A week ago, I was given a project by my professor to do part of it and then give it to another student. After soldering the power supply section of it and testing the resistances/voltages, I get a voltage of -10.11V as an output of a -20V voltage regulator(LM337). The regulator is connected up to a variable resistor, although I get a voltage range of 0.729 to -10.11V on it.

I am not able to ask my professor what might be affecting this , as I don't have class until next Thursday.
The assembly manual is here.

Edit: I was tired yesterday, and as such, wasn't really thinking about what to do. I had gone through the troubleshooting guide on page 19 under "No voltage at variable output", and all of the causes were not it. This morning, after studying the board, I realized that two diodes below VR2 were soldered with the opposing polarity. I just soldered it, and I got a voltage of .7V CCW and -22V CW, which is within the 30% tolerance. Thanks you for your answers!

  • 1
    \$\begingroup\$ please rewrite your post to make it a bit more professional ... make a reference to the page where the schematic can be found .... refer to the regulator as U5 ... refer to voltage measurement testpoints by number, since the schematic has them included ... refer to the resistor by its name \$\endgroup\$
    – jsotola
    Jan 24, 2020 at 1:22
  • \$\begingroup\$ Go through the trouble shooting guide present in the document. \$\endgroup\$
    – User323693
    Jan 24, 2020 at 2:22

1 Answer 1


The datasheet of the LM337 shows what the parts should be (the schematic is correct) and how to calculate the setting of the potentiometer and the input voltage to get an output voltage. The 15VAC input has a peak voltage of 15V x 1.414= -21.2V and the rectifiers reduce it to about -20V. The 120 ohm resistor and 2k pot can be calculated for the LM337 to produce at least -18V.

Make sure that the input to the LM337 is at least -18V and check the values of the 120 ohms and 2k pot.


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