Alt-BEAM Archive
Message #01948
To: Sean Rigter rigter@cafe.net
From: Steven Bolt sbolt@xs4all.nl
Date: Thu, 1 Apr 1999 08:28:47 +0200 (CEST)
Subject: [alt-beam] Re: 74HC240
On Wed, 31 Mar 1999, Sean Rigter wrote:
---8<---
[ Increased supply current when plain 74HC logic inputs are in
linear region ]
> This high current also makes it difficult to design a 5V solar engine
> using logic gates. The problem is much less severe at 2V supply
> voltage, and this "linear region" current drops down to a few hundred
> uA. This is one reason why the Steven Bolt's Sun Eaters circuits must be
> operated at 2-3V. It is also the reason why a 5uA supply current opamp
> designed to operate in the linear region is such a good choice for a
> solar engine (nuff said).
The about 2V trigger level was the first design choice, made largely
because:
- The usual pager and small recorder motors tend to waste much
energy if fed a higher voltage to accelerate from stop.
- The convenient small solar panel (BP243318) supplies 3.5V
unloaded, but only in good light; it gives much better
performance when the SE triggers near its nominal voltage (1.8V)
Which also explains why very few SE designs use opamps. The TLC27,
for instance, is intended for a supply voltage between 3V and 16V.
Opamps are usually unhappy at lower than specified supply voltages,
so we may expect performance to suffer during most of a charge
cycle and at trigger levels well below 3V. The 74HC logic family is
intended for 2V < Vcc < 6V and is known to work quite well at 1V,
if frequencies are low. I've actually seen 74HC circuitry behave
correctly at 0.5V and even less.
That said, it is certainly true that opamps are at their best when
doing linear work. Logic circuits aren't. The SunEaters go into
linear mode only during the very brief pulses of their generators,
and achieve very low average supply currents. If your circuit has
more linear aspects, and operates at 3V or higher, cmos opamps are
very attractive.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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