Alt-BEAM Archive
Message #05212
To: beam@sgiblab.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Fri, 09 Jul 1999 19:52:02 -0400
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?
Terry Newton wrote:
>
> 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.
Hmmm, there are open collector 1381's available as well.
> 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
As base current into the NPN is no problem, I reccomend using 2 diodes.
One between the 1381 and the PNP, and another between the PNP collector
and the NPN base.
This stops you from beating on the poor 1381, and also allows the circuit to
operate with higher current solar cells. For those who want higher switch off
thresholds, add more diodes to the base of the NPN.
Oh yes, and I've just found something odd. Apparently an intermitant problem
with one of the storage caps, or the cap may be about to die. Its leakage
current
seems to go way up sometimes.
Maybe its a coincidence that the cap happend to be happier when connected to
the
thru-hold SE's? Right now, I cannot rule this out, but earlier today the same
SE
based photovore started having low light level problems.
I started thinking this might be due to the change in resistor values (now
2.7K) but
I've ruled this out. Its the caps it seems, and its intermitant. This problem
comes and
goes without apparent rhyme or reason.
The cap bank in question is a pair of recycled 3,300 uf caps in parallel. I
had tested them
on an old digital capactiance bridge, and noted their value and leakage, etc.
Checking them
again I saw something different. Also one of my tests showed that I could not
duplicate the full sun lockup with a 4,700 uf cap. So I have to suspect the
cap bank until I rule that out.
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