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Do you leave your computer on?

13 January 2021

As someone who has been an electronics tech/engineer for many, many decades, I can testify to the fact that your valuable technology is most likely to break when you apply or remove power from it.

Most people who find their expensive gear broken usually say "it was working before but when I turned it on last time, it just didn't go".

So why is that?

And if that is the case, is it better to leave your electronic devices, including your PC, running 24/7?

The answer to that question is...

The answer is... it depends.

Firstly, let's look at why electronic equipment is predisposed towards failure during the moments after you flick the on-switch.

Modern electronics are generally very reliable. Solid state devices mean there are no valves to wear out and smaller solid-state capacitors don't suffer the same age-related effects as older electrolytics based on a liquid dielectric.

So if there are no moving parts and no "wear" related degradation, just why do these devices fail at all?

Well a great many failures are related to thermal cycling and the physical effects of that cycling.

As we all know, most materials expand when heated and contract as they cool. There are exceptions to this rule (such as water when it becomes ice and natural rubber) but it's a pretty safe assumption.

Another well-known property of many materials, especially metals such as the conductors inside the many integrated circuits found in modern electronics, is that of fatigue. If you physically distort a metal it can cause the molecules of that metal to realign slightly. Distort it far and often enough and eventually you get a form of crystalisation that makes the metal stiffer (it's alled work-hardening) but also far less ductile. Once this happens, further distortion can cause the material to fracture.

Now imagine that every time you turn on your favourite bit of electronics, all the ICs and other components start heating up and expanding. This causes minute physical distortion of things like the tiny gold bonding wires that connect the chip's silicon substrate to the outside world (via the bits that actually get soldered to the circuit board).

Now, what happens when you turn the device off?

It cools down, eventually reaching ambient temperature. This process causes a contraction of most componets, effectively applying a reciprocal physical stress to the components.

In the vast majority of cases, the stresses involved are so minimal and the number of power/thermal cycles so low that they are of no consequence. However, sometimes, perhaps due to a flaw in the manufacturing process or bad design, these cyclic stresses can cause chips to fail internally or (more likely) the solder bond that joins the chips pads to the circuit board to fracture. This last scenario is uncomfortably common in the case of large chips with BGA connections to the outside world. Many people know about the XBox's "red ring of death" and one of the most common causes was solder-fracture due to this form of heat-cycling.

In theory, the chances of suffering such a failure are significantly reduced if you leave your electronic devices (especially those that get very hot) on all the time. Having said that, any device that gets very hot is obviously using a reasonable amount of power so if you only used such a device irregularly or infrequently, the cost of leaving it on 24/7 may exceed the value of the device long before it would fail from thermal cycling issues in normal use.

Another reason for failures at power-on is the way most power supplies work.

Power supplies turn AC into DC, at one or more different voltages that are used internally by the device.

In order to accomplish this conversion, the incoming alternating current must be rectified and turned into a nice, smooth DC voltage. A rectifier will convert AC to DC but it is not smooth so capacitors must be added to filter out the "hum" that would otherwise exist. These capacitors are one of the main failure points in modern electronics, for several reasons.

Firstly, they are often electrolytic types that use a liquid as part of the dielectric that provides internal insulation. Over time, this liquid can evaporate, even though manufacturers do their best to ensure that the devices are totally sealed. These capacitors are often mounted very close to other devices that generate a lot of heat and this heat can accelerate the loss of electrolyte. From this perspective, NOT leaving your device on will reduce the total heat exposure and would be a good idea. However...

When the equipment is first turned on, the capacitors are "empty" and hold no charge. This means that quite heavy currents can be generated as they "fill". That high current can cause localised internal heating which, in turn, can break down the very thin dielectric insulating layer inside.

That high current also flows throught the rectifier which places it under significant thermal stress, until such time as the capacitors are charged. That thermal stress produces the effects we've already discussed. That sudden inrush of current can also be the exact moment that devices decide "I've had enough" and fail.

These are just a couple of the factors that can contribute to the "it was working last time I turned it on" types of failures that are so common but I guess we still haven't answered the question of whether it's better to leave gear on 24/7 or turn it off and on as required.

As mentioned above, you should perhaps do some simple calculations based on frequency of use (minutes per month) and energy costs to determine how long your device would have to be left on to consume more than its total value in electricity.

Another increasingly important factor, when it comes to computers, routers, etc... is the risk of being hacked. I suspect that few people consider that while they're sleeping at night, their computer is sitting online, a ready and willing target for any little script-kiddy who might choose to try and try to squeeze past its defenses and rattle through your files. From this perspective, it might be a good idea to turn it off when not in use... or at least disconnect the network cable or wifi.

What are readers' experiences? Do most of your electronics seem to fail at power-on or not?

Do you leave all or some of your gear on 24/7 and if so, why?

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