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

 

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.

 

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?

 

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.

 

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.