Question about the LM317

Discussion in 'General Electronics Chat' started by jrdoner, Apr 16, 2015.

  1. jrdoner

    Thread Starter New Member

    Feb 21, 2012
    I am trying to build a regulated power supply controlled by a microprocessor. I have the following arrangement working. I operate a CAT5111 digital resistor from an Arduino board, and the 0 - 5 volts variable that I can get out of that is run through an LM358 opamp configured as a 5 to 1 noninverting amplifier, so my 0 - 5v. is now 0 to 25v. Next I take that variable voltage and feed it into the adjust leg of an LM317, effectively as the ground level seen by the adjust pin. When this variable voltage is 0, the LM317 is configured to deliver 5 v. When I move the variable voltage to the adjust pin up, I get an LM317 range from 5 - 30 v., which is exactly what I want.

    Now it appears to me that the load current for the LM 317 in no part flows through the adjust pin, but if it did, I suppose the OPAMP output, which looks like ground to the LM317, would not sink current, or at least only a little. I am hesitant to load test this arrangement until I know that it won't cook my opamp.

    Can anyone enlighten me as to the reliabilty of this circuit?

    Thanks in advance for any insights.
  2. Papabravo


    Feb 24, 2006
    You need to show us a schematic because I cannot visualize your ceircuit from the verbal description. You do know that the output of an opamp cannot assume a value that is larger than the supply voltage -- right?
  3. #12


    Nov 30, 2010
    IIRC the 317 dumps about 50 ua through the ground leg.

    100 ua +/- 5 ua
    Last edited: Apr 16, 2015