This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM258: LM258

Part Number: LM258
Other Parts Discussed in Thread: INA180

Hi Sir,

        I am trying to make a battery current sensing circuit using LM258.

.Problem Statement- At no load condition also, the output of circuit is showing 3.8V. My requirement is for input Battery current of 0-200A (peak current) , the output voltage should be 0-5V. I have tried with both current sensing resistor as well as Mosfet (Rds). In both cases , result is same.

Please refer the attached schematic for above mentioned problems.

pic1 - Battery current sensing ckt using MOSFET

pic2- Battery current sensing ckt using shunt resistor

pic3- Typical Battery current waveform

 

  • Hello Ratnesh,

    I see input voltage near 12V and the op amp supply voltage is 5V. Input voltage should be 0V to VCC-2V (0V-3V). 3.8V is the VOH of the op amp. So the op amp could not possibly provide 0V to 5V. Lastly the unwanted signal is 12V and the desired signal is 0 to 400mV so the difference amplifier needs very high common mode rejection. 

    You need an extended common mode current sense instrumentation amplifier. I will suggest one later today.

  • Hello Ratnesh,

    The INA180 is a good place to start.

  • Hi Ron,

     Thanks for response.

    • For the similar condition discussed above ,I have tried supplying LM258 with 12V as well as 24 V. But in both cases output voltage (while even not connecting load) was near to VCC (ie in case of 12V , it is nearly 11.8v and in case of 24 V, it is 23.8V).  So, I concluded 12V which is common mode voltage( as you also pointed out ) is getting amplified (Gain = 6). But as supply voltage of op-amp is just 12V or 24V. Hence output is getting saturated near these values. Please tell if my understanding is correct? If yes, is there anyway to reduce this common mode voltage gain?

    •  INA 180 is a good choice. I had this in my mind. But I have to design this circuit at lower cost. So,do you think 3 op-amp instrumentation amplifier will work in this condition?

    Regards:

    Ratnesh 

      

  • Ratnesh,

    For any supply 20V or greater.  Can you record the voltages on all the op amp pins?  

    Here is a simulation of the middle and final op amp output vs. load current. See how the middle op amp is higher than the 12V sense voltage level. 

     

  • Hi Ron,

    I will share op-amp pin voltages surely by tomorrow. But can you please share the schematic or TINA simulation file for which the above graph is plotted? The graph details are much closer to my desired output.

    Regards:

    Ratnesh 

  • Ratnesh,

    Here is the tina model. Despite the model result, I expect the minimum output voltage will be about 0.6V because the the sinking current required by the feedback resistor will be over 100uA. See section 3, figure 7 of the application note to see why. 

    VBB.TSC 

    Is this circuit a prototype build?  I suspect that something is connected incorrectly (not matching schematic)

  • Hi Ron,

    I am trying same circuit for few days as well as I have tried 3 op-amp instrumentation amplifier for similar condition. But I am not able to achieve desired gain, no matter how much resistor values I change.There is no output at all. Its a prototype circuit which I am making on General purpose board,with SMD components and hand soldering ( not on PCB).

    What is wrong exactly? Is it about circuit or the condition I am doing? Because in simulation circuits are working. Can You suggest anything.

    Regards:

    Ratnesh

  • Ratnesh,

    It is debug time. Start with left amplifier. Check both input voltages and output voltage then ask yourself does that make sense?  If IN+ > IN- then output should be high. 

  • Hi Ron,

    Now I am using 3 op-amp instrumentation amplifier circuit for high side current sensing application . It is working perfectly. Side by side, I am trying to work on 2 op-amp circuit also. Thanks for your co-operation.Your suggestions very appreciated. thanks man. I hope in future ,we will discuss more analog circuits.

    Regards:

    Ratnesh