Group Design

On Monday, August 10, 2020 at 9:43:06 AM UTC-4, David Brown wrote:
> On 08/08/2020 01:37, Chris wrote:
> > On 08/07/20 14:57, Phil Hobbs wrote:
> > 
> >> I know it isn't what the cool kids do, but that doesn't make it
> >> unreliable or cause it to fail safety inspections AFAICT. Medical may be
> >> different, but it would pass UL with no issues. What regulations nwould
> >> you expect it to fall foul of?
> >>
> >> Cheers
> >>
> >> Phil Hobbs
> >>
> > 
> > It's not the regulations, as afaics approvals are more concerned with
> > *process*, how the design decisons were made and why. Would not
> > necessarily catch design errors that resulted in a regulator running
> > near it's design limit, though avionics or mil temperature cycle
> > testing might  cause a failure because of lack of margin for a device.
> > 
> > Whatever you might think, it's well known that high temperatures and
> > cycling shortens the life of electronics and no, I don't have
> > references,but it's all on the web. Some prefer to skimp every penny,
> > others prefer conservative design rules. It's the difference between
> > consumer electronics and pro kit. may look like the same parts, but
> > the design tradeoffs are different. Engineering design is always an
> > approximation and compromise: performance, cost, reliability, pick any
> > two...
> > 
> > Chris
> > 
> 
> I think it's fair to say that this 70 C case with poor thermal contact 
> with the board is an indicator of bad design - bad schematic design 
> (using a linear regulator rather than a switch mode), and bad layout 
> design (poor cooling).  And poor design in such an obvious case suggests 
> poor design overall - which is not going to go well for any kind of 
> safety requirements or regulations where the design process is considered.

But you're not judging, right? 

No one supports the use of a switching regulator... well, until I did a power budget.  Seems that the two LCD displays use some 10x the power of the rest of the circuit ex motor.  With the motor using 2 amps at some duty cycle the LCD displays appear to be more of a drain on the battery.  We are looking at the issue now.  Even with a switcher the savings is only 50% of the LCD power.  

Personally I think LCDs suck with a contrast ratio of 3:1, but the rest of the world seems to like them. 


> It is also correct to say that the reliability of a system is a function 
> of the reliability of all the parts, and that high temperatures and 
> temperature cycling shorten lifetimes and increase the rate of failures. 
>   But you can't say how significant this particular part would be 
> without a lot more information about the rest of the system.  As Phil 
> says, a part rated for 125 C is not going to see much increased failure 
> risk at 70 C compared to 50 C.  But if it sits next to a part that is 
> rated to 85 C and the final system is mounted in a closed box and used 
> in the Qatar summer, perhaps it will have a relevant effect.
> 
> All in all, this looks like a rush job put together by people with no 
> experience with high reliability or safety-related systems.  That's the 
> general problem, rather than one particular hot regulator.

That's certainly true.  The other day I was in a conference call with the project lead and the new requirements guy.  The requirements are more of a list than anything formally analyzed.  The ultimate reason for this seems to be the realization of the large amounts of personal time being invested in the project and in particular the time invested by the project lead.  

For me the project is recreation during the pandemic isolation.  For others it competes for their free time against families, etc.  That doesn't change the fact that to do the job right requires using the proper tools and methods. 

I am nearing my bail out point.  I'd like to get through a design review of the schematic and the layout.  Then I'm happy to go. 

-- 

  Rick C.

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