Resistor wattage

[Um...Me123]

I need a 80 ohm resistor at 1.1 watts.  When you combine reisistors in series does the wattage combine.  So would two 40 ohm 1/2 watt resistors in series be equal to 80 ohm 1 watt instead of using a power resistor?
Thanks

 

[audioguru2]

Yes, but resistors operated at their max rating are extremely hot, enough to melt plastic or char a pcb. In circuits I design I never allow a resistor to operate at more than 0.75 of its max power rating.

 

[Alun]

Mounting the reisitor 3-10mm away from the PCB and fixing it vertically in your project's case will help a lot. 1.1W isn't that much power and you might be able to get away with overloading it by 0.05W for short periods, if in doubt use three 27ohm resistors in series and audioguru is wasting 25% percent of his resistors. ;D

 

[audioguru2]

audioguru is wasting 25% percent of his resistors.

Resistors are very cheap. I derate them to keep them cooler and reduce any change of their value caused by heat.

 

[bwriter]

Of course, you could achieve the same result by connecting two 160 Ohms resistors in parallel. Just in case it's not obvious to everyone..

 

[OutToLunch]

Resistors are very cheap. I derate them to keep them cooler and reduce any change of their value caused by heat.

Lifetime reliability would also be in question - reliability of the resistor in question and surrounding components that will be absorbing that heat. I suppose I would fall into the category of wasting 50% of the potential of the resistors I use cuz I always derate by that much. You always want a margin of safety - personally, I believe a factor of 2 should always be strived for.

 

[indulis]

Sure, your MTBF would be affected running any component at it's limit. What determines a resistors wattage... like everything else, a certain temperature rise!!! As long as you can keep it "cool" your OK.

Convection, while better than radiation, still isn't as good as conduction, for thermal cooling, so why would you want to move the resistor further away from the PCB. If it's a power run, it should have nice BIG & FAT copper traces which should have much more surface area than the resistor itself.  I can't be 100% certain about thru-hole components (been away from them too long), but SMT resistors have to be de-rated over temperature.

While it would be nice to have 50% de-ratings, there can be times when things like that can actually hurt you... take a SMPS primary switching FET for example. Say you look at the drain waveform and see a 120V spike. Now, a 150V MOSFET might have a Rds on of say... 50 milliohms and work just fine, while a 250V MOSFET might have a Rds on of 150 milliohms, so that's a factor of 3 higher for power dissipated...  which one would you use??? Of course it would be the 150V MOSFET!!!!

 

[OutToLunch]

you've gone off into tangential land - a resistor is not a FET.  Still, though, depending on the architecture of the SMPS, I would usually recommend starting with a FET that has a max voltage rating at least twice that of the input rail.

 

[Alun]

Lifetime reliability would also be in question - reliability of the resistor in question and surrounding components that will be absorbing that heat. I suppose I would fall into the category of wasting 50% of the potential of the resistors I use cuz I always derate by that much. You always want a margin of safety - personally, I believe a factor of 2 should always be strived for.

Most resistors already have quite a big safety margin, if you solder them 3mm to 10mm away from the board surface and run them at 100% (or even slightly higher but don't run it for long periods) of their rating then as long as their is enough air circulation (they're not packed too close to neighbouring componants) then they should come to no harm. Normally it's better to give one resistor more space rather than packing lots of them into a small area.

 

[OutToLunch]

i suppose i'm looking at this from a different perspective. every where i have ever worked has attempted to squeeze as much wattage per square/cubic inch as they possibly could while at the same time adhering to safety and shipping standards. At the last place I worked, if I had placed a tombstome power resistor on a board or a radially leaded power resistor mounted with clearance to the board, it would have meant slopping it with RTV to keep it stationary so the leads don't snap during vibration testing. Also the ambient temp inside an enclosure can many times be expected to be 70C so any power dissipation on parts has to account for that as well.

If you have an open circuit with a fan blowing at least a few hundred lfm across anything dissipating power, then, sure, run at 100% rating.

 

[cynicmonster]

Hi All,

Just curious as how to determine the power rating of a resistor....a bit more info...I have picked up a box of electronic bits and pieces recently and have a quantity of what are large, old resistors. They are constructed of a dark tan material, almost like a PCB material, the smaller ones are 10mm x 3mm radius, while the other lot are bigger again.

And on the point of old parts, do parts such as resistors, caps, etc. have a shelf-life (besides improvements on tolerances, are they unusable after a certain period of time - I wouldn't have thought so...).

Picked up an old A.E.E brand capacitor - the code is RKA1230  1 uf. I can't imagine using it anywhere (about half the size of a brick!!) but curious to find out if it contains PCB. I haven't been able to find any info from this end specific to this model.

Anyone need a Mullard Brand ZM1000 valve???....

Hope to hear back soon.