Alt-BEAM Archive

Message #05049



To: beam@sgiblab.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Tue, 06 Jul 1999 13:40:14 -0400
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?


Steven Bolt wrote:

> On Tue, 6 Jul 1999, Sean Rigter wrote:
>
> > And hopefully learning something about electronics in the process.
>
> I'm sure there is a lesson to be learned here. But can we be so
> optimistic as to say that this is significantly advancing anyone's
> knowledge of electronics?
>
> Methinks BEAM should become a little more efficient as a teaching
> aid...

Personally, I think teaching debugging is as important as electronics
theory.

I'll admit to being chronically rusty at analog transistor circuitry, but I
think we are
missing something here.

This SMT SE lockup problem is not being reported, and cannot be duplicated
with identical component values and conventional parts. This problem is
being
reported on several SMT SE's built by different people.

One helpful email suggested that lead lenght and EMI might enter the
picture if any
real difference existed, but I've looked at this carefully, that this is
not a factor.
When using the SufBoard for SMT prototyping, you actually have much longer
lead
lenghts than with a free-formed SE.

Yet the SMT SE's do behave a bit differently, as a matter of objective
fact. I'm testing
this right now, and I see a difference. This difference is harder to see
with less efficient
motors however, but this makes some sense after some study.

If you read the list archives, there is a body of thought (supported by
some evidence)
that lowering the 2.2K resistor to 1.8K often solves the whining motor
oscillation
problem. I agree that this trick works in practice, but the circuit theory
might lead you
to beleive otherwise, that it narrows the operating range of the circuit.

So all the detailed circuit theory is very helpful, but when we seem to see
conditions where
the theory does not match the evidence we need to find out why, and debug
the actual problem, not the theory. In my experiance, the 1.8K resistor
'trick' is a sure fire fix for the
whining motor full sun lockup condition, with walkman style motors. The
motor type
makes a big difference here.

The fact remains that the same measured component values produce different
behavior
with the SMT parts. The 'flaws' in the SE are an issue here, but they
alone are not the cause of the problem.

I've swapped the actual solar cells motors and, and the capacitors as well,
emperically.
I've measured the resistor values, and tested 3 full sets of SMT
transistors and triggers.

There is a difference.

Now that we are all brushed up on the circuit theory, and failings of BEAM
as a teaching
tool, would anyone care to look at the actual problem?


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