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mains volt sensing amplifier problem using lm324

shams iqbal1
Prodigy 160 points
Other Parts Discussed in Thread: LM324

Hi

i have seen this circuit in power supply product

now in this circuit V1 and RV1 is just for simulation testing and actual mains input goes to R10 and R128 resistors 

if i tries to change C8 and C9 values to 1nf then this circuit does not work properly 

in this circuit changing various manufacturers LM324  ic  gives strange (improper functioning) results  

i am trying to build analog front end for micro-controller adc input stage for true rms volt reading 

where my objective is to provide circuit isolation from mains by adding high value resistor to input stage and very low phase lag of mains signal  should be appear on amplifier output

i have these question in this diagram 

  • what is the impact of C8 and C9 value 
  • why there is 1k (R27 ,R6 ) resistor to opamp input stage and what is importance for these resistors 
  • how to calculate R50 and R49  for desired output amplitude 

so plz guide me 

Regards 

  • 0
    TI__Guru**** 165526 points

    Shams,

    The Xc of C8, C9 is 1.45M ohms at 50Hz. So some attenuation is expected. The 2M and 22.7K set the maximum input voltage while maintaining a good common mode voltage (0V to 10V, 5V with no input). As drawn +/-440V not counting cap effect. The resistors also set the gain. 22.7k/2.001M (DC gain) 11.3mV/V

    The caps will reduce problems from transients on the line. The 1k resistors set the differential gain of the voltage seen at the capacitors 22.7k/1k = 22.7V/V

    What problem did changing the capacitors cause?