2 outputs of same op amp gives different voltages

I am using lm358p op-amp for 2 different heaters I connected both of them in a single op-amp and it didn’t work firstly I thought that the problem was thermocouples I swapped them but the problem kept going then I made the connections in a breadboard when I used the first amplifier it worked just fine and then I swapped the amplifiers and 2nd amplifier gave a different value (note: I didn’t change anything else and I tried with a 2.0v power supply too) I changed the resistors the capacitors checked all of them but it still doesn’t works could you help me here is my schematic

Hi,
Welcome to the forum.

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OPs schematic;

Tom... :slight_smile:

Hi,
Can you please name the pins on the dual opamp please.

You have TR1 in one feedback circuit do you have a similar trimpot in the other feedback?
If so are they adjusted the same?

Thanks... Tom... :slight_smile:

TomGeorge:
Hi,
Can you please name the pins on the dual opamp please.

You have TR1 in one feedback circuit do you have a similar trimpot in the other feedback?
If so are they adjusted the same?

Thanks… Tom… :slight_smile:

Yeah, I adjusted them the same but my problem is when I have 0v at the input of the opamp but it still gives 50 on Arduino’s analog input I double-checked the input voltage it is zero but at the meantime, the output is different on the Arduino serial port.

As I know usually both should give 0v while the input voltage is 0v (trimpots or resistors shouldn’t affect them) but they are not

Here is the pinout of lm358 opamp

I also added the interrupted trimpot below

unnamed.jpg

This was my experience trying to amplify low voltages across a sense resistor for input to an arduino. 0V in and still reading 25mA current through the sense resistor.

The 358, although an industry standard, is a pretty poor op-amp for that purpose. Look at the offset voltage specs. Depending on the model it can be anything from 0.3 to 7mV or more. There is your reading.

I switched to a NCS2333 (pin compatible) which has offsets in the uV range and 0V in actually reads 0mA output. They are a lot more expensive but sometimes you have no choice.

Willem.

Hi,
This what I was trying to get you to draw, it has all the necessary info for a proper schematic.

What gain are you expecting from the LM358s?
If you are trying to make an amplifier for thermocouples, this type of amplifier will not do the job.

google
arduino thermocouple shield

You will find there is a special Instrumentation Amplifier for the job and assembled to be as stable as possible.

Tom... :slight_smile:

LM358_NANO.jpg

Those 10k input resistors will convert the opamp's bias current into another source of offset voltage error.
I presume you are using them as RC filter with the 0.1µF caps.

Changing them to 1k and the caps to 1µF will keep the same time constant but reduce bias offset by a factor of ten.

Some key parameters you have to understand when using opamps are the input offset voltage and current and
the input bias current.

In general having high value resistors in series with inputs causes issues, you have to do the basic sums.

JFET input opamps and CMOS input opamps have much much lower input currents which can finesse the issue,
but the voltage offset remains.

The LM358 is a pretty poor performer, most modern opamps can better it in several ways.

Thanks to all of you I'm gonna change the opamp you helped me a lot

I’m using Arduino Nano (old bootloader) and a 7812 to step down the voltage for Arduino’s Vin pin but when I do that the analog reads from the opamp (powered from Arduino’s 5v starts giving different voltages.

I tried 7809 7805(directly to 5v pin)
I tried removing 78xx and giving the 24v still works fine (Arduino on USB)
I powered Arduino with 7805 from a breadboard still works fine

I removed all possible broken components which can make shorts (transistors MOSFETs etc.) didn’t work
I tried analogReference(EXTERNAL); AREF PİN 5.000v didn’t work
I tried analogReference(DEFAULT); didn’t work

I changed arduino 3 times no difference
I changed opamp 3 times no difference

when Arduino is powered from USB it works fine but external power doesn’t

I am stuck this thing makes no sense at all

Note: Arduino is clone ch340g and Ams1117 used.

Also here is my schematic (colors are air wires same colors are connected):

Please show us a schematic.

BTW

An IRFZ44 is not a logic level MOSFET ?

asdfdsgtegb:
I tried analogReference(EXTERNAL); AREF PİN 5.000v didn't work
I tried analogReference(DEFAULT); didn't work

They are the same.
"Default" switches/connects Aref to VCC.

VCC is about 4.6volt on USB supply, because of a USB back-flow protection diode.
VCC is about 5volt on external supply, providing V-in is >=6volt.
The only two 'stable' voltages are the 3.3volt supply and the INTERNAL 1.1volt Aref.

12volt on V-in is very high. There have been reports of failing Nanos if powered >9volt.
At 12volt, a Nano (with insufficient 5volt regulator heatsinking) can't deliver any significant current to sensors.

Yes, we need a schematic, and is seems not enough separation between 220volt mains traces and low voltage traces.
Very dangerous layout.
Leo..

Edit: OP's other thread seems to be the same project.

@asdfdsgtegb

TOPIC MERGED.

Could you take a few moments to Learn How To Use The Forum.
Other general help and troubleshooting advice can be found here.
It will help you get the best out of the forum.

larryd:
Please show us a schematic.

BTW

An IRFZ44 is not a logic level MOSFET ?

I was thinking about making a simple schematic but then I thought the error could be everywhere so I posted the whole PCB could you tell me schematic of where exactly do you want

ballscrewbob:
@asdfdsgtegb

TOPIC MERGED.

Could you take a few moments to Learn How To Use The Forum.
Other general help and troubleshooting advice can be found here.
It will help you get the best out of the forum.

I did line by line what I'm making wrong

Please do NOT cross post / duplicate as it wastes peoples time and efforts to have more than one post for a single topic.

Continued cross posting could result in a time out from the forum.

Hi,

Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html.
Then look down to item #7 about how to post your code.
It will be formatted in a scrolling window that makes it easier to read.

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom.. :slight_smile: