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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Thu, 13 Jun 2024 07:51:59 -0400
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On Wed, 12 Jun 2024 12:26:51 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>On 6/12/2024 9:13 AM, legg wrote:
>>>>> Would you expect a cell phone to operate in the same nominal environments
>>>>> that a human being would encounter in their normal living?  If it FAILED
>>>>> to operate "above 93F" (which is likely most of the lower 48, at some
>>>>> portion of the day, lately), you'd likely be looking for another device
>>>>> as you would always have to be in an air conditioned environment to make
>>>>> that guarantee.
>>>>
>>>> I used to be in the iPhone design team. At the time we used a 40 deg C as the
>>>> maximum ambient temperature.
>>>
>>> It's that, here, now (103F) -- and another 5-10 degrees expected before
>>> temps start back down.  And, I imagine a few million people are experiencing
>>> that same sort of temperature.  If 20% have iPhones, that's a shitload of
>>> devices operating at or above their design maxima.
>>>
>>> Where did the "40" come from?  Why not 41C (was that not "round enough"?)
>>> Or, "100F"?  I.e., was it arrived at by deliberate thought or just picked
>>> out of the air as "good enough"?
>> 
>> 40C is the ambient temperature where public health safety warnings are
>> issued in most jurisdictions. Schools will cancel track meets etc.
>> Some ammend that to wet bulb temperatures, which can be reached in as
>> low as 36C environment.Don't confuse it with the Fahrenheit scale.
>
>I'm not "confusing it" -- rather, deliberately using "100" as a "nice round
>number" (note my comment re: 41C vs 40C)  Is it just "coincidence" that
>the "safety warnings" are issued at 40 and not 39?  Or, 41?  Is there
>something "magical" about "40" -- other than it ends in a '0'?

It has something to do with the human body at reat, in the shade,
failing to be cooled by sweat in moving air at a certain temperature.
Cooling has then to be obtained by artificial means. It probably IS 
a 'nice round number', as some are affected earlier than others.
A body will already be avoiding direct sun exposure with hats /
umbrellas, shade trees etc.

>
>It's been above 100F (38C) for 16 days, already.  People are still working
>their normal outdoor jobs, looking at cars on dealer lots, walking 100 yards
>across uncovered parking lots to gain entry to stores, etc.
>
>Are phones NOT expected to work in these environments?  (Gee, I wanted to
>telephone emergency services to get help for my buddy who collapsed in
>the heat but it was too hot for my phone to make the call)

Sorry to hear about your buddy. Senior citizens or health compromised 
people have to take special care in the heat.

How do you know that was the problem with the phone? 
What were the phone's symptoms?
>
>We expect cars to continue to operate in those temperatures.  What's the
>criteria that we use to determine what should and shouldn't be expected
>to remain operational?

Already said - automotive component temperature limits are set by 
other standards. That being said, it's a test of your vehicles 
integrity to do hard work at elevated temperatures. For the benefit 
of passengers, some attempt is made to keep the cabin below 40C.
>
>And, how do these limits differ from the (somewhat arbitrary) figures
>that we publish as specifications?

A company will have stated limits for a project in the earliest design
stages. They will use limits expected by their industry, unless there 
are reasons to do otherwise. Stupidity would be a pretty bad reason.

>
>>>> As part of the development we would run a "thermal virus" software to cause the
>>>> CPU to dissipate an approximation for the maximum possible.
>>>>
>>>> Under those conditions the internal temperature could get to the 70 deg C
>>>> region. If excessive temperatures were reached the CPU would be throttled to
>>>> avoid damage.
>>>
>>> Makes sense.  But, has limits to its applicability.  I.e., if the CPU
>>> couldn't support the load of running the cellular radio, then you've
>>> prevented damage but still rendered the phone inoperative.
>>>
>>>> I was working on the display/touch hardware; LCD displays stop working at about
>>>> 75 deg C (they just turn black)
>> 
>> You'd probably find that surface touch temperature limits are exceeded
>> before that - screens having direct access to the external
>> environment.
>
>You don't need to use a bare finger to activate a screen.  And, with most
>phones, can use voice dialing.

Are you serious? Touch temperature is the surface temperature in 
 contact with operator, usually the hand. If it's not meant to be 
held, you can get away with visible surface temperature warnings.
Anything made to be touched doesn't get an exemption.
>
>>> Yes.  The technology has lots of environmental limits.  And, too cold
>>> and it gets sluggish (not a good thing for an AC device).
>> 
>> LCD screens also become unreadable at ~ -20C. Again - don't confuse it
>> with the Fahrenheit scale.
>>
>>>> Since the display was within a couple mm of
>>>> the CPU there was not margin.
>>>>
>>>> Under less stressed conditions the internal temperature was much lower.
>>>>
>>>> I suspect that the battery is probably the most sensitive item for storage
>>>> temperature while not operating, especially if fully charged.
>>>
>>> And the battery's failure mode can be spectacular.  So, do they
>>> rely on the printed specifications to bail them out of any liability
>>> lawsuits?
>> 
>> He's talking about aging, loss of capacity and charging voltage
>> tolernce restrictions. Lithium battery 'cook-off' temperatures
>> are much higher.
>
>So, a battery stored 1C beyond that maximum storage temperature
>is crap?  Of course not.  For *components*, we have graphs that
>make these relationships more explicit.  So, I can trade off
>temperature vs. performance, etc.
>
>For *devices*, we rely on a single number to express a limit
>(operating or storage).  It's fairly obvious that these
>limits are not hard and fast and have fudge factors built in.
>How large those are is up to the person designing the product and
>writing the published specification.


>
>>> Note that you don't tend to see different "grades" of consumer kit
>>> as you would encounter in commercial/industrial markets -- where
>>> the consumer can buy an option/upgrade/upsell to address a market
>>> that he feels more typically reflects his usage.
>>>
>>> How many consumers actually are aware of these parameters for
>>> the kit they've bought (often at very dear prices)?
>> 
>> Industrial grade components and equipment are designed for
>> higher operating ambients - but equipment designed for personal
>> (hand-held or pocket) use assume the limits for human physical
>> comfort and safety.
>
>But that's not true.  Sit outside in 93F temperatures (the published
>spec for the iPhones I researched) and you will neither be comfortable
>nor safe.  Why doesn't everything operate at 25C which is where we
>tend to be most comfortable and feel most safe?  Ans:  because we
>live in conditions that extend well above and below those limits.

Safety standards are pretty explicit w/r to operator environment.

Humans do not, in fact, operate well over any environmental extreme.
That's why we wear clothes; live in temperature-suitable dwellings,
don space suits etc.

>
>I can recall being outdoors in -26F and +117F.  I was much more
>uncomfortable at -26F than at 117F (I was actually hauling 20T of
>stone and, aside from the weight of the stone, more comfortable
>than I'd been at 80F in New England)
>
>So, what value writing a published specification limit that
>you KNOW will be exceeded, REGULARLY?  Are you looking for a
>legal leg to stand on when the user tries to dial "911" and
>the phone fails to perform as expected -- because it was 94F
>at the time?

Commodities respond to market requirements.

>
>>> When we were last looking at vehicles, we noticed many of the
>>> "front-facing technology" would throw errors, before you even
>>> made it onto the road for a test drive.  "Oh, the electronics
>>> are overheating from being out in the sun..."  "WTF?  So, can
>>> I only drive at night?  And, how many kilobucks for this bit
>>> of kit??"
>> 
>> Vehicular operating environmental limits differ from consumer
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