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CPU Processor - How hot is too hot?

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This question has surely been asked before:


How hot is too hot for a G3 processor in a G3 MT?


The processor has a simple passive heatsink. There isn't much room for anymore more extensive nor is there anywhere to plug in a fan (unless I use one of the Molex plugs from the PSU).

The 300MHz G3 gives a reading in Gauge Pro and Metronome of 39C. When I overclock it to 400MHz, the temperature rises to 46C. The ambient temperature is currently 27C.

Normally I try to keep my Intel processors 30C or under but I have more possiblities with PCs I build myself.



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46c is probably fine for a G3, they're fairly low wattage. There's probably several ways to improve that, including adding a fan, adding CPU heatsink paste, or refreshing the paste if it was ever there.


Intel refers to the maximum temperature on their chips as "TJMax" - that's the top temperature the chip "can" run at. (There's an intel-era Mac laptop joke that the fans are set to kick in at literally TJMax.)


That spec is between 95 and 105C on every Intel chip for the past ~10-15 years.


MacBook Pros have pretty much always run hotter for whatever reason than any other contemporary laptop. It's been getting more severe in the past few years, as the chasses of these machines get smaller but they continue to have 100w of hardware in them.

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IBM's own datasheet on their PPC740 and 750 (the G3) CPUs mentions temperature a few times. (find it at http://datasheets.chipdb.org/IBM/PowerPC/7xx/PowerPC-740-750.pdf )


Some relevant parts:

Storage temps are OK from -55 to +150C

Recommended Operating die junction temp is from -40C to +105C

Page 38 about the thermal assist unit notes values provided for reading temperature, and that temperature readings of the chip are of the junction temperature.

Page 49 has some calculations for heatsink/airflow, with the sentence "resulting in a junction temperature of approximately 71°C which is well within the maximum operating temperature of the component."


The datasheet references 300-533MHz G3s - I don't know precisely how close they are to the G3MT CPUs from a bit earlier, but I'd say it's fair to say at 47C you have a fair bit of room.



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The thermal paste is a good idea.

There is none between the processor and the heatsink. A fan on the heatsink would be a good idea too, if I can find one small enough. There isn't much room around the processor in a G3MT once it is closed.

Thanks everyone for your feedback.

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