margin power of resistor

[wanna_eat_1kg_of_elecons]

Hello, have got a (maybe) simple question. I can calculate the power for a resistor. If i calculate the power dissipated will be 0.2 watt (i^2 *R) for instance and i have got a 0.25 watt resistor that could be fine. But what is the safety margin normally used?

To put it in other words what safety margin is used for what kind of application?

 

[alec_t]

what is the safety margin normally used?

There is no 'normal' margin. It varies according to the application. In an aircraft, hospital, military or other mission-critical use you would expect a wide safety margin. For domestic use a narrow margin may be acceptable.

 

[wanna_eat_1kg_of_elecons]

ok i understand that so what would be a margin be for

military
hospital
toys
automotive
audio

i have not got a idea to be honest but i want to know

 

[crutschow]

Military margins are typically 50% and I would expect automotive and medical margins to be similar. Others are probably more like 25%.

Note that a power rating is often at an ambient of 25C. Anything higher due to higher room temperature or rise in the electronics box from internal power dissipation requires further derating.

 

[colin55]

In addition, each type of resistor operates at a different temperature. The manufacturer sets the temperature and this pruduces the allowable wattage for the resistor. This is why you can get very small 3watt resistors.

 

[audioguru]

You do not want to touch nor have anything that melts near a resistor that operates near its max power rating.
Resistors are designed to get very HOT when at their power rating! A resistor operating at half its max power rating is still pretty hot but maybe is acceptable.

 

[cachehiker]

I'll push them right to their rated limit for many of my own projects since most reputable manufacturers are already building in a 20% margin.

That said, I'll also subtract 20% or more if the ambient temperature is over 40C, 20% or more if nearby devices or materials don't appreciate the extra heat, 20% or more if the application is professional and intended to be very reliable, 20% or more if subsequent replacement or troubleshooting will be especially difficult, etc. That's easily 1/3 to 1/2 of rated power if multiple factors apply. And don't forget to leave a healthy gap between the resistor and the PCB, maximum rated temps for resistors are at least 100°C hotter than those for PCB's.