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Current Limiting

  1. May 8, 2007 #1

    cepheid

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    Do you know of any easy way to limit currents in electronic circuits? For example, if I'm using an H-bridge driver chip to drive a particular actuator, and I want to ensure that this load (actuator) doesn't draw more than, say, 1 A so as to ensure the chip doesn't get fried, how would I do that? A fuse isn't really ideal, as it would be nice to be able to reset the system remotely in the event of over-current.
     
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  3. May 8, 2007 #2

    berkeman

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    You can make a current limit circuit based on the p-n junction voltage of a BJT, and make it so that the BJT turning on robs current from the main path (or does something else to limit or cut back on the main current path).

    You use a low-value resistor that is sized to make a diode drop voltage when the current gets up to the limit value. For 1A and 0.6V, that would mean that you would place a 0.6 Ohm resistor in the main current path. Then connect the base and emitter of an NPN BJT across this current sensing resistor, and use the collector to do something that will limit or shut off the main current whenever the B-E voltage gets up enough to turn on the BJT.

    There are plenty of things to think about when using this simple circuit, of course. Like, Quiz Question -- what is the tempco of the p-n B-E junction of the BJT? How will that affect the turn-on voltage over the operating temperature range of your circuit? And you might want to add in some hysteresis to the current limit, and even make it a true fold-back current limit of some sort.

    Does that give you some ideas?
     
  4. May 8, 2007 #3
  5. May 8, 2007 #4

    berkeman

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  6. May 8, 2007 #5

    cepheid

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    If there is some sort of obstruction blocking the shutter being actuated, and this current limiting circuit is in place, then doesn't that mean that the actuator will simply jam with the limited current value flowing through it, and will burnout before I know that something has gone wrong? Maybe what I want is more along the lines of a circuit breaker.


    Edit: But I don't know how you'd implement such a "breaker" suitable for electronic circuits.
     
    Last edited: May 8, 2007
  7. May 8, 2007 #6

    berkeman

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    That's what "foldback" current limiting does. If you have a linear circuit where damage will occur if you just limit the output current to some value, then you have the overcurrent trip a holding circuit that limits the current to something like 1/10 of the trip current, and stays there until the current demand falls below the lower limit. Basically it forces the load to go light (or the circuit to be power cycled) before it will allow full output current operation again. Foldback current limiting is common in linear power supplies.
     
  8. May 8, 2007 #7

    cepheid

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    Thanks for your help! I'll look into foldback current limiting and post back here if I have any questions.
     
  9. May 8, 2007 #8
    Yea I usually google images, that way google skips over websites with no images you would find when just googling. A picture is worth a thousand words or a schematic :smile:


    Foldbacks are perfect for this application, you don't need to replace fuses, or manually turn on the circuit braker, plus circuit brakers for 1 Amp are very hard to find. And third, current limiting is all automatic, you don't have to be around tending the circuit.
     
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