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LM317: LM317 operation in cold

nader niakan90
Intellectual 470 points
Part Number: LM317

Hi,

I have a LM317 in my circuit. The normal operation of my circuit is : Input Voltage : 20VDC, current draw: 30 mA, Output: 5 VDC. My circuit functions normal at Room temp. But, in cold (below -10C) acts strangely. When I place my PCBA circuit in the cold chamber while it is already connected to power supply, as soon as I turn on the PS, the PS ramps to 13 VDC (instead of 20 VDC, which I set the PS at) and current hikes to 55 mA (which supposed to be 30 mA). This abnormality is not transient and in fact stays for as long as I keep it running.

On the other hand if I keep the same condition and situation (cold temp), but this time I hot connect my circuit to PS (connecting my circuit to PS, while PS is already on and is at 20 VDC), then everything is normal, i.e. input is 20 VDC, current is 30 mA. I repeated the test with various PSs from different vendors and I still get the same result. I think this must be due to intitial start up / cranking of LM317.

I have attached partial circuit schematic, where LM317 is located.

I have also attached scope captures of the above two situations.

I appreciate any little idea to enlighten me on this, because I am baffled on it !

Thanks

  • 0
    TI__Guru 55940 points
    Hi Nader,

    Thank you for providing the scope shots. This is always helpful when attempting to help debug remotely.

    Do you have any scopeshots that include your output voltage? It would be good to know what that rail is doing during the startup sequence.

    Additionally could you double check your power supply's current limit?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 470 points

    Hi Ryan,

    The scope shots were taken by Keysight's App Eng., when he came to our site. We were not interested in the output value, as we were focusing to see why the hot connection would make the circuit work normally, while the other option (PS already connected to the circuit, before PS is turned on).

    This phenomena must be due to start up conditions of LM317, which I am not aware off and do not have the knowledge.

    Thanks

  • 0 in reply to nader niakan90
    TI__Guru 55940 points
    Hi Nader,

    Without a scopeshot showing the output voltage as well as the input voltage there is no way to tell what the linear regulator is doing during your event. Your scopeshots provided so far show that the input voltage rail reaching a voltage and regulating with no indication of what could be holding it at a lower rail. As you stated there do not appear to be transients on the input rail; therefore, in order to help debug additional information such as the output rail of the regulator is needed to give an indication of what could be causing the input rail to not reach the intended voltage.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Guru 55940 points
    Hi Nader,

    Since it has been several weeks without a reply, I assume that this thread is resolved. If this is not correct, please feel free to post again.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 470 points
    The setup in our lab has been torn down already, and unfortunately I am not sure when i will be able to repeat the test and record the output. But, I like to leave this issue open for a little more while. Thanks