Over-voltage protection for LM338

Discussion in 'General Electronics Chat' started by buzaiandras, Aug 20, 2012.

  1. buzaiandras

    Thread Starter Member

    Jul 18, 2011
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    0
    Hi all,

    I have a question about over-voltage protection for the output of an LM338K adjustable voltage regulator.
    Is there a way to create an over-voltage protection so that the protection will kick in at Vout + Vthreshold, where Vout will vary?
    What I mean is that no matter what output I select for Vout the protection circuit will kick in at Vout + the treshold voltage and not at a fixed predefined voltage.

    Is this a common protection method for lab adjustable power supplies? Or lab power supplies usually don't have a over-voltage protection?

    Thank you,

    Buzai
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    Why do u seek the impossible?

    There is a way.
    If u want to protect an output of a PSU tht is variable, u should make an over voltage protector that tracks the out put voltage.

    Of course this is do able but I believe it is out of ur reach.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Most lab supplies don't have overvoltage crowbar protection but some old ones did. I don't think it's very useful.
     
  4. buzaiandras

    Thread Starter Member

    Jul 18, 2011
    54
    0
    That is exactly what I am trying to achieve. Unfortunately I don't know how, yet :). So any help is more than welcomed.

    I am not sure what do you mean by: "Of course this is do able but I believe it is out of ur reach".

    If it is not a very common practice I will probably not use it, but I would still like to learn how can I achieve what I am trying to do :).

    Thank you,

    Buzai
     
  5. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Most supplies use current limiting instead. Regardless of the output voltage the output current is limited and sometimes folded back.
     
  6. buzaiandras

    Thread Starter Member

    Jul 18, 2011
    54
    0
    Thank you. What does exactly mean that "sometimes is folded back"?

    Thank you,

    Buzai
     
  7. buzaiandras

    Thread Starter Member

    Jul 18, 2011
    54
    0
    I researched on the net about the "foldback" term :). So this is clear to me now.
    But, back to the original issue: how can this "tracking" over-voltage protection be achieved?
    I assume the key is in the adjust resistors for the LM338 (but I may be very wrong), since you set the output voltage level from there. But I can't figure it out.

    Any ideas?

    Thank you,

    Buzai
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U see building a tracking supply to track a varying voltage is not easy.

    Do some search on tracking supplies first.
     
  9. buzaiandras

    Thread Starter Member

    Jul 18, 2011
    54
    0
    Already done that. All I could find were twin power supplies.
    And according to Wikipedia, the term tracking power supply refers in fact to twin power supplies.
    http://en.wikipedia.org/wiki/Regulated_power_supply

    Quote from the Wikipedia article:

    I don't expect a readily made schematic or something like that.
    But I consider that sending me to search/google for a response is rude. I already done that, and because I could not find much information about it , I started this thread (who knows ... maybe I missed some resources in my search ...).
    I wouldn't open a new thread without first trying to find the answer by myself

    So if somebody knows how this can be done (and is willing to share ...) please advice :).


    Thank you,

    Buzai
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    This is not done AFAIK.

    Tell u what, why don't you come up with something. U see the real cool thing abt AAC is tht no body will do anything for u. No ****. But if you start from scratch and present it for corrrection/improvement, then there will be loads of replies.

    So first draw up something from ur mind and post it, then we'll by all means tell u to get lost :D or hmmmm ya , u might have something...:cool:
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
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    69
    I really don't see this as a real problem. I would use a voltage divider on the input to the 338 as one input to a comparator. On the output of the 338 use another voltage divider to the other input of the comparator. Install a high voltage transistor between the input to the 338 and the filter capacitors. Use another op amp to control the base of the transistor, with the output of the comparator to control the in/out of the transistor. The first voltage divider I spoke of should be on the output of the filter caps.
     
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