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Overloading a resistor

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#1

I am working on a pre-charge/discharge circuit.
I need to choose a resistor based on these values. I was thinking about a 1k 50W resistor.
It says overload 5X for 5s. What is the peak power rating for this resistor, HSA501K0J, RES CHAS MNT 1K OHM 5% 50W?

It will be carrying 32W normally but for precharge and discharge time it will reach peak power of 195W.

So my main concern is it will be given for a temperature. How this resistor will react, for this time period? This overload graphs for a given temperature. Should I also take into account the rise in temperature? Or how can I make this calculation according to the datasheet?

Thanks for your help

#2

Welcome!

A big consideration is how this design is going to be used.

Will this design be:

  • Just one, or a few units hand made.
  • Hundreds or thousands made on an assembly line and you’ll expect them all to perform reliably in many different environments.

Glancing over the data sheet I noticed that to operate the HSA50 series at 50 Watts it must be connected to a chassis/heat sink of 53500 square mm (230 x 230 mm, a rather large metal surface). With just the aluminum housing of the part it is only rated for 20 Watts.

Assuming you are going for the first option, “Just one, or a few units hand made”, then the easiest way to get things working is to figure it out experimentally. Build up prototypes with temperature sensors thermally bonded to the resistor housing. Then monitor the temperature rise while operating the system(s). If you plan to use the device at temperatures above your normal ambient temperature, you should also test the prototypes in an oven set for the highest ambient temperature you want to operate at.

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#3

Hi @layda,

As Paul states, all of the specifications for the HSA501K0J are given assuming it’s mounted to a 230 x 230 mm (9.1" x 9.1") chassis for a heat sink. If you have no heat sink, then it can only handle 20W. If you are somewhere in-between, then you would have to interpolate, which is not trivial.

Note that the graph showing power overload capabilities are assuming a 20°C ambient air temperature. So, if you have a heat sink of the specified size and you start with an ambient air temperature of 20°C, it will handle 250W for 5 seconds or 200W for about 7 seconds. However, if your ambient temperature is higher, the overload capability will be reduced. If the 195W peak you mention is only a small portion of 5 seconds, you may be OK, but otherwise, you are likely coming very close to its limits.

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Another thing to remember is that you should be using either heat sink grease or a thermal interface material between the resistor and the chassis to allow good thermal conductivity. Otherwise, the tiny airgaps between the resistor and the chassis will significantly reduce thermal conductivity between the two.

If your ambient temperature will always be near typical room temperature (around 25°C) and you properly heat sink the resistor, it will likely work, based on the datasheet specifications. However, if you need to assure reliability, you will either need to perform testing, as Paul describes above, or give yourself some more headroom by upsizing your resistor to 100W or more.

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