As many others before, I'm aiming to build my own simple CC CV lab bench PSU based on the LM317 regulator. Now, I've defined most of the circuitry, but I'm stuck with a particular design choice regarding the Adj pin of the LM317.

Since I wanted to use an Op Amp to drive the Adj pin, I wanted to know what are the pros and cons when:

  • using the output voltage of the Op Amp at the Adj pin as negative feedback


  • using the output-voltage regulated by the LM317 as negative feedback.

In my mind the latter would be able to account for the voltage reference fluctuations of the LM317, but I assume that on the other hand it might be somehow unstable. If this is the case, how can I compensate for these instabilities?

Added the schematic for the Adjust-Pin driven LM317 circuit

CC CV LM317 Bench PSU Schematic

  • 3
    \$\begingroup\$ What is the point of driving the adj. pin with an op-amp? The whole point of the LM317 is that it is simple. \$\endgroup\$
    – TimWescott
    Mar 16, 2019 at 21:44
  • 2
    \$\begingroup\$ You should include a schematic of what you want to do. The LM317 already has a build-in error amplifier, adding an external one sounds pointless to me but you can maybe convince me otherwise by showing a schematic. Explain why the LM317 voltage reference fluctuations are an issue. You investigate stability by doing a stability analysis. There are many Lab supply designs to be found, explain why you need to design your own. If you intend to design your own then I think that you should be able to answer your questions by yourself. \$\endgroup\$ Mar 16, 2019 at 21:53
  • \$\begingroup\$ you can use an opamp for cc. however, in my experience, the time and effort spent in adapting the lm317 may not be worth it. You may be better off using a TIP122 and designing both the CC and CV. True, there will not be thermal shutdown, so if you really require it, you will have to implement it yourself at the heatsink. If this is your first power supply, use two LM317, the 1st one does CC (with an additional BD139 as variable resistor), and second one does CV. \$\endgroup\$
    – Indraneel
    Mar 16, 2019 at 22:10
  • \$\begingroup\$ I was using the LM317 over forty years ago when it was relatively new. It's not new anymore and it is as inefficient now as it was then. I recommend that you use a modern device and a modern design - unless you are trying to gain first-hand understand of why using the LM317 for a CV+CC PSU is such a bad idea. \$\endgroup\$
    – Transistor
    Mar 16, 2019 at 23:37
  • 1
    \$\begingroup\$ Added Schematic. I'm aware that it's not the most elegant design as well that other people have alredy done similar (better working) circuits, but that was never my point. I'm trying to design my own PSU to learn some of the basic design and working principles. I'm using a LM317 because I have even less experience with discrete BJT o MOSFET linear regulator circuits. I'm using an external Op Amp to add CC CV which could (it isn't right know) digitally driven by an Arduino. The whole intention behind this post is to question one of my design choices, as described in the question above. \$\endgroup\$
    – C. Daniel
    Mar 17, 2019 at 10:56


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