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I am using 12V , 2A DC power supply as an input to LM338T and LM1085-IT 3.3 , for 5V and 3.3V output respectively.

The issue i am facing here is when these both voltage regulators are connected in the circuit , LM338T only gives 1.26V output , which is expected as 5V and LM1085-IT gives 3.3V output (which is correct as expected ).

When i disconnect the ground wire between the 12v ground and LM1085-IT 3.3v regulator , the LM338T gives proper 5v but when this ground wire is connected it gives only 1.26V.

Capacitors are connected as given in the datasheet of LM338T, but have not shown in my image.

Please guide me , i am a newbie in electronics and lacking to solve it.

Here i am attaching the image of the connections.

enter image description here

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    \$\begingroup\$ Where are your caps? \$\endgroup\$
    – Trevor_G
    Commented Jan 22, 2018 at 19:24
  • \$\begingroup\$ they are connected. haven't shown here in the image. \$\endgroup\$
    – KarmaCoding
    Commented Jan 22, 2018 at 19:25
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    \$\begingroup\$ THen edit the question and use the schematic editor at the top of the editor to show a real schematic... How is anyone supposed to answer this when you only give half the information...... \$\endgroup\$
    – Trevor_G
    Commented Jan 22, 2018 at 19:29
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    \$\begingroup\$ Your schematic shows measuring the 5V output between OUT and ADJ. it should be between OUT and the negative of the input voltage (ground). \$\endgroup\$
    – DoxyLover
    Commented Jan 22, 2018 at 19:33
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    \$\begingroup\$ In short, the hook-up of LM338 is completely wrong. Please see Datasheet for proper connections and recommended resistor values. \$\endgroup\$
    – Ale..chenski
    Commented Jan 22, 2018 at 19:38

2 Answers 2

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The LM338T regulator is a very old design, and it has very substantial "adjustment" current, 50 uA. As such, the feedback circuit has to be of relatively low impedance. Check with datasheet and application diagram on page 12, it needs about 820 Ohm and 270 Ohm resistors to operate properly.

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  • \$\begingroup\$ Actually i searched on google to calculate the R1 and R2 on couple of websites, Hence used the available resistor values. But if i disconnect the ground wire of power supply and LM1085 3.3 , the LM338T is giving me proper 4.96V output. \$\endgroup\$
    – KarmaCoding
    Commented Jan 22, 2018 at 19:40
  • \$\begingroup\$ @KarmaCoding, apparently you used wrong Google sources for your calculations. The correct resistor values are 270 Ohms instead of "green 51k", and 820 Ohms instead of 15k. Manufacturer's datasheet always should be trusted first against Internet hackings. \$\endgroup\$
    – Ale..chenski
    Commented Jan 22, 2018 at 19:44
  • \$\begingroup\$ Thanks for helping so fast. I really appreciate it. I will check it with fig16 and get back soon. \$\endgroup\$
    – KarmaCoding
    Commented Jan 22, 2018 at 19:47
  • \$\begingroup\$ @KarmaCoding, you should be checking with Fig.16 15 mins ago :-) And keep in mind that the regulator is designed to produce 5V (with 270/820 Ohm divider) between common GROUND (negative terminal of your 12-V source) and OUTPUT. You need to correct your diagram and measuring method. \$\endgroup\$
    – Ale..chenski
    Commented Jan 22, 2018 at 19:51
  • \$\begingroup\$ Seems like a newbie in electronics like me have a lot things to learn. thanks for the response. \$\endgroup\$
    – KarmaCoding
    Commented Jan 22, 2018 at 19:57
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The Ground wire for your "Point B" should connect to the negative side of the 12 volt supply, not to the LM338 ADJ terminal. The LKM338 attempts to maintain about 1.25 volts between ADJ and OUT terminals.

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  • \$\begingroup\$ I connected according to your suggestion , then it is showing 2.30v at POINT B. \$\endgroup\$
    – KarmaCoding
    Commented Jan 22, 2018 at 19:38

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