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Old 4th September 2014, 12:21 AM   #1
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Default 1/2W rated to 1W

I was recently in the process of acquiring some resistors and came across the following description in a listing; "1/2 watt rated to 1 watt". It was related to some Vishay metal film resistors. What is the true wattage rating of the resistor, 1/2W or 1W? I fail to see how it can be both without some kind of caveat governing when it is which one.
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Old 4th September 2014, 12:26 AM   #2
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If you read far enough into the data sheet they are rated for 1/2W below a certain resistance. They also emit more heat than a standard 1 watt if I remember correctly so if you are running them near their limit they might scorch your pc board.
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Old 4th September 2014, 06:46 AM   #3
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Also depends on the ambient temperature. Military/aerospace equipment often is required to operate at temperature extremes and the part has to be derated accordingly.

What is the part number you were looking at?
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Old 4th September 2014, 09:04 AM   #4
DF96 is offline DF96  England
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The resistor will survive 1W, but it will get so hot that the PCB or adjacent components might not survive?
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Old 4th September 2014, 03:59 PM   #5
marce is offline marce  United Kingdom
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De-rate at least 50% and preferably 80% if space and funds allow.
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Old 4th September 2014, 09:22 PM   #6
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Originally Posted by Damon Hill View Post
Also depends on the ambient temperature. Military/aerospace equipment often is required to operate at temperature extremes and the part has to be derated accordingly.

What is the part number you were looking at?
The Vishay numbers are CCF60150RFKE36 and CCF60200RFKE36, 150 Ohm and 200 Ohm respectively. From what I can gather from the data sheet it looks like the magic temperature of 70 C may be a governing factor after which serious derating occurs.
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Old 4th September 2014, 10:26 PM   #7
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I'd say it's good for 1 watt given it's size and construction unless it's used in a genuinely hot location. If in your application it gets too hot to touch, then use a 2 watt or higher resistor, or use in series or parallel (adjusting resistance values accordingly).

That's my rule of (burnt) thumb.
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Old 5th September 2014, 08:26 AM   #8
marce is offline marce  United Kingdom
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The derating is also done to get decent MTBF figures, resistors running at 50% power rating give 10X better figures than ones running at 80% of their power, and running them at 20-30% gives even better figures.
The other problem is heating of other components, when the PCB is flat (horizontal) then its not to bad as the heat will rise from the resistor, when a PCB is vertical this rising heat can heat other components above it so this needs thinking about during the design process.
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Old 5th September 2014, 10:23 AM   #9
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Quote:
Originally Posted by Damon Hill View Post
That's my rule of (burnt) thumb.
I treated myself to a hand held laser thermometer (about 40 from Maplin in the UK). You target the component with the laser spot and get an instant digital temperature reading.
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