Be careful handling computer racks
ard at p850ug1.demon.co.uk
Tue Jan 19 14:30:28 CST 2010
> Tony Duell wrote:
> > Off-topic, but modern consumer electronics (particularly the cheap 'own
> > brands' tends to be assembled in thin sheet-metal cases with edges that
> > will give you a nasty-ish cut if you catch them.
> I've stopped using the really cheap PC cases. The Antec 300 I'm using
I wasn;'t just thinking of comptuers. Things like VCRs, DVD players, etc,
have nasty sheet metacl covers now that draw blood.
> now seems to have had the metal edges rolled over at least once, which
> pretty much leaves it with smooth rounded edges that have the cutting
> ability of a wet paper towel.
> > Modern components shouldn't get hot enough to burn you :-).
> Exceptions: power transistors (esp. BJTs). SCRs. Big diodes. Well, just
> about anything that's handling more than about ~10W of power really...
Hmm.. I prefer it if even those are cool enough to touch without a nasty
burn. I always feel that too-hot == unreliable.
A diode should be dropping 0.7V or so. For that to dissipate 10W, it must
be carrying about 14A. That's not small. Ditto for thyristors.
> That said an un-heatsinked 7805 running at 1A output with a 12V input
> will run surprisingly hot, too, and that's only 7W. I've seen 7805s hit
True. The only ime I run a 7805 without a heatsink is when a 78L05 would
have done :-), but I'm using the larger-cuirrent version becasue I had
one in the junk box.
> > Glassfets are another matter.
> Mmm, vacuum tubes. "After turning off main power, wait 15 minutes for
> valves to cool."
That sounds about right. Big output tetrods/pentodes get very hot...
> > And an IC that has developped internal shorts can get hot
> > enough to burn you, some DRAMs were prone to this.
> A few old 1k*4 SRAMs did this too. 2114 anyone?
> Those things were nasty little buggers. If it wasn't a bad bit, it was a
> dead bitline, address line or just a dead chip shorting the power bus.
> Must be one of the least reliable SRAM chips ever made.
Oh, you've noticed that too. I don't like shotgu debugging as you know,
but if I see 2114s in a device and have unexplained problems, I often
replace them with known-good ones just to see. And most of the time that
cures the problems.
My HP9836CU would randomly fail to detect the intenral FDC card during
the power-on tests. It turned out (and this is not in any manual that I
can find) that the test for the presense of an FDC was to read and write
the sector buffer RAM on the board. And this RAM was a pair of 2114s (it
could also be 2112s, in the former case only 1/4 of each chip is used).
> > Yes, I was thinking of bruns from the soldering iron, or something heated
> > by it Most of the time it's when I am soldering a piece of wire to a
> > large-ish metal object (pin of a 4mm plug) and I don't let it cool for
> > long enough before taking it out of the vice.
> I seem to do that with startling regularity... Somehow the concept of
> objects touched by soldering irons also being hot hasn't quite sunk in yet.
It's suprising how long a metla pin will stay hot after soldering.
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