking the dissipation because it is 5V input . \$\endgroup\$
    – Java Main
    Nov 7, 2015 at 11:25

2 Answers 2

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You don't have any of the datasheet-recommended capacitors in your circuit. I bet your 317 isn't regulating.

Also, what's your input voltage?

And are you really connecting a 20K resistor between ADJ and out? That can't possibly draw enough current to regulate (min guaranteed for regulation is 10mA). I suspect you've read the resistor backwards and you think it's 120 ohm, but it's probably not (2% precision resistors are rather uncommon, whereas the blue-bodies ones usually are 1% metal-film). Best measure it to be certain.

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  • \$\begingroup\$ Input voltage is 5 volt . Thank you for explanation about the relationship between the LM317, the current and the resitor . However i do not understand the necessety of the capacitor unless for makind stable the voltage and that is now what i am experiencing in the measures . I will add capacitors to see if it makes diffrence . \$\endgroup\$
    – Java Main
    Nov 7, 2015 at 11:23
  • \$\begingroup\$ LM317 needs to drop about 3V, so with 5V input you cannot expect more than 2V regulated output. \$\endgroup\$
    – Fizz
    Nov 7, 2015 at 11:25
  • \$\begingroup\$ And since minimum output is going to be 1.25V... that doesn't give you much range. \$\endgroup\$
    – Fizz
    Nov 7, 2015 at 11:36
  • \$\begingroup\$ Thank you again for the information about the voltage drop . But somehow i am getting 3.74 volts . I used 200R do you know how wast it possible to get such voltage knowing the pot drop? \$\endgroup\$
    – Java Main
    Nov 7, 2015 at 11:37
  • \$\begingroup\$ Yes, but it will probably vary with the load. That's the definition of it not being regulated. For 3.6V output (from 5V), look at an LDO (low dropout regulator) or a switching (buck) regulator. Alas 3.6V is not a very standard voltage (you can find 3.3V-output regs easily though), so you'll have to use an adjustable one. \$\endgroup\$
    – Fizz
    Nov 7, 2015 at 11:38
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Based on your picture I think you put 20K ohm resistor between Vout and ADJ pin change it to 240 ohm and it would be better to connect pin 1 and 2 of your trimmer together (and connect to ADJ) and pin 3 of it to ground

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  • \$\begingroup\$ yes you are right, i did as you said . It seems to work but not so precisely . \$\endgroup\$
    – Java Main
    Nov 7, 2015 at 11:19
  • \$\begingroup\$ You should put capacitor same as datasheet and Vin-Vout should be more than 3 volt \$\endgroup\$
    – Mohammad Yousefi
    Nov 7, 2015 at 12:28

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