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I have designed a circuit to measure 0-12.2V and Scale it to 0-10V from a PLC input to read. I have put an LM741CN in the circuit as a voltage follow to isolate the test circuit from the measurement circuit.

All is well except that when I input less than 2V into the op amp it stops following the voltage and continues to output about 2V I have input 0V into it and I still get about 2 volts out.

I can't find anything on the spec sheet about there being a minimum voltage output. What am I doing wrong?

schematic

simulate this circuit – Schematic created using CircuitLab

Transistor
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TheColonel26
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  • My guess is that you will receive a lot of answers telling you about the output swing limits. Have you grabbed up a datasheet PDF and done a search for that term, "swing," to see what it says? – jonk Sep 09 '17 at 23:43
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    Have you read the LM741 datasheet - particularly the "Output Voltage Swing" line under "Electrical Characteristics"? – Peter Bennett Sep 09 '17 at 23:44
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    @TheColonel26: I edited your schematic and used GND symbols to eliminate some of the criss-cross wiring. The answer you are looking for is max and min output vs supply voltage. Check the datasheet. – Transistor Sep 09 '17 at 23:46
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    Related: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp – MarkU Sep 09 '17 at 23:57
  • "*I have put an LM741CN in the circuit as a voltage follow to isolate the test circuit from the measurement circuit.*" You are not *isolating* it but rather buffering it. Is there any reason you can't use a 1.22:1 potential divider? – Transistor Sep 10 '17 at 00:04
  • It isn't just the output range. The input range is limited as well. The 741 is just plain and simple outdated. – JRE Sep 10 '17 at 00:07
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    **Stop using the LM741. It's obsolete.** If you want a general-purpose op-amp, use a more modern part. (LM358 and TLC274 are two cheap options.) –  Sep 10 '17 at 01:14
  • @Transistor: this circuit is actually to measure voltage drop, through another device. Supply voltsge for thst device is 12V, When the device is open, I need it to read 12V, however with just the divider it will pull down the Voltage to 11.88ish. – TheColonel26 Sep 10 '17 at 12:17

1 Answers1

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The 741 is not a rail to rail amplifier.

The thing on the spec sheet you want is the minimum output voltage specification, which depending on things like rail voltage and load will always be a few volts above the negative supply rail.

In your case, the negative rail is at 0v, so the minimum output voltage you can expect is 2(ish) volts if it doesn't have to sink a current, and 3(ish) if it does.

Use a 'single supply amplifier' like the LM324 if you want your inputs and outputs to include ground, or a 'rail to rail' (R2R) amplifier if you want to swing all the way to both rails.

In all cases, 741 is never the right amplifier to use these days.

Neil_UK
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