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Old 29th January 2008, 01:22 AM   #1
d3imlay is offline d3imlay  United States
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Default Max dissipation on 1/4W resistor

What is the safe maximum dissipation on a 1/4 watt resistor? I don't want the resistor to turn colors over time or discolor the board.
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Old 29th January 2008, 01:38 AM   #2
AKSA is offline AKSA  Australia
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For complete reliability and no discoloration, I generally rate to one third nominal. In the case of a 1/4W resistor, this is 85 milliwatts continuous.

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Old 29th January 2008, 01:45 AM   #3
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I'm a little rougher than Hugh...I'll use a resistor up to about half of its rated wattage.

Grey
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Old 29th January 2008, 02:20 AM   #4
d3imlay is offline d3imlay  United States
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My application is 160mw. Switch to 1/2W?
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Old 29th January 2008, 02:43 AM   #5
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Realistically, the resistor can take 160mW pretty much forever. But if you're concerned about long term discoloration and so forth--and can spare the space for the larger footprint--yeah, sure, use a 1/2W. It's not like they're a whole lot more expensive, so it usually comes down to a question of size.
I routinely use RN60 resistors. They're technically 1/4W parts, but in military speak the way to make things reliable is to use them waaaaay below their ratings. With that in mind, if you hold up an RN60 next to a 1/2W resistor, you'll find that they're the same size. In this limited sense, I use "1/4W" resistors as 1/2W resistors. They're really 1/2W parts, anyway, at least as far as we're concerned out here in audio land.
You can get into discussions about what the resistors are made of, but the major determinant of how much heat a resistor can safely dissipate is surface area. Most resistors aren't mounted on heatsinks, so what you see is what you get. The surface of the part is the heatsink. The more surface area, the more heat it can dissipate, assuming the same materials and construction techniques.

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Old 29th January 2008, 02:43 AM   #6
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I'd happily run a 1/4W metal film at that level, but not a carbon film or comp. A lot has to do with the traces it's soldered to. If the heat sinking through traces or lugs is good, the resistor will run much cooler. There are also very small MOX resistors that can run high power levels for their size, but the penalty is they also run *hot*. IMO, they're not good for audio, and without enough trace width, they can unsolder themselves from the board! As with any component, ask yourself where the heat goes and is there some airflow. What are ambient conditions inside the chassis like. Sometimes you have to seriously derate parts if you want a long service life. Other times not so much.
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Old 29th January 2008, 03:15 AM   #7
gootee is offline gootee  United States
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Default Re: Max dissipation on 1/4W resistor

Quote:
Originally posted by d3imlay
What is the safe maximum dissipation on a 1/4 watt resistor? I don't want the resistor to turn colors over time or discolor the board.
It depends on whether you really meant "maximum", or (more likely) "maximum average".

The actual "maximum" could probably safely be hundreds of watts, if it were only for a few milliseconds, once in a while. Some resistor datasheets do have information about those types of applications.

I use a 5 Watt 1-Ohm metal film resistor in a soft-start power supply application where it dissipates more than 800 Watts, for some tens of milliseconds each time the unit is powered on, and it never gets detectably warm to the touch.
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Old 29th January 2008, 08:51 AM   #8
Donk is offline Donk  Europe
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Check this out for temp/diss derating curves
http://www.amri.net/index.html#list
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Old 29th January 2008, 10:28 AM   #9
Mooly is online now Mooly  United Kingdom
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Resistors have a maximum voltage rating as well, even if the power disipation is well below it's rated maximum. Many a "start up resistor" in switch mode P.S.U.s failed for this reason.
Regards Karl
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Old 29th January 2008, 02:16 PM   #10
Nordic is offline Nordic  South Africa
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Resistors expected to carry realively close to their max load benefit from a little space between them and the board.
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