Alt-BEAM Archive

Message #05210



To: beam@sgiblab.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Fri, 09 Jul 1999 18:11:51 -0400
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?


Bumper314@aol.com wrote:

> I dont know if we are still talking about the 1382 here, but i just made one
> that is working fine, i dont konw why. My other two are haveing that save
> problem of not charging, i first brought this up a while ago and it was
> cleared up in a few messages but this one is going on and on...ill
> investigate my board a little to see if anything is different and measure the
> current coming out of the cell..i just thought this was worth saying =)

Well, I for one am still looking into the problem.

Here's what I've found so far:

The reccomendation of raising the resistor values above 2.2K may work, but
I think there is a much better solution.

Try this, place a diode (I'm using a shottkey, 1N5817) between the base of the
2N3904 and the connection between the output of the 1382 and the collector of
the 2N3906. If you have no schottkey's on hand, try a 1N4148 style diode.

This will raise the cut-off voltage by about 0.4 volts. Please let me know if
this
solves your problem. (non-Schottkey diodes may raise this a bit more than .4V)

Another approach I'm toying with is to place a current limiting diode between the

solar cell and main cap. A 1N5314 will limit the charge current to 4.7 ma.

Now this will slow down charging under bright light, and some will cry that this
wastes energy, but we can turn this into a real feature...

Add as many solar cells as you wish, not only to drive your photovore, but also
to
power any payload it may carry. Add a supercap on the solar cell side of the
current
limiter diode. Call this the 'fat' cap.

What happens now is that the photovore is powered by the fat 'energy reserve',
and the
photovore will run at a nearly constant speed under a wide range of lighting.
When light
is lost, or drops below usable levels, the photovore will still run from the
reserve energy
source until it drops to a level just above the trigger point of the SE's.

When light returns, the photovore will 'sleep' a while, until it has an energy
reserve again, then
it will move at its nearly constant speed under changing light conditions.
Energy waste is very small in this case, and we added functionality at the same
time.

If you cannot locate a current limit diode, short the gate and drain of a MPF102
JFET, this
will limit maximum current from source to gate and drain to the Idss of that
particualr FET.
This parameter varies between 2 and 20 ma for MPF102's, so you might want to
select for
the fastest charge rate that still prevents lockup.

Either of these approaches can resolve the problem without resorting to other SE
circuits.



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