Electronics > Beginners
LM741 as comparator 1.9V problem
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PerranOak:
Hi,
I'm experimenting with an LM741 as a comparator.
I have a simple circuit with pots to sweep the voltage on both inputs (with meters) and I watch the output with a meter.
Powered with 5V and the inv input at 3V all is well as I sweep the non-inv input above and below 3V the output goes high and low as expected.
However, if the inv input is below 1.9V the output will not change no matter what the non-inv input.
How can this be?
Thanks.
capt bullshot:
This is pretty normal for a common Op-Amp.
Read the datasheet and find the common mode voltage range of the input. If one (or both) inputs exceeds this range, the OpAmp won't work as expected. For a non-RRIO (rail-to-rail In/Out) like this 741, the input common voltage range usually goes from about neg. supply+2V to pos. supply-2V (2V ... 3V with a 5V supply). See the datasheet for exact values. There are some Op-Amps that have a common mode voltage range including the neg. supply (e.g. the LM358), or modern variants that allow from neg. to pos. supply (rail-to-rail input).
Audioguru:
The 741 opamp was designed 50 years ago to use a 30V power supply (plus and minus 15V). Many will not work when the total supply voltage is less than 10V but yours is only 5V so of course it does not work.
Zero999:
As mentioned above, you're exceeding the common mode range of the op-amp.

Ideally a comparator IC should be used, such as the LM393 or LM311, rather than a generic op-amp, which will be designed for linear operation.
orbanp:
You got solid advice above: do read the datasheet!

Also, here is an app note that gives good info on using opamps as comparators:
http://www.analog.com/media/en/technical-documentation/application-notes/AN-849.pdf

Peter
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