current limiter advice

Discussion in 'The Projects Forum' started by justinesmithies, Aug 21, 2013.

  1. justinesmithies

    Thread Starter New Member

    Aug 4, 2013
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    I am trying to make a circuit that will supply a maximum of 2Ma 12vdc that will with stand circuits and still only allow the max of 2Ma output without damaging the psu supplying the 12vdc.
    Can someone just verify that i have done this correctly please ?


    https://www.circuitlab.com/circuit/783ath/2ma-current-limiter/

    Thanx in advance.

    Justine
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    I've used that kind of circuit, and they work well. It looks right to me. However, you should be aware that it relies on the Vbe of a transistor for its operation, and that's not a high-precision figure. It'll limit current, but it's not a very accurate way to do it.
     
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  3. justinesmithies

    Thread Starter New Member

    Aug 4, 2013
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    Thanx John for your input.

    Do you have a better schematic to do the same that would be more accurate ?
     
  4. #12

    Expert

    Nov 30, 2010
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    I'd just scratch around in a drawer and get a jfet to make a constant current generator.
    Way more stable than a Vbe and doesn't have the waste current problems of an LM317.
     
  5. justinesmithies

    Thread Starter New Member

    Aug 4, 2013
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    I need the circuit to never supply more than 12vdc 2Ma max even under short circuit conditions.

    All suggestions welcome.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    And let's be clear that we're talking about 2mA and not 2MA, which is a billion fold bigger problem.
     
  7. #12

    Expert

    Nov 30, 2010
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    Don't use more than 12 volts for the supply and it will never deliver more than 12 volts. Use a current limiting circuit and it will limit the current. (I already drew one for you.) Of course, I'm assuming you didn't mean 2 Mega Amps.
     
  8. justinesmithies

    Thread Starter New Member

    Aug 4, 2013
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    Sorry guys typo's which yes do matter.

    2mA

    #12 I see in your circuit you have a variable resistor which of course would allow for more accurate adjustment , Would that survive a short across the outputs ??
    Also i need to add in the other transistor so i can switch this circuit on / off using a 12v high for on and 0v for off.

    What is the equation to work out the value of the resistor in this circuit ?
     
  9. #12

    Expert

    Nov 30, 2010
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    It's complicated. You have to choose a jfet that has an Idss greater than 2ma. Then the resistance depends on the gain of the jfet, the actual Idss, which varies from one jfet to the next, even in the same batch. You can't just calculate a resistance for all jfets, even if they all have the same part number. I declared a 2N4416 because the Idss is 5 ma to 15 ma and I have a drawer full. I don't even know for sure if 5k would cover the right range. You have to measure as you build it. More resistance = less current.
     
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