Non-Inverting LM358 Circuit Not Working

3 years ago by Drich98 to c/askelectronics

Since I never understood op-amps from reading or practicing problems I wanted to build a circuit to probe around and use different resistor values to set the amplification.

Currently I am attempting to build an LM358 Non-Inverting Op Amp. I am using my power supply for a +/- 12V rail, and my Arduino Uno for my 5V supply at the V+ input pin. I have chosen two 1k resistors to amplify the signal to 10V at the output and put and led as a indication that the circuit is working.

My questions are as follows:

Is the ground for the voltage rail and input signal the same?
What exactly is wrong with the circuit I built? I want the LED to only turn on when 5V is supplied at the input, right now the LED can turn on if I connect the ground to the voltage rail supply even without an input voltage.
I've seen the post on Adafruit with the feedback resistors connected to the same ground as the rail supply, but the circuit diagram does not show where the input voltage ground is? Link: https://blog.adafruit.com/...

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Susan_B_Good 4 points 3 years ago

Just a word of caution - education is a process of diminishing deception. Books provide a simplified version of real World electronics. Universities and colleges put a lot of effort into designing lab practicals that will actually work and give the predictable results that students expect.

So the normal learning process when it comes to op amps - is to read and understand the theory. Then complete those crafted lab practical exercises - having been introduced to the added complication of systemic and random errors. Then do your own thing, when all the remaining Real Life complications hit you like a brick.

So, if you can find a course in analogue electronics, even a distance learning one, you might find the steps are smaller and more easy to assimilate.

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Drich98 0 points 3 years ago

I understand your caution, however I understand the theory behind OP-Amps, theory can only go so far which is why I'm building a circuit on a breadboard now. I should clarify that the basic rules for ideal op-amps I have a grasp of, although I can never seem to remember these rules. For example, I have the formulas for a BJT and MOSFET transistors memorized because I spent a lot of time reading and using them in practical applications. Op-amps I have spent a lot of time reading but no time building circuits, which is essentially what I am trying to do now. I have a degree in EE, and at this point this is one of the basic components that wasn't covered much in university, nor did reading or doing practice problems help. I'm very much a hands on learner, I can read formulas and equations all day but if I don't apply what I learned I'll forget it after several days unless I repeatedly practice.

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Susan_B_Good 4 points 3 years ago

What worked for me, that may not do so for anyone else - is to take an existing circuit (usually a reference one provided by a manufacturer) and build that. Get that working (sometimes, it hasn't worked- the manufacturer's technical support department has often been very helpful, especially when their reference design has a design fault or has been misprinted - after doing that, they used to send me unmarked, pre-production chips/etc to play with and provide feedback).

Then modified that design, to test my understanding. Tried different board layouts, guard rings, etc and documented the effect. When it didn't work as expected - took that back to their tech support to see if we could work out why.

So, for me, taking something that works and keep modifying it, just a little.

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