Re: Group Design

From	Dimiter_Popoff <dp@tgi-sci.com>
Newsgroups	comp.arch.embedded
Subject	Re: Group Design
Date	2020-08-07 20:42 +0300
Organization	TGI
Message-ID	<rgk3q9$o2f$1@dont-email.me> (permalink)
References	(5 earlier) <nnd$10d6570d$45c5d306@c70ced0050c4d383> <041d35fe-f5a5-4b7a-a78b-291104e34220o@googlegroups.com> <rgi24o$1has$1@gioia.aioe.org> <cc6f131d-e8e7-387a-fe5e-907478e75062@electrooptical.net> <rgje0v$g3r$1@gioia.aioe.org>

Show all headers | View raw

On 8/7/2020 14:30, Chris wrote:
> On 08/07/20 02:50, Phil Hobbs wrote:
>> On 2020-08-06 19:01, Chris wrote:
>>> On 07/08/20 23:20, Rick C wrote:
>>>
>>>> I'm also concerned about the board layout.  The board is not at all
>>>> dense but there are several
>>>
>>> parts that need copper area for heat sinking.  They are using a linear
>>> regulator to drop 12+ volts
>>>
>>> to 5V and 3.3V.  The back lights for the LCDs draw 300 mA!  So the 5V
>>> regulator gets quite warm,
>>>
>>> 70°C!!!
>>>
>>> You should be concerned. The whole thing about safety critical kit is
>>> that it should be conservatively designed, with all parts running well
>>> within their ratings. If the hw bod is happy with a regulator running
>>> at 70c, he must be clueless. Sorry, but it needs to be said.
>>>
>>> If you care about your professional career and any litigation
>>> issues, you should either have enough influence on the project
>>> direction to stop design errors like that, or just walk away from it...
>>>
>>> Chris
>>
>> Hmm, interesting. Most linear regulators I'm familiar with are rated to
>> 125 C junction temperature, and some higher. What safety or regulatory
>> issues have you run into that were caused by running some part 55C below
>> its rated temperature?
>>
>> Cheers
>>
>> Phil Hobbs
>>
> 
> That's not the point and reliability is affected by temperature. It may
> not matter for a short lived down hole item where failure has no safety
> implications, but not good enough otherwise. The constant temperature
> cycling will affect wire bonds and reliability. Cheap tacky design,
> using a linear regulator to go from 12v to 5, then 3.3, considerable
> current, where a point of use switcher would be a far more efficient and
> probably more reliable solution as well Wasted power needs a bigger 
> backup battery etc...
> 
> Chris
> 
> 

A 7805 at 70C is not too hot, nothing like that. I'd be worried of any
nearby electrolytic capacitors, I have seen these fail if put too close
to some hot part. Once on a product of mine - though the board was "too
hot" only for one cap of a particular series, other units have worked
for well over 5 years 24/7. And once I repaired our TV set (some 15+
years ago, a CRT one), it had an electrolytic cap next to a "red" hot
7805.
I have *never* seen a 78xx regulator fail because of overheating
and I have designed in quite a few. One can have them run hot
within reason, just keep in mind that they have temperature protection
and if you overheat one it will protect itself by more or less
cutting off the power at its output.
Another thing to keep in mind is to not forget to put a diode
in parallel if the output voltage is > say 5-6V; they do die
if the input goes 0V or so with the output capacitor charged.

Dimiter

======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

Thread

Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-07 18:46 -0700
  Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-07-07 19:02 -0700
    Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-07 20:36 -0700
      Re: Group Design David Brown <david.brown@hesbynett.no> - 2020-07-08 09:31 +0200
        Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-08 07:08 -0700
  Re: Group Design Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-07-08 14:13 +0200
    Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-08 07:19 -0700
      Re: Group Design Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-07-08 18:57 +0200
        Re: Group Design Theo <theom+news@chiark.greenend.org.uk> - 2020-07-08 18:28 +0100
        Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-08 11:12 -0700
          Re: Group Design Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-07-08 23:34 +0200
            Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-08 15:20 -0700
              Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-07-08 23:00 -0700
              Re: Group Design Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-07-10 10:09 +0200
              Re: Group Design Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-08-07 00:01 +0100
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-06 17:37 -0700
                Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-08-07 03:06 -0700
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-06 21:50 -0400
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-06 19:12 -0700
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-06 22:27 -0400
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-06 19:36 -0700
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-06 23:39 -0400
                Re: Group Design Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-08-07 12:30 +0100
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-07 09:57 -0400
                Re: Group Design Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-08-08 00:37 +0100
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-07 20:29 -0400
                Re: Group Design upsidedown@downunder.com - 2020-08-08 08:59 +0300
                Re: Group Design David Brown <david.brown@hesbynett.no> - 2020-08-10 15:43 +0200
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-10 09:18 -0700
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-07 07:12 -0700
                Re: Group Design Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-08-08 00:00 +0100
                Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-08-07 20:30 -0400
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-07 17:57 -0700
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-07 17:56 -0700
                Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-08-07 18:10 -0700
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-07 19:43 -0700
                Re: Group Design Dimiter_Popoff <dp@tgi-sci.com> - 2020-08-07 20:42 +0300
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-08-07 13:48 -0700
      Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-07-08 14:11 -0700
      Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-07-09 04:25 -0400
        Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-09 11:37 -0700
          Re: Group Design Paul Rubin <no.email@nospam.invalid> - 2020-07-09 11:54 -0700
          Re: Group Design Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-07-09 20:02 +0100
          Re: Group Design Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2020-07-09 15:31 -0400
            Re: Group Design lasselangwadtchristensen@gmail.com - 2020-07-09 15:36 -0700
              Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-09 18:36 -0700
                Re: Group Design Clifford Heath <no.spam@please.net> - 2020-07-10 16:17 +1000
                Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-09 23:36 -0700
                Re: Group Design Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2020-07-10 10:22 +0200
      Re: Group Design David Brown <david.brown@hesbynett.no> - 2020-07-09 14:40 +0200
  Re: Group Design John Speth <johnspeth@yahoo.com> - 2020-07-09 07:59 -0700
  Re: Group Design Don Y <blockedofcourse@foo.invalid> - 2020-07-09 18:33 -0700
    Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-09 19:01 -0700
      Re: Group Design Don Y <blockedofcourse@foo.invalid> - 2020-07-09 22:32 -0700
      Re: Group Design George Neuner <gneuner2@comcast.net> - 2020-07-10 17:36 -0400
        Re: Group Design Ed Prochak <edprochak@gmail.com> - 2020-07-14 11:09 -0700
          Re: Group Design George Neuner <gneuner2@comcast.net> - 2020-07-16 00:26 -0400
  Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-09 22:48 -0700
    Re: Group Design Don Y <blockedofcourse@foo.invalid> - 2020-07-09 23:53 -0700
      Re: Group Design Rick C <gnuarm.deletethisbit@gmail.com> - 2020-07-10 01:15 -0700
        Re: Group Design Don Y <blockedofcourse@foo.invalid> - 2020-07-10 02:22 -0700

csiph-web