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Choosing resistor value

Discussion in 'General Electronics Discussion' started by steve_venkat, May 25, 2017.

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  1. steve_venkat


    May 22, 2017
    I noted this in PIC16F877A controller.They mentioned into current maximum to VDD is 250mA. While choosing resistor across power supply and VDD means we having use 250mA maximum value to find out the specific resistor value or have to use any lower value below 250mA...?

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    No resistor is required. You must design your system to keep within the budget.

    It's a bit like the maximum load rating for a truck. You don't ask what cost per pound you need to charge to avoid overloading the truck, you weigh the boxes and make sure the total weight is ok.
    Arouse1973 and steve_venkat like this.
  3. steve_venkat


    May 22, 2017

    Thanks for your kindly reply...
    I can't understand this line "you weigh the boxes and make sure the total weight is ok"..

    Are you suggesting to calculate something else..?
  4. Audioguru


    Sep 24, 2016
    The maximum allowed current into the IC is 250mA. You do not use a resistor to reduce the current, instead you do not overload the IC. If the IC has no load then its current is only a few mA.

    The battery in a car can supply 400A to the starter motor when it is very cold. But the clock in the car does not use 400A, instead it uses only about 10mA. A resistor does not limit its current, instead its design and loading determine its low current.
    steve_venkat and davenn like this.
  5. davenn

    davenn Moderator

    Sep 5, 2009

    read AG's post --- specifically the first bit

    you need to consider the loads on the outputs of the micro and ensure that they do not, when combined, exceed 250mA
    I would make sure they don't exceed around 200mA ... I wouldn't want the micro operating anywhere near its max

    steve_venkat likes this.
  6. steve_venkat


    May 22, 2017
    1.In PIC16F877A they mentioned like this..If i used all ports as OUTPUT means the IC will be overloaded right..?
    2.If I used PORTB as INPUT and PORTC is used as OUTPUT means,the output current will depend on VDD..?
    3.Or output of each port is depend on VDD..?

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  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    No. All of the ports might be outputs, but the current drawn from them might be a fraction of a milliamp.
    Arouse1973 likes this.
  8. BobK


    Jan 5, 2010
    Calculate the current drawn from pins that are outputs and high and add them up.
    Extra credit: Calculate the max current drawn from all pins that might be high as the same time.

    Arouse1973 likes this.
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