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Current limiting before voltage regulator

  1. May 9, 2009 #1
    I am making a power supply and am using a lm338k to regulate a variable voltage from 2 to 30 volts. I also want to put in current limiting, preferably variable, up to 5 amps since thats what the lm338k is rated at. Since the resistances in the current limiting circuit need to vary when the voltage varies i want to limit the current before the voltage regulator where the voltage stays constant. I was wondering if limiting the current before the regulator will cause a short circuit on the outputs to allow more than 5 amps to flow through the voltage regulator. I don't see how it can since the regulator drawing an excess of 5 amps will still cause the transistor in the current limiter to shut down which will keep the regulator safe, but ive definitely been wrong before.
     
  2. jcsd
  3. May 9, 2009 #2

    MATLABdude

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    Take a look at the National Datasheet for the LM338 (page 12):
    http://www.national.com/ds/LM/LM138.pdf [Broken]

    I think some voltage regulators with short circuit protection have built-in self-resetting fuses. If you look at the first set of graphs, you can see how the device operates near the limits of its operation.

    If you can, buy the steel can (TO-3) version (STMicroelectronics also makes one, but you'll have to compare and contrast the datasheets in case there are some unexpected nasty surprises). Much better thermal characteristics, and easier to heat sink. Also pricier than the TO-220, but worth it.
     
    Last edited by a moderator: May 4, 2017
  4. May 9, 2009 #3

    vk6kro

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    You can put a current limiter before the voltage regulator and you might do it if you were using a three terminal regulator for voltage regulation.
    Don't forget that this will chew up some of your supply voltage and you will need two or three volts above the output voltage for the Voltage regulator. So, you could have to supply 16 volts just to get 5 volts out.

    However, dedicated regulators like the LM723 have built in foldback current limiting as well as sensing their voltage directly across the load (so you don't get poor regulation due to the current sensing).
    They also have superior voltage vs temperature characteristics and allow you to use high powered transistors to carry high currents.
     
    Last edited: May 9, 2009
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