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LM324: True RMS Detector (AN008474)

tejaChintalapati
Expert 1945 points
Part Number: LM324
Other Parts Discussed in Thread: LM118-N

Hello!

I'm trying to measure RMS Voltage for Sin wave using this design. I've designed the circuit using LM324 and am simulating AC voltage(5 Vpp, centred at 2.5VDC) using Waveform Generator. Even I try changing the amplitude on Input, output is staying constant at around 500mV. There's no documentation available for this design. Can you help me understand this design better?

I know that I can use transformer to bring the voltage down and interface it with an MCU to get the readings. But, I don't want an MCU in my design and would like to go purely with hardware design. Do you've any other solution you can provide me for my use case?

Thanks

  • 0
    TI__Guru**** 166906 points

    Teja C,

    That is a design from 1973 using an all but obsolete op amp, LM118-N; so there are better ways to perform this function now.

    What is your LM324 supply voltage? +/-15V is recommended for this design.

    I'll start with A1, for positive input voltage, D1 is on and D2 is off so A1 is disconnected. 

    For negative input voltage, D1 is off and D2 is on so A1 makes D2 cathode be VIN * -1

    See if your circuit does this.

  • 0 in reply to Ron Michallick
    Expert 1945 points
    Hello Ron!

    Thanks for the reply. We don't have negative voltage in our design.

    Does TI has some offering(s) in this area? Some IC perhaps? Can you please elaborate a bit about the better ways you were mentioning?

    Thanks
  • 0 in reply to tejaChintalapati
    TI__Guru**** 166906 points

    Tega,

    Searching the TI site, I only found the LM118 design and a software solution reference design. I did notice RMS IC's on other sites.

    I can help you debug the circuit. I need to know the voltage at all nodes so I can figure out what is not working.