Alt-BEAM Archive

Message #09685



To: "'beam@sgiblab.sgi.com'" beam@sgiblab.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Tue, 1 Feb 2000 07:33:02 -0800
Subject: [alt-beam] Re: Sim D1 notes and questions



The problem is indeed the 74HC04 which can draw several tens of miliamps
>>>when Vdd is above 3V<<< and the input is in the "linear region". This
"linear range" is actually highly non-linear since the leakage current
increases when the input voltage is between 1/3 and 2/3 of Vdd but sharply
peaks when that input voltage is right at the threshold (~1/2 Vdd). This is
easily demonstrated by placing a 1M potentiometer between 0V and +5V with
the wiper (middle terminal) to the HC input. Then measure the 74HCcurrent on
the amp scale of your multimeter (with Vdd @ 5V) while slowly rotating the
pot. Note that when the current is maximum, the voltage at the input is at
the threshold. At that point HC devices often (and AC devices always) burst
into oscillation. This leakage/oscillation problem usually occurs when
unused HC inputs are left un-terminated and the voltage on the input can
float to the threshold. In other cases, the design is responsible and that
is certainly the case with the simplest SIMD1 when the solar cell output has
dropped to 2.7V and voltage of the fully charged storage cap (and Vdd) is
still at 5V. Note that the problem virtually disappears at Vdd = 3V and
below, and I have tried to point out that the simple circuit SIMD1 is only
useful with a single 2.5V cap and a lower voltage solar cell. BTW The
subsequent series of designs for driving LEDs from with low source voltage
was a natural progression of using the output of such a low voltage source
for pummers etc. I did publish a slightly more complex SIMD1 design which
uses positive feedback to help "push" the input voltage through the
threshold region and clamp the solar cell output until the cap is almost
fully discharged. Invented by Steven Bolt (and adapted by Ben Hitchcock),
the SUNEATER circuit samples the threshold voltage with short pulses (using
a oscillator) which greatly helps to reduce leakage current. Not
surprisingly, Steve is also a great proponent of using as low a voltage as
possible for Vdd. BTW the leakage for 4000 type CMOS is an order of
magnitude lower than HC logic and would be quite suitable for driving a
piezo chirper.

Hope this helps.

regards

wilf

> -----Original Message-----
> From: cactus@dynamite.com.au [SMTP:cactus@dynamite.com.au]
> Sent: None
> To: beam@sgiblab.sgi.com
> Subject: Sim D1 notes and questions
>
> I haven't had a chance to check yet but I think the power is bleeding
> through the hex inverter (it's an HC so it shouldn't use up too much).
>
> Any thoughts on how I can conserve the voltage in the cap even as the
> light dims in the evening and still have it there when the SIM D1
> triggers?
>
>



9686 Tue, 01 Feb 2000 09:09:38 -0500 [alt-beam] Re: Flux cleaning (was: Bicore question) beam@sgiblab.sgi.com Richard Weait At 06:22 PM 2/1/00 +1100, Rob
wrote:
>What about using low temperature 2% silver solder with 'no clean' flux?
>The flux residue is next to invisible but can be evaporated off if you
>should so choose. As far as I know it causes no problems at all. The lower
>temperature quality is an advantage to someone using a lower wattage iron.
>It melts quickly with my 25 watt one saving the components from changing
>colour.
>
>Rob

Hi Rob,

Just a brief comment on soldering in general. If the components
are changing colour, something is wrong. I have found that soldering
with the temperature too low can be a problem. When it is 'way too
low' the solder won't melt; that problem is immediately obvious and
easy to fix. Just turn the iron 'on.' :-) When it is 'just too low'
the solder will melt, but freeze when applied to a pad because the
pad (and traces) act like a heatsink. This can result in an iron
stuck to the circuit board! Pulling it away will wreck the pad, and
turning the heat up will likely overheat the board because of the
time it takes. The size of the iron tip also effects the temperature
choice; a smaller tip cools faster when applied to a solder joint.

All of this said, my preference is to use a higher temperature, with
a smaller tip and count on completing the connection quickly.

_The Art of Electronics_ (Horowitz and Hill, Cambridge Uni. Press)
has a good outline on soldering.

Cheers,

Richard.

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