Got any 28V current limited switch ideas?

Discussion in 'The Projects Forum' started by ErnieM, May 8, 2012.

  1. ErnieM

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    Apr 24, 2011
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    I'm working on a new test fixture that has to drive several lines with a switched voltage. As happens with test fixtures, bad parts or bad connections can short these inputs, so I want to make a current limited switch, with a limit of perhaps 125 mA. As I'll need lots of these I'm looking for something small and inexpensive.

    What I've come up with is using an old school LM723 voltage regulator in the basic high voltage scheme, and adding a transistor to short out the NI pin to disable. The device is cheap at $.60 each, just a little large as it's only available in DIP packaging.

    Does anyone know of anything else cheap and handy that could do this?
     
  2. #12

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    Nov 30, 2010
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    I've seen the 317L used as a current limiter just because its internal thingy works like you want it, or close to what you want. Uses minimal external parts, too.
     
  3. ErnieM

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    Thanks #12 for the suggestion. It does limit current and survive repeated attempts to kill it, but I don't see a simple way to shut it off.
     
  4. #12

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    Maybe something like this. Still a compromise, but how much to compromise is your judgement call.

    There are also thermistors that go high resistance when overcurrent happens but I'm not very familiar with them, and they aren't switches, either. Currnet limit + switch seems to be two different things.
     
    Last edited: May 8, 2012
  5. ErnieM

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    317's work very nicely as you suggest in your sketch. I actually did just that once centuries back in a model railroad controller where to motor voltage was a variable 12V peak pulse width. It still leaves the (I forget exactly, 1.2VCD?) reference voltage on the output when the resistor junction is shorted. (Model railroad engines just love pulsed power especially when just creeping slowly forward.)

    As far as the thermistor idea goes, I did drop a polyfuse in there, the kind that self-heal, thinking a fault would trip that but things would soon be working again.

    However, the pass transistor I used (SOT-89) would open to protect the fuse.

    Murphy was watching.

    Thanks for idea bouncing.
     
  6. crutschow

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    Many years ago I read that the transistor is always faster than the fuse. ;)

    Edit: The corollary is that the transistor will always open to protect the fuse.
     
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