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Topic: I can't make working non inverting op amp (Read 307 times) previous topic - next topic

Intech0921

Jun 19, 2019, 11:31 am Last Edit: Jun 19, 2019, 11:34 am by Intech0921
Hello!

I need to use non inverting op amp in my project. I'm complete beginner and I've never used any op amp before. I didn't get expected results from op amp in a project so I decided to build simplest schematics I can think of so I could test it. Unfortunately, it still doesn't work. I need help, I really cannot understand what I'm doing wrong.

So I used parts I had in home:
-op amp lm2902n data sheet
-22k resistor
-12k resistors
-arduino uno

I made it that way:
schematics

Here is how I understand this:
Arduino sends 3.3V signal to op amp plus input. Op amp is powered by 9V battery. I connect one 22k resistor between minus input and output and three 12k resistors to the ground. So I think I should get output voltage of V2 = V1(1+22/36) = 4,5V. That signal is send to arduino analog input which should give me number close to 1000 as 1023 is 5V. I however get something around 60 and op amp is getting hot.

I ask for help with understanding what I'm doing wrong.

wvmarle

That's not a schematic, that's a prime example of how useless a Fritzing style wire diagram is for showing circuits.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Intech0921

Yes, sorry I didn't give you normal schematics but my problem is that I can't get any schematics to work and I think I may be connecting wires wrong way. I just wanted to show how I connected it and I thought that fritzing is more readable than picture.

But if you prefer regular schematics I tried to make it and this is what I got:
schematics


I'm sorry if that fritzing style of showing circuits is difficult to read for you, but I'm learning and Fritzing is only tool I know how to use right now. I will do better in the future but that is all I got at this moment. I would apprecaite any help.

PerryBebbington

#3
Jun 19, 2019, 01:00 pm Last Edit: Jun 19, 2019, 01:02 pm by PerryBebbington
Hello INtech0921.

Your proper schematic looks correct. In your Fritzing diagram you have the +9V from the battery connected to 0V on the Op amp and 0V from the battery connected to the Vcc pin of the Op amp. Or at least, as far as I can see that's what you have done.

I'm not surprised it's getting hot.

MarkT

Hello!

I need to use non inverting op amp in my project. I'm complete beginner and I've never used any op amp before. I didn't get expected results from op amp in a project so I decided to build simplest schematics I can think of so I could test it. Unfortunately, it still doesn't work. I need help, I really cannot understand what I'm doing wrong.

So I used parts I had in home:
-op amp lm2902n data sheet
-22k resistor
-12k resistors
-arduino uno

I made it that way:
schematics

Here is how I understand this:
Arduino sends 3.3V signal to op amp plus input. Op amp is powered by 9V battery. I connect one 22k resistor between minus input and output and three 12k resistors to the ground. So I think I should get output voltage of V2 = V1(1+22/36) = 4,5V. That signal is send to arduino analog input which should give me number close to 1000 as 1023 is 5V. I however get something around 60 and op amp is getting hot.

I ask for help with understanding what I'm doing wrong.
That opamp cannot drive to the +ve rail.  You are asking it to do so.

Try giving the non-inv input 1V, not 3.3V.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

wvmarle

That is not a preference. That is about being able to understand the schematic in the first place.

The one you now linked to is less bad. At least it shows the way the OpAmp is connected, that's progress. The highly unusual layout still makes it very hard to read; the values of the resistors are placed very awkwardly. This arrangement should give you 5.3V on the output, dangerously close to the limit of the pin. Are you sure the power to the OpAmp is connected the correct way around?

No power for the Arduino, that won't work well.

No decoupling cap for the OpAmp.

No pin numbers given for that IC, not even where pin 1 is.

Stop wasting your time on learning Fritzing. Spend a little extra time and learn how to read and draw REAL schematics - use KiCAD or EagleCAD, two popular and free schematic drawing programs.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

wvmarle

That opamp cannot drive to the +ve rail.  You are asking it to do so.
OP is powering it at 9V so 5.3V is comfortably below the rail.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MarkT

OP is powering it at 9V so 5.3V is comfortably below the rail.
Well in that case the Arduino analog pin is probably fried now, there's no protection resistor between the opamp output and the analog pin.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

wvmarle

Fair chance.

Unless the Arduino is powered at real 5V, not USB 5V which is ~4.6V, so it's still within spec (max is Vcc+0.5V) albeit scarily close to not.

Or if the OpAmp power was indeed connected the wrong way around and indeed produced 0V output, which could mean the OpAmp is the part that's fried.

Or maybe the OP is just darn lucky and it all still works but was just wrongly connected - then OP still has a chance (but should at least swap out the OpAmp's resistors to decrease the gain, and maybe also lower the input voltage).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

DVDdoug

I'm gonna' throw a couple more things at you...

Do you have a multimeter so you can actually measure voltages?

Since the op-amp has a 9V power supply, it would be wise to add a over-voltage protection circuit between the op-amp and Arduino input.   (I'd recommend a 1K - 10K resistor instead of the 100 Ohm resistor they show in the examples.)   I use an op-amp with +/-12V power supplies as an audio peak detector.   It should never go negative and I don't expect  it to ever go over +5V, but I have the full-protection circuit anyway.

The Arduino actually has built-in low-current protection diodes but I'm not 100% sure of their current rating.  So, you can probably get-by simply adding a 1K - 10K resistor in series with the Arduino input (to limit the current and allow the internal diodes to safely-work).   That's better-than-nothing...

If you want to test the op-amp there are two super-simple circuits you can build.   A buffer amplifier and a comparator.   You'll need to make a voltage divider for the comparator reference and you MUST have a protection circuit if you connect it to the Arduino because it will "slam" between ~9V and ~0V.

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