Metal chassis resistor power dissipation

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Discussion Overview

The discussion revolves around the power dissipation characteristics of metal chassis resistors, specifically a 47 ohm, 50W resistor used at 24V. Participants explore the thermal performance of these resistors, including temperature rise and the implications of using them without heatsinks.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the resistors reach temperatures of 48-55 deg C after a few minutes of use, despite being rated for 50W at 25 deg C without a heatsink.
  • Another participant suggests that the 50W rating is applicable only with a standard heatsink, proposing that without it, the resistors are rated for about 20W.
  • A participant questions how to calculate the temperature rise for resistors and refers to a datasheet that indicates a significant temperature rise when dissipating 12W of power.
  • It is mentioned that the temperature difference from the datasheet should be scaled based on the power rating with and without a heatsink.
  • Some participants express surprise at how hot the resistors get, indicating that they are not designed for situations where they can be touched.
  • Concerns are raised about the failure of the resistors depending on internal temperature, with a note that relevant data for this calculation is not provided in the datasheet.
  • One participant humorously suggests that touching the resistors could lead to burnt fingerprints, emphasizing the need for caution.
  • The utility of an IR thermometer for measuring temperatures in circuit work is discussed, with recommendations for its use in both low voltage and high current applications.

Areas of Agreement / Disagreement

Participants express differing views on the power rating of the resistors without a heatsink, with some agreeing on the lower rating while others remain surprised by the observed temperatures. The discussion does not reach a consensus on the implications of these findings.

Contextual Notes

Limitations include the lack of detailed data on internal temperature calculations and the specific conditions under which the resistors are tested. The discussion also highlights the dependency on the definition of power ratings with and without heatsinks.

likephysics
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I need some information on power dissipation of metal chassis resistors. I have 47 ohm, 50W resistors. (digikey part #A102169-ND)

I hooked them up to 24V. So current flowing is about 0.51A
But the resistors get too damn hot to touch(48-55 deg C, just after a few mins(~5mins). The power dissipation is 12W. But these are rated 50W, 4x more. This is not the first time I am using these. But I am surprised by how hot they get.
Any guidelines or app note on these resistors and power dissipation?

The ambient temp is 25 deg C. Resistor datasheet says 50W at 25 deg C, without heatsink.
 
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As I read this the HSA 50 types dissipate is 50W WITH a standard heatsink. - without the HS these are good for 20 W - which seems inline with your experience.
 
Windadct said:
As I read this the HSA 50 types dissipate is 50W WITH a standard heatsink. - without the HS these are good for 20 W - which seems inline with your experience.

Ok. How do you calculate the temp rise for resistors.
 
Jony130 said:
Look at data sheet page 4

Note those graphs are for the resistors on a standard heatsink (i.e. an aluminum plate with the dimensions given on page 1).

For a resistor without a heatsink, it's probably a reasonable assumption the temperature difference will be scaled by (max power rating with heatsink)/(max power rating without heatsink), i.e. for the JSA50 multiply the temperature difference from the chart by 50/20 = 2.5

But I am surprised by how hot they get.
Note from the chart that this type of resistor is "not even getting warm" running at a temperature of 50 degrees C. They are not meant to be used in situations where people can accidentally touch them. They work fine at temperatures much higher than boiling water!
 
Note the failure of these will depend on the internal temp - and the data needed to calculate this is no provided. Probably because at that point you are well beyond the SOA - the surface temp is the easy thing to monitor so that is the parameter they specify.
 
likephysics said:
... I am surprised by how hot they get.

AlephZero said:
They are not meant to be used in situations where people can accidentally touch them.

I'm guessing here, but I bet there's a burnt finger print on your resistor. It's a good learning experience that "power resistors shouldn't be tested by touch". I'm fairly sure any future touches WILL be accidental. :biggrin:
 
dlgoff said:
I'm guessing here, but I bet there's a burnt finger print on your resistor. It's a good learning experience that "power resistors shouldn't be tested by touch". I'm fairly sure any future touches WILL be accidental. :biggrin:

No, not this time. Burnt my finger enough times. Not happening again!:cool:
 
An IR thermometer is a valuable tool for any circuit work where temperature can be used for evaluation or diagnostics - RS has them for about $20 - there are some on Amazon for less.
 
  • #10
Windadct said:
An IR thermometer is a valuable tool for any circuit work where temperature can be used for evaluation or diagnostics - RS has them for about $20 - there are some on Amazon for less.
They are indeed a valuable tool for low voltage circuit diagnostics, but also for high current power applications such as substation bus, switch, and re-closer connections.
 
  • #11
I would sat that fits into my SOW... although I prefer real IR cameras for that level / professional work. Started with a ProbeEye - required Argon Gas - was a PITA
 

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