Connecting Ic's

Discussion in 'General Electronics Chat' started by martilla, Sep 28, 2013.

  1. martilla

    Thread Starter New Member

    Sep 28, 2013
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    Hi all,
    I have constructed a circuit where the output from an ic HCPL-3760 is at 100/uA inputs into a Mosfet driver requiring maximum 10uA. I have found that a 540K resistor would do the trick. Is this critical or will the driver IXDI-604 just take the current it requires?
    When an IC supply is quoted at 3mA is it important to reduce current to this level or will the IC take what it requires?
    Sorry in advance for such a basic question but circuit diagrams just give a voltage without supply current limiting.

    Regards
    Martilla
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,381
    The IXD-604 is basically a voltage operated device, normally a resistor is required to bias or supply current for the output of the Opto isolator.
    Max.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Most ICs just take the current they need at their rated voltage so they don't need a resistor in series. The input ant output of the HCPL-3760 do require resistors to limit the current.

    540kΩ is a rather high resistor value and will cause slow circuit operation. How fast does your circuit need to switch?

    The 100μA input current of the driver is just leakage current and you normally don't limit for that.

    Post a circuit diagram of what you are trying to do.
     
  4. martilla

    Thread Starter New Member

    Sep 28, 2013
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    crutschow,

    circuit attached. I have built this circuit and it does work but I am concerned about the high resistor values.
     
  5. #12

    Expert

    Nov 30, 2010
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    It seems you have read a leakage specification and mistaken it as a requirement for that amount of current. (Mosfet gates are often driven with whole amps of current when you want them to switch really quickly.) You have proven that a mosfet can be driven by tiny amounts of current, but that will only work for fairly slow circuits. The gate to source capacitance becomes significant..no, dominant at high speeds.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1,394
    You are running your green LED at almost 30mA.
     
  7. #12

    Expert

    Nov 30, 2010
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    Can't see inside the MOSFET package IXDI604PI but there is a 12 volt supply going in there, too. (Risk of LED burn.)
     
  8. martilla

    Thread Starter New Member

    Sep 28, 2013
    3
    0
    Yes you are both correct about the green LED @ 12V. I added these LED's for trouble shooting.
    I will change it to 500 Ohm.
    Thanks
     
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