From: Winfield Hill
Subject: Re: 741 question
Date: 17 Nov 2002 04:11:51 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.10
> What sort of current consumption can I expect from a 741,
> configured as a differential temperature switch, running
> off of a 9volt battery.
The venerable 741 is not often used these days, unless one
is seeks an absolute bottom-dollar price. It's a very poor
choice for use with a 9V battery. First because it operates
poorly with low supply voltages, and is not specified at all
below +/-5V = 10V total supply. If you examine the curves,
you'll see that the internal biasing circuitry responds to
low voltages by reducing the operating current, which makes
gain, bandwidth and slew rate suffer badly. In addition the
opamp's output range suffers badly at low supply voltage.
Keep in mind that a 9V battery only has 5 to 6V available at
end-of-life, even less when you draw momentary high currents.
The second reason a 741 is a poor choice is supply current.
You can learn a great deal about these issues by borrowing
a copy of The Art of Electronics, by Horwitz and Hill, and
reading Chapter 14 which is all about Low-Power Design.
To choose a good low-power amplifier peruse this excellent
table at National Semiconductor. Sort on various parameters,
like supply current, operating range, or output capability.
You might find a classic LM358 or LM324 to be a good choice.
Sadly, many of the most interesting low-power opamps these
days are only available in small surface-mount packages, so
if you want to make easily-soldered breadboards with miniDIP
and DIP packages, you may need to stick to older parts. Two
other classic parts to consider are the LM4250 and LM346.
BTW, NSC offers free samples. :-) Have fun.