Current Limiting

Discussion in 'General Electronics Chat' started by newton, Apr 19, 2007.

  1. newton

    Thread Starter New Member

    Apr 19, 2007
    9
    0
    Hello,

    I have a microcontroller that has an input voltage pin (vdd) that is specified at 4.5-5.5vdc with a max input current of 250mA.

    First, how do i make sure the input current is < 250mA?

    Second, the Vss pin says that it's max output current is 300mA, do i need to do anything specikal on this pin, or just hook it to the negative side of my power circuit? (the microcontroller is dsPIC30F4011-20E/P), the power supply circuit is from a 7805 voltage regulator.

    I appreciate any direction, or any other advice.
     
  2. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    For the VDD current you could add a fuse, a resetable fuse or a current limit circuit.
    I would suggest a resetable fuse. Check out Littelfuse for device specifications.

    If there are fault conditions in your system where you could sink enough
    current to exceed the 300mA of VSS current you could use current limiting resistors or
    output buffers.

    (* jcl *)
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,147
    1,791
    Generally speaking you dont need to worry about this limitation UNLESS you have outputs that are sourcing or sinking large amounts of current, or you are running at a very high clock rate say greater than 20 MHz.

    You should not be using the processor to source or sink large amounts of current. If you need to do this you should use a transitor or FET driver.

    The fuse probably wont save the processor anyway because they act too slowly and they require a substantial overcurrent in order to blow.
     
  4. newton

    Thread Starter New Member

    Apr 19, 2007
    9
    0
    I am only using this processor as a protocol changer driving a max232cpe serial line driver chip. will this cause a large sink or source of current? i will also be switching two led's on and off.

    Thanks for the help so far guys.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,147
    1,791
    It does not sound like it. What current are the LEDs running at?
     
  6. newton

    Thread Starter New Member

    Apr 19, 2007
    9
    0
    they are at 10 mA with a 470ohm resistor in series
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If you are only driving an RS232 interface IC and two 10mA LEDs you probably do
    not need any current limiting.

    Check out the IOL specification for the uC. You may want to use a transistor to buffer the outputs that drive the LEDs. 10mA sounds a little high for the PIC.

    (* jcl *)

    ---
    www.luciani.org
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,147
    1,791
    If you try to draw 10 mA and the PIC does not want to supply that much, the voltage at the output will rise to relieve the stress. The only effect is that the LED won't be quite as bright. Using a transistor switch, the PIC output has plenty of current for the base of the transistor which will then have more than enough schtutz for the LED.
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Most PIC would sink/source 20mA, and can be used to drive LED directly. Just don't drive too many LEDs simultaneously. Two are fine. Ten or more might be not.

    May I ask why do you choose the dsPIC for your application? It seems over the top. Those are 16-bit DSP like controller.
     
  10. newton

    Thread Starter New Member

    Apr 19, 2007
    9
    0
    yeah, i chose the dspic b/c i needed two uart ports, and in the future i will need pwm and qei. i also will need a gob of memory for all the data i need to store and use. i am not comfortable wiring up external memory, or anything for that mantter, so i wanted everything on one chip (memory). future expansion without development... hope i chose wisely.
     
Loading...