CPU TEMPERATURE

3/28/98

CPU Temperature is a complex function of the CPU clock speed, CPU activity, and the ability of the Computer to remove heat from the CPU chip. in the 1980s CPUs were typically specified at maximum operating temperatures of 70C or 75C rather than the 65C specified for most semiconductors. Heat is removed from the CPU primarily by conduction to the motherboard and to the ambient air which is typically moved by fans in desktop computers and may not be moved at all in laptop computers. 75C is painfully warm, but is well below the boiling point of water. The rule of thumb often used by technicians is that if you can not hold a finger on the CPU for several seconds, it is running too warm. Pentium III CPUs include an on chip temperature sensor that is specified to operate in the 80-90C range.

As a first approximation, CPU temperature changes linearly with CPU clock speed. Modern CPUs have been using progressively less voltage in a attempt to hold power consumption, and therefore heat, down while permitting higher clock speeds. Some CPUs -- especially 5 volt 60 and 66MHz Pentiums (rated at about 20 watts) push the limits of permissible temperatures.

Many CPUs including 5V 486DX2 66MHz and faster, 5 V Pentiums, and 3V Pentiums faster than about 100MHz are conventionally run with a heat sink. The efficiency of passive heat sinks is widely debated as is whether the use of thermal "grease" compounds significantly improves heat transfer. Fans are frequently used on heat sinks with ball bearing fans exhibiting much greater reliability than less expensive fans. Fans are subject to a failure mode where they oscillate rather than spinning. The two can look alike, but oscillation may not move a lot of air. Peltier Effect heat pumps are also used although there have been cases reported of condensation from the cold side of these heat pumping devices damaging computers. Good airflow from computer fans is needed when fans are not mounted on the heatsink. Heatsink design is a black art and enormous differences are claimed in the performance of different heat sinks of similar size and surface area.

Temperature of Voltage Regulators used to drop voltages for CPUs running at less than 5 volts have sometimes proved to be more of a problem than CPU Temperature.

Some Intel Pentiums will automatically slow the CPU speed if the chip starts to overheat. Some BIOSes will beep repeatedly if the CPU exceeds a user determined threshold.

Tests using infrared temperature measuring devices have shown substantial differences in CPU temperatures (20C) when running different operating systems and applications. The File Menu of EDIT.COM in MSDOS 5 or 6 is reported to be especially stressful thermally. Software with names like CPUIDLE and WATERFALL is available that drops the CPU temperature by significant amounts -- 5C to 15C -- when the CPU is idling.

Temperature sensors with associated alarms in heat sinks have been available for several years. It is expected that these will alert the user if a fan fails or if the combination of CPU temperature and ambient air temperature on a hot day (the two add) is too much for the CPU. As of 1998, some motherboards are being built with built in CPU temperature sensors and automatic shutdown circuitry. Typically, these are built around the LM75 or a similar temperature sensing chip combined with motherboard monitoring chips such as the W8378xD series. It has been reported that the distance from the CPU to the sensor -- typically several mm -- is sufficient to cause temperature readings on these motherboards to be low by as much as 20C. CPU temperatures and fan speeds are often reported by BIOSes as well as by Windows programs such as MBM.

Heatsink temperatures can be read with IR probes, thermistors or "Thermal Labels" that indicate temperature by color. Thermal labels may be purchased (part number I-602) from the manufacturer: American Thermal Instruments, Inc., 9 Huffman Ave., P.O. Box 353, Dayton, OH, 45401 USA. Phone: (800)648-6339. Fax: (937)252-6509

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