Resistor thermal power loss

Discussion in 'General Electronics Chat' started by Prach, Aug 8, 2014.

  1. Prach

    Thread Starter New Member

    Aug 8, 2014
    4
    0
    Hello,

    I am using two paralel ordinary 0,6W 5k6 resistors in my application. The resistors are wired as pull-ups for switching using transistors with open collector, and for the switching voltage is 24V. All of this is located on a PCB with a fan located cca 10-20cm from the resistors. My concern is this - is it safe in a long term stability to operate these resistors considring quite high thermal power loss (~0,1W @ 24V)?

    I have actually ran this wiring for over a year now (with very high uptime) and no problems have emerged so far. But since I'm doing this again on another machine I would like an opinion on this matter.

    I'm attaching simplified wiring scheme.

    Thanks,
    Prach
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    I would say that less than 20% of specified power is a lot more conservative than most designs you see around and it should last for years.
     
    kubeek likes this.
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Those resistors will most likely not even get warm, as you dissipate 0.1W on a 0.6W resistor. Even in industrial +85°C ambient temperature and 25% overvoltage they should be still nice a comfortable .

    BTW welcome to our forum, fellow Czech! Care to post on what is this going to be used for?
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    A "typical" safety factor used in most industries is to size a resistors wattage at 2 to 3 times its dissipation..
    So if its dissipating 1W you use a 2 or 3W resistor.
     
  5. Prach

    Thread Starter New Member

    Aug 8, 2014
    4
    0
    Thank you for your answers! The resistors do actually get a bit warm, by the feel I'd guess ~40-50°C.

    @Kubeek: Thanks for the welcome. This wiring is part of a PCB that converts 3.3V Arduino output to 24V logic using photocouplers. All this is used to control step motors.
     
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