LM317T reverse voltage.

Discussion in 'The Projects Forum' started by irpheus, Dec 15, 2010.

  1. irpheus

    Thread Starter New Member

    Feb 15, 2009
    2
    0
    Hello,

    Just a quick question which I hope you can help with: Any of you know if the LM 317T will accept a positive voltage on the output that is ~20 volts HIGHER than the input without breakdown? I can't see this from the datasheet...

    Help appreciated :)

    Jesper
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Yes, the LM317 will accept up to a 40V differential between input and output. I had a little trouble finding it, but it is on page 4 of this datasheet.

    http://www.national.com/ds/LM/LM317.pdf

    Welcome to AAC!
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Actually this is mentioned in the section "Protection Diodes" of the datasheet.

    Most if not all three terminal voltage regulators will not tolerate this kind of reverse voltage differential and the semiconductor junction will break down. The data sheet advise to connect a diode from output to input to safely bypass the current path away from the semiconductor junction. Though the LM317 makes this mandatory only when the differential voltage is higher than 25V as its junction can sustain the abuse. I would never take this risk because of the consequence of overvoltage to the circuit that it was driving.

    This is an abnormal situation and is not the usual input/output voltage differential as pointed out by Bill above.
     
  4. irpheus

    Thread Starter New Member

    Feb 15, 2009
    2
    0
    Hi,

    & thanks for looking into this. I apparently have missed the implication of this in the datasheet so appreciate that 6 eyes & 3 minds were better than two (one) :)

    My reason for looking into this is that I actually have connected the protection diodes, however, one of the went up into fumes yesterday because I happened to short-circuit the input to the LM 317. I use a smaller solarcell panel to charge some batteries (13 volts) and when the wires from the solarcells short-circuited, the diode conducted a lot of current - which it couldn't sustain.

    The short-circuit might happen again so was looking to find out if the LM 317 would accommodate the battery-voltage on the output, and over-looked the information in the datasheet.

    Thanks again!

    Jesper
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Jesper,
    Use a diode on the output of your charging circuit so that the regulator output will never "see" the battery voltage if it's higher than the regulator's input voltage. Without such a diode, your regulator circuit could be draining your battery bank.

    A low-VF diode (Schottky) will consume less power than a standard diode.

    See the attached for an example. D6 is a 3A 20v Schottky diode.
     
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