Alt-BEAM Archive

Message #05021



To: beam@sgiblab.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Sun, 04 Jul 1999 15:17:00 -0700
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?


Hell Bob,

Almost certainly a problem with "holding" current. Your solar cell is
putting out too much current(grin). The SE latch current must drop below
some minimum level before it will "unlatch" and allow the cap to charge
up again. The SMT devices or the solar cell may have a little more gain
and this makes this version more sensitive to the holding current. Need
some ideas to fix it? You could try to increase the resistor ie from
2.2K to 4.7K. That will also reduce the maximum load current above which
the circuit will oscillate. Adding a 10uf cap in parallel with the
resistor may help to overcome that problem by providing sufficient
positive feedback to latch under the initial "motor inrush" current
condition.

I will experiment a bit to get the feel of this circuit

wilf

Bob Shannon wrote:
>
> I've just been playing with a SMT SE, and noticed something odd I've never
> encountered
> before.
>
> These surface mount solar engines are built on 'SurfBoards', small commercial
> SMT prototyping boards that resemble a Basic Stamp SIPs, but are blank.
>
> I build several SE's with these boards, using 1382-J voltage triggers and real
> McCoy SMT 3094's and 3906's. My capacitor is 6,600 uf, and I'm using a 3733
> solar cell.
>
> The problem is lock-up under full sun. The problem does not happen with indoor
> lighting
> unless the photovore is held abut 5 cm from a full spectrum desklamp.
>
> I changed the resistors from 2.2K to 1.8K, with no effect.
>
> Trouble shooting with a scope gave me a supprise or two. First, the SE is not
> oscillating.
> Secondly, the lock up condition happens at low cap voltage, rather than just
> before the trigger
> point.
>
> The main cap voltage rises to the trigger point, a motor fires and the voltage
> drops down, then never rises again. The cap does not charge until the light
> level drops, a lot.
>
> I have never seen this failure mode with 1381-J's, even using identical motors,
> capacitors, and
> solar cell. (in fact, I'm duplicating a photovore I gave away as a gift, only
> using my SMT engine boards rather than the free formed SE's used in the
> original)
>
> Has anyone notices a difference when using SMT components rather than thru-hole
> parts?
>
> I'll search the FAQ again, but I dont recall ever reading about a fails to
> charge in bright light problem before, or not one where the circuit works well
> in modest lighting. I can only presume that one of the SE's is not shutting
> down as it should.
>
> I hate the thought that I might need to add a resistor between the solar cell
> and main cap if thats whats going on here. Is there another solution? Or is
> it possible that this is an internal resistance (ESR) problem with my drive
> capacitors?
>
> This particular photovore has larger caps than many, and higher powered, but
> more efficient
> motors (as compared to pager motors) beacues it needs to carry some extra
> cargo. This extra cargo casts a shadow over the main drive 3733 solar cell, so
> I was going to add a small booster cell (2224) on the tail, facing the rear.
> This will provide additional current when the 'bot is facing away from the
> light, and compenstate for the shadow on the main cell.
>
> With this high light level lockup problem, I worry that this additional cell
> will only make things worse. (I'll add them in parallel, with 1N5817
> schottkey's to prevent back feed)
>
> My next question is this, is there a way to control this effect? Can I get a
> photopopper to stop
> once its found a really bright spot to feed? (so the cargo systems may operate
> till the light level drops off and its time to go hunting again)
>
> Last question, what is the limit of a photopoppers ability to climb over
> obstacles?
>
> How many can roll over a dime, or quarter, or the thicker US nickel? How many
> can roll
> over a 1/8th watt resistor and other bench clutter? Ever seen a photovore turn
> a 2N3906 into
> road-kill on the bench?
>
> This has a lot to do with wheel diameter and torque.
>
> Pager motors are efficient, but have little useful torque. There are many
> motors that are just
> as efficient, or more efficient, but they need a bit more power than a pager
> motor to produce far more torque (walkman motors are a good example of this).
>
> I find that these slightly higher powered, but more efficient motors will drive
> wheels up to
> about a centimeter in diameter without gear reduction. The result is a
> photovore that covers
> more ground per minute, and a greater range of terrain as well.
>
> (this explains the 6,600 uf cap and -J type triggers I'm using with salvaged
> motors from laptop
> CD-ROM drives)

--
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http://www.geocities.com/Area51/Nebula/1710/
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