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I'm looking at the specifications of an ohmite shunt resistor: http://www.ohmite.com/catalog/pdf/10_series.pdf

It is a 0.005 ohm resistor, rated at 2,3,5 and 7 Watts (4 different parts in the same datasheet) at 25C ambient. But there is no information about thermal resistance.

If my current over this resistor is going to be, say 30A, then power dissipated on this resistance is going to be 4.5W. Does this mean that if I pick a 5W or 7W resistor, I will be OK in free air (no cooling requirements)? What if my ambient is not 25C? Since there is no thermal data in the datasheet, how can I go about calculating cooling requirements for an ambient temperature of 50C? Are shunt resistors never cooled?

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Steven has commented on the temperature coefficient of resistance.

The data sheet also does provide thermal derating information. It says:

  • Derating Linearly from 100% @ +25°C to 0% @ +275°C.

  • Power rating: Based on 25°C free air rating.

  • Overload: 5 times rated wattage for 5 seconds

While this is not precisely the desired degrees C/Watt rating it does provide a guide as to what to expect temperature wise. A degrees C/Watt rating can be adumbrated from this, possibly incorrectly.

  • Power rating is at 25 C air temperature.
  • Derating occurs linearly to 275 C.
  • So C/W = (275-25)/5 = 250/5 = 50 C/W for 5 Watt version.

The 2, 3 and 5 Watt versions have different physical sizes but share a common derating formula so arguably

  • the 2 Watt is 250/2 = 125 C/W
  • the 3 Watt is 250/3 = 83 C/W
  • the 5 Watt is 250/5 = 50 C/W as above.

Odds are this is all "a bit rough" but it seems to provide a guide to their thinking.

Note that while the above derating formula allows of 275C operation (at no power input) the spec sheet also sets 200C as max operating temperature.

To answer your specific original question:

At 50C the 5 Watt part would be rated for

  • 5 Watt - (50 C - 25 C)/50 C/W = 5-0.5

  • = 4.5 Watt at 50C.

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The datasheet has a graph showing the temperature coefficient:

tempco

So given a nominal resistance and a power you should be able to calculate the resistance at higher temperatures.

A possible reason why thermal resistance to ambient isn't given is that it's too much dependent on the way the resistor is mounted.

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