# Metal chassis resistor power dissipation

1. Dec 3, 2013

### likephysics

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.

2. Dec 3, 2013

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.

3. Dec 3, 2013

### likephysics

Ok. How do you calculate the temp rise for resistors.

4. Dec 3, 2013

5. Dec 3, 2013

### AlephZero

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

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!

6. Dec 3, 2013

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.

7. Dec 3, 2013

### dlgoff

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.

8. Dec 3, 2013

### likephysics

No, not this time. Burnt my finger enough times. Not happening again!

9. Dec 4, 2013

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. Dec 4, 2013

### dlgoff

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. Dec 5, 2013