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I am currently trying to learn to read resistor datasheets, specifically the thermal ratings.

I want to have a general idea of what the temperature of resistor will be at certain power.

Is there some place I can look up and learn to read resistor datasheets in detail?

I found these graphs online when looking to buy a resistor I need and I am having trouble understanding the graphs. I am unsure to what exactly they represent. Can you help out?

Here are the graphs:

This is graph 1. The power consumption curve is unclear to me.

Graph 1

This is graph 2. The power reduction curve and surface temperature are unclear.

Graph 2

JRE
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komrad2236
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    Those curves show that if the ambient temperature gets higher you can't dissapiate as much power as at room temperature without overheating the resistor. The value is given in % of rated power. – kruemi Nov 03 '22 at 12:54
  • I agree with @komrad2236. So for example, for resistor (100W 4Ω), max voltage V40c you can apply at room temperature below 40C is Power40c = V40c x I40c = (V40C ** 2) / 4Ω. Now at temperature 220C, P220c = 20% of P40c, ... – tlfong01 Nov 03 '22 at 13:22
  • It will be easier to answer with an example. How much power will the resistor dissipate? What is your maximum allowable temperature rise? Do you have enough space for the "standard heat sink"? I personally don't want a heatsink that can burn me, but sometimes it is unavoidable. I like to limit the temperature rise to 40 degC. – Mattman944 Nov 03 '22 at 13:31
  • Power resistor fundamentals: https://www.electronics-tutorials.ws/resistor/res_7.html – tlfong01 Nov 03 '22 at 13:48

1 Answers1

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First question is, is there some place I can look up and learn to read resistor datasheet in detail?

I don't know of any better place than this site.

I want to have a general idea of what the temperature of resistor will be at certain power.

You can predict the heat power generated because that is the average of voltage x current. As to what temperature this causes the resistor to rise to depends on how you extract that heat: -

  • No heat extraction means that the resistor will carry on ramping up in temperature at a rate determined by the power and the thermal lag or capacity of the resistor until it melts.
  • In other words, a resistor dissipating power in a vacuum with no other from of heat extraction will continue to rise in temperature because nothing is extracting the heat
  • Of course, in air, there is a cooling effect and this will stabilize the temperature at a value that ensures electrical power in equals heat power out.
  • When mounted on a heatsink, the effective surface area of the resistor increases dramatically and this means with a flow of air (either natural or forced) the resistor will run cooler.

Second, I found these graphs online, when looking to buy a resistor I need, and I am having trouble understanding the graphs or I am unsure to what they represent exactly, can you help out?

The power consumption curve is based around the resistor being mounted (or not) on a heatsink in a particular air-flow condition (forced or natural) and, it's telling you that as ambient temperature rises above 40°C you cannot dissipate the same power as below that temperature

The power reduction curve is pretty much the same as the power consumption curve except it is showing options of mounting the resistor on a heatsink (curve B). Curve C and D are for different body size resistors it seems.

The surface temperature rise graph doesn't indicate what the various plots are so I'm stumped on that one.

Andy aka
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