June 08, 2007
Loose EndsEd Nisley
Ed catches up on reading and responding to his (e)mail.
Ed is an EE and author in Poughkeepssie, NY. Contact him at ed.nisley@ieee.org with "Dr Dobb's" in the subject to avoid spam filters.
To a good first approximation, all e-mail is now spam: Something over 91 percent of all messages, as of late last year, are unwanted trash. Attack probes against my firewall dropped to one every 146 seconds shortly after the December 2006 earthquake near Taiwan knocked out undersea data cables between Asia and the U.S., but returned to one a minute in short order.
E-mails from you folks provide one reason to not pull the plug. In this column, I'll go into more detail in response to thoughtful inquiries and suggestions.
Power Kills
A reader aliased as Naj Hajek asked the age-old question of whether 'tis better, from a longevity standpoint, to leave consumer electronics turned on all the time or to turn them off when not in use. This turns out to be one of those simple questions with a complex answer.
From an energy-saving standpoint, turning off unused equipment has an immediate payback. Although most contemporary equipment dissipates a few watts even when nominally off, simply because what was once the power switch has become just another microcontroller input bit, power dissipation in standby (or idle or sleep or whatever) mode is much less than in fully active operation.
At my rule-of-thumb rate of $2 per always-on watt per year, many cheap consumer devices cost more to run than to buy. Inexpensive plug-in power monitors, such as the P3 Kill-A-Watt, show the actual power consumption that affects your utility bill.
In terms of reliability, though, the answer may be different. Almost by definition, higher power dissipation corresponds directly with higher operating temperature; just put your hand on the exhaust duct of your PC's CPU to verify that assertion.
It should come as no surprise that metals and plastics change size with changes in temperature. A material's coefficient of thermal expansion (
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) is the fractional change in length per degree of temperature change, usually expressed in parts per million per degree. For example, copper has 
