LM385 differential amplifier for current sensing

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bmandl

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Hello!
It's been a long time, since I last time posted on this forum. And here I am with a problem. I have build myself an v-a meter. V-meter works great, now a-meter it's causing me some trouble. It is like that. I am using LM358 as differential amplifier for current sensing. My calculations are, that I should get approx. 8V on the output of amplifier at 1.41 V of input (3A through 0.47R shunt resistor). But I am getting just 1.43V, same as the input. I also noticed, that I get 1.41V on - input of amplifier, instead of 0V. Can someone at least give me a clue of figuring out my problem, please. Below it's schematic file. One note: I am supplying amplifier with 15V DC, not 5V as it is on schematic.
 

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I don't see where you are measuring the current. :?: The input to the LM358 is open. Where is the 0.47R resistor? Also it is not configured as a differential amplifier.
 

As far as I know, that's not how you set up a single supply differential amplifier. This is how it's supposed to be:






You can find this and more here:
www.ti.com/lit/ml/sloa091/sloa091.pdf

Hope this helps.
Tahmid.
 
There is no shunt resistor on this schematic, becouse this is just for U-I meter. Voltage drop across shunt is connected to + input of amplifier.
So in this case, there is no option to make amplifier without two resistors on + input?

---------- Post added at 18:19 ---------- Previous post was at 18:05 ----------

What if I use LMC6482 for operation amplifier?
 

You can use a voltage follower (analogue buffer) with an op-amp after the shunt (using the shunt voltage as the input voltage).

Then, you can use the 2 resistors.

As for the LMC6482, I'm downloading the datasheet and will take a look and let you know.

---------- Post added at 23:33 ---------- Previous post was at 23:30 ----------

The LMC6482 is a rail-to-rail input and output op-amp. So, yea, you can use it.

Hope this helps.
Tahmid.
 
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    bmandl

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Ok, thank you very much. I will tested it next week and then I will post full project with pictures.
 

Ok, I used LMC6482 and tested it. It works, but the output voltage is floating too much, so consequently, displayed current is floating from 1 to 3 Amperes. Why is that and how can I prevent it?
 

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