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just got my lockergnome newsletter and this was posted in it as a reply to the editor.
so i am posting it here for all to see.

Chris Rolf writes:


I was interested to read the letter sent in by the Gnomie who posed the question: I have heard several different answers to this question and I don't know what to believe; is it better to leave my computer running or shut it off?

Well I have some groundbreaking news for that Gnomie! Yes, all the answers given by Lockergnome were unarguably correct, but there is a lesser known phenomena that needs to be addressed and that is component response to varying levels of heat. First of all, I must point out that I have had 45 years of experience in the field of electronics, both military and consumer.

One needs to understand that when electronic components are in use, they heat up, and when they heat up, they expand. This is not a problem if they are constantly expanding and contracting from the frequent experience of the host equipment being turned on or off. However, what has been found from extensive study is that passive components - resistors and capacitors and, to a lesser extent, transistors - tend to part company from the connectors. For example, resistors are manufactured from carbon, and therefore the only means of connecting metal to carbon (for a solderable connection to the pcb) is the age-old pressfit solution. This is okay if the equipment is regularly switched on and off, but woe befall the transgressor who leaves his equipment switched on for an extensively long period of time!

What happens is this: because the component connections are pressfit, when electrical current is applied to the passive components upon switch-on of the equipment, all of the components expand in size due to the heat that is generated by the running of the equipment. If the equipment is left on continually for a long period of time, the metal connectors that are pressfitted to the resistors and capacitors experience metal fatigue. If the equipment is switched off and electricity ceases to flow through the main body of the component, the main body contracts as it cools down. However, the metal pressfit lead-in is suffering metal fatigue and it parts company with the main body of the component, therefore causing equipment failure! My advice, for what it is worth, is: TURN IT OFF WHEN YOU ARE FINISHED!

(By the way, I did not tell you about the company that installed a stand-by generator, and when they tested out the generator, 60 plus computers went "on the blink" from metal fatigue that was generated in the passive components in the "victim" computers!)



Archived topic from Iceteks, old topic ID:1824, old post ID:15156

Interesting... I heard about this, but the opposite, that turning on and off causes the contraction to sooner or later give way, while if it's always on, there's less chance of it happening as it will only expand to a certain point. I don't know who to believe anymore. I think all of this is conspiricy since my PC is always on, but I did turn it off for extended time and turned it back on and nothing happened.

Archived topic from Iceteks, old topic ID:1824, old post ID:15157

I don't think it matters one way or the other, most people upgrade before the pc wears out anyway.

Archived topic from Iceteks, old topic ID:1824, old post ID:15160

I worked for a welding research institute for a couple of years. About 6 months before I left, they were doing some very nice experiments. They were exposing pieces of steel to thermal cycles, about a few tens of thousands, to see the effect of thermal fatigue on the material. Cycles were about a few minutes to a few dozens of minutes long, temperature variations was usually of a few hundreds of degrees. Some experiments lasted for months.

Steel exposed to such thermal fatigue became so brittle after a time that it needed no shock or external pressure to fall into pieces - was very interesting to watch.

At the same time, experiments with composite materials, particularly metals containing various types of fibers (carbon or glass fibers in metal) were done - such materials are very nice to use for various applications, but when the fibers detach from the base metal they're f...inished. It was also found that such materials were hurt significantly by thermal stress, because the different degrees of dillatation with temperature of the base metal and the fiber material caused huge local stresses at the contact surface. If the temperature didn't vary, after some time, depending on the properties of the base material, microscopic deformations and relaxation led to reduction of such stresses.

Therefore I cannot give credit to the statement that thermal cycles are not bad but constant higher temperature is. Indeed, gold and copper are not elastic as steel is, and the buildup of micro-stress in these two metals due to thermal cycles is a lot less than in more ellastic metals, however, the contact with a rigid or even crystallin surface, as carbon or semiconductors are, enhances the effect of thermal cycles.

Also, statistics says that 80% of all failures of electrical equipment happens at start/stop.

Which reminds me of a joke: a jewish business man has a big problem, so he asks the Rabi for advice:
- Rabi, I'm a business man, I have a flight booked almost every day, terrorists are placing bombs on planes, it's only a matter of time until I'll be sitting on a plane with a bomb. What should I do?
- Right you are, you're in great danger. You do this: whenever you fly, take a bomb with you. The probability of two bombs being on the same plane is a lot lower than a single bomb to be on that plane.

Archived topic from Iceteks, old topic ID:1824, old post ID:16558

That was interesting, and that's a funny joke. Welcome to the forums.

Archived topic from Iceteks, old topic ID:1824, old post ID:16559