Alt-BEAM Archive
Message #05242
To: beam@sgiblab.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 10 Jul 1999 22:08:14 +0200 (CEST)
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?
On Sat, 10 Jul 1999, Bob Shannon wrote:
> Or a reading barrier.
Call me thick; I'm just a Dutchman.
> Please take no offence here, but I dont beleive your in the right foot yet.
No offence taken, but I think I was on the right foot to begin with :)
> You see, with the only (known) variable being the packaging of
> the voltage trigger and transistors, the lockup problem reliably
> follows the SMT parts, not the solar cell, not the caps, and not
> the motors (resistor values being measured as equal).
>
> Why is this so hard to express here? The same parts dont have a
> problem with thru-hole parts, but lock up with SMT parts.
Let's assume that the SMT parts are not actually broken, and that
you proved the other parts' health with your swapping procedure.
The first key question is then: Is the performance of the SMT parts
as per datasheets reasonably similar to the performance of the
thru-hole parts? Of course you checked that, and the answer is yes.
In which case we're back on the design. It quite simply should
tolerate substitution of any reasonably similar transistors. If it
doesn't, the design is at fault, and we shouldn't take to much
notice of the SMT / thru-hole dividing line, which is accidental.
The datasheets rule. Designers and their designs have to live with
them.
> > Anyway, I still wonder what you expected from that curve tracer, if
> > you were diagnosing by swap.
>
> Well, I'm suspecting that the Vbe of the SMT NPN part might be
> different, or the gain of the PNP, or something along these
> lines.
Differences in Vbe can't be significant. Gain varies enormously
between transistors (SMT or thru-hole) with exactly the same type
number, as you can see in the tables and datasheets. Ranges like
"between 100 and 600" are common.
Your suspicions may well be correct in that such differences can
make the 2-transistor latch/driver fail in all sorts of nasty ways,
but if they do, the design is at fault - not the parts, as long as
their variation is within the limits specified by the datasheets.
> I am not 'expecting' anything, I am investigating ~if~ any
> electrical difference actually exists.
Your tenacity has finally made me curious. Could you table and post
the values you found?
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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