How dimension a Resistor

[roiberts]

Dear Forum

In my design, a resistor need to dissipate 100mW ,so,normally i diemnsion it for 1/4w

Now,i have this dilemma:

2 test signal ,of not ordinary value , nust be considered. :

Test 1 signal create a 0.5w for 60second on my resistor
Test 2 signal create a 1.5w for 5 second on my resistor.

Qestion is:
What power is more important = What resisir i buy = 1.5w oe 0.5 ?

Thanks

Roberto

 

[barry]

I think you need to use a resistor greater than 1.5W. 5 seconds is a long enough time for a resistor to get damaged from being over-powered. It might be something as subtle as a resistance change, or it may affect its life.

At least, that's what I would do.

 

[roiberts]

Thanks for reply

Could be intresting to know what influence have a overpower of tot
second on the life of a resistor...
What calculation or data can i use for get a idea of this effect….

Thanks again

Roberto

 

[BigBoss]

Resistor Manufacturers publish "Admissible Power vs. Pulse Width" curves. in their datasheets.
You can refer them to understand what kind of resistor you have to use.

 

[wwfeldman]

i agree with barry
1.5W for 5 seconds is more than enough to fry the resistor
use a 2.5W or 3W, depending on easy availability.
if its wire wound, you probably want a non-inductive resistor