Alt-BEAM Archive
Message #05078
To: beam@sgiblab.sgi.com
From: Bob Shannon bshannon@tiac.net
Date: Wed, 07 Jul 1999 18:33:51 -0400
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?
Wilf Rigter wrote:
> Hello Bob,
>
> you wrote:
>
>
> I assume the whining problem refers to the START of the discharge cycle and
> I can understand that lowering the 2.2K resistor value will solve the
> problem by increasing the PNP and NPN base currents and cause the NPN to
> saturate for that specific motor current thereby avoiding the "whining
> motor" syndrome which is caused by instability of operating the SE on the
> edge of saturation.
Exactly correct. This change (from 2.2K to 1.8K) has been discussed on the list
some time back. Some people felt that 1.8K was an all-around 'better' value to
use.
I tested this with several (conventional) SE's, and it seems to be true over a
range
of different motor types.
As a result, I adopted this value when I built my SMT SE's, and ran into the
problems
that started this thread.
When the 1.8K vs. 2.2K discussion was ongoing, no one ever mentioned the full
sun
lockup problem. In reading the older email by Dave H, reposted by Steven, I now
wonder
why this problem is being seen with their magbot kit only (apparently).
I asssume that this kit uses conventional (thru-hole) parts. Does anyone know
the value
of the resistor in the Solarbotics magbot kit?
I have also read (in the FAQ) that for magbot applications, people should start
with very
low values, beginning at 47 ohms, and work upwards from there for good
operation.
No mention is made of the full sun, low voltage lockup problem there.
> However you earlier described a different problem which was resetting the
> latch at the END of the discharge curve during which in fact you really want
> the NPN to come out of saturation and turn off.
Correct. The lockup voltage is between .70 and .74 it seems.
> I suggested that increasing
> the resistor could solve that problem but could cause the problem of motor
> oscillation at the beginning of the discharge. So far everything fits my
> theoretical model of the SE but requires confirmation by experiment (just to
> keep me honest).
>
> Did you actually try increasing the 2.2K resistor to 4.7K?
In progress now. I had to 'borrow' some assorted values of SMT resistors, and I
was
also reluctant to try to rework the SE's that are already attached to my chassis
until I
had practiced removing them from a junk board first. I'd hate to ruin the
whole bot
at this late stage. I may just cut an etch and add a second resistor in series
rather than
try to remove the existing 1.8K parts.
I also plan on testing the LED version you posted. I'll let you know what I
find.
> You wrote:
>
> >Personally, I think teaching debugging is as important as electronics
> theory.
>
> Yes, it is useful to learn the theory and operation of these very simple yet
> quirky SE circuits.
>
> Nevertheless it is good engineering practice to design circuits which are
> insensitive to "normal" variations in passive component values or to
> transistor gain variations or to changing parameters caused by temperature,
> humidity etc. If you are a beginner and want to play it safe, build a well
> designed circuit and use it as recommended. It will work as advertised but
> you may not learn much about the underlying electronics.
Ahhh, excellent point! I've been waiting for someone to mention this.
If we started using 'better' SE designs, we stop a lot of the learning.
Based on my testing, there is far more hype about which SE is better than
there is actual evidence to support any opinions. There are as many
misperceptions
on this point as there are flaws in all of the SE's.
------------------------------------------------------------------------
eGroups.com home: http://www.egroups.com/group/alt-beam
http://www.egroups.com
- Simplifying group communications
Home