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Resistor Wattage Rating

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djsfantasi

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I've read that you should use a resistor with a wattage rating that is 20% greater than the load. Then, I've read double the wattage... Which is correct? I'd normally go for double, but when price and size are a concern, I question it and go for the 20% overhead.

Specifically, as part of a 7 segment display module, which eses 2 LEDs in series for a total Vf of 4VDC and running it at 25mA. My calculations call for a 330Ω resistor. The power through it is 0.193W. No heavy duty switching - either on or off. Is a 1/4W 330Ω resistor ok? And why? 0.194 is 77.6%, under the 80% (20% greater than required) mark.

Thanks,
dj
 
It affects the reliability of the resistor.
The hotter the resistor operates, the greater chance of failure or significant change in value.

Another consideration is that the power rating is for a 25°C ambient temperature.
If the circuit is in an enclosure and/or operated in a higher temperature environment, then you need to use a higher power rated resistor.

So I would stick with the 50% derating unless there's a good reason not to.
 
Depends on what may be good reason. Already ordered the 1/4Ws? Nope, not a good reason... Have to redesign the PCB and relocate the power bus and all sixteen resistors? Ummmm, still not a good reason, but one that took me a couple of hours, because DipTrace Beta kept crashing whenever I tried to reassign nets.

Thanks for pushing me in the direction I was trying to talk myself out of, even though I felt it was the right one (using 1/2Watters)
 
50% is the general number when you are designing something that will work in indurstrial temperatures, i.e. +70°C ambient.
Yours at 0.193 out of 0.250W seem rather ok, but still, I would try to estimate what will the tempereature inside your device be when you use it all day, and look what the manufacturers graph says about power dissipation vs temperature, but most likely you will be ok, that margin sounds about good to me.
 
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