Alt-BEAM Archive
Message #05211
To: beam@sgiblab.sgi.com, beam@sgiblab.sgi.com
From: Terry Newton wtnewton@nc5.infi.net
Date: Fri, 09 Jul 1999 18:29:56
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?
At 06:55 PM 7/8/99 -0400, Bob Shannon wrote:
>This circuit is proven to operate reliably with (measured) identical
>component
>values. Something different starts to happen when SMT parts are used
>however.
>...
>I heard today that SOT23 transistors are not made from the same dies used
>for the
>TO92 packages, due to bonding wire attachement issues. It seems that they
>are not quite the same
>parts.
Ok... looking for that circuit...
Could be SM 1381's don't like having their low output being
pulled high by something else, that's not exactly something
you want to do with output stages. If you try that with any
kind of power you'll likely get smoke or other flakiness.
If that's the problem then maybe just a single resistor,
say 330-470 in series with the 1381 would do the trick, letting
the base resistor of the PNP limit the drive current after
triggering. PNP HFE still matters, size base resistor to
compensate, with a resistor in series with the 1381 it could
probably be 10K and it would work fine. Another solution that
makes transistor HFE not matter much at all is to use two
resistors, one in series with the 1381 out and one in series
with the NPN base (say both 220-330), leaving the PNP base
resistor around 2.2K. But if the 1381 and transistor sat.
characteristics fighting is the problem then the single
resistor solution should work. Make the PNP base R as high
as you can get it to avoid excess loss into the NPN base,
since it no longer has to waste current fighting the 1381.
A 1N914 in series with the 1381 output (anode to 1381, cathode
to PNP col and NPN base (or its series resistor if into correct
design:) should also work and accomplishes about the same thing,
perhaps even more efficiently.
Otherwise.... I have no idea...
Terry Newton
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