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Power resistor temperature de-rating seems too harsh

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Hi Treez,

In short, no, I don't. :) Most resistor datasheets I've seen from 1/4W to 5W ones are always the same, derate from 70°C... The self-heating curve might give a clue if you can figure out the connection, I wasn't able to. If you work out the temperature rise of the resistor with your applied voltage and current (may need to obtain Rth from manufacturer, that °C/W figure), that'll show where to derate from, if you get me. Tamb + Trise = Tmax @ Vout & Iout I suppose.
 

Thanks, this is only for a cheap, qhick load bank, and i reckon we can run these resistors white hot and it'll be fine.
We'll put a fan blowing on them anyhow.
I reckon the 70 degc figure is all about the resistance change over time which occurs if you run them at full power at >70degc...but we arent worried about small changes in resistance.
Any thoughts?

I mean, ive worked on power supplies for down hole apps...where ambient was 180degc......and you have no fans.....and the components run at well over their rated powers...and it works fine... at least for the 48 hours or so that these things have to survive for.
 

Hi,
I am sure i have run these at 150degc
Click to expand...
The datasheet also says you may operate them at 150°C.
So I see no problem.

But to be more detailed: 150°C is ambient temperature...but only ate 60% of rated power amd then the resistor temperature may be about 250°C.

Klaus
 

Hi,

Won't post endless links, can upload some pdfs of resistor derating web pages and pdfs of same topic I've been reading, if you want. Pretty interesting stuff.

Regarding the why of the 70°C, this page led to these standards: JIS C 5201-1 and IEC60115-1, which seem to be where the 'rule-of-thumb' value comes from. I don't have a spare > €200 to peruse the jis one here.

It seems a few variables compose what can be done with/to a resistor and 48 hours or 2,000 hours depends on several of them.
 

The 70 degree bend is just an arbitrary spec. Physically, the resistors have a maximum internal temperature and a specific Rth. What if you delete the horizontal 100 % line and extend the derating curve to the left above 100%? No more "harsh" derating?

You are looking at the curve from the wrong side. The essential parameter is the 300 degree limit, I presume based on long term lifetime considerations. And Rth, which can be empirically verified.

You may want to question the 300 degree limit temperature, but please consider that datasheets target longer lifetimes than 48 hours...
 

Thanks, ours is just a cheap load bank, and if it lasts 48 hrs in total, then to be honest , thats fine
 

At 70 deg C ambient at full power the internal hot spot is max, running at full power above a 70 deg C ambient ( lots of free air around it ) will raise the internal hot spot temp and shorten the life of the resistor...

btw, the surface of the R is way above 70C, the 70C refers to ambient ... heat flow out of a resistor is prop to Rsurface - Rambient
 
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