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LM124: Working principle of the current source

user4090349
Intellectual 770 points
Part Number: LM124

Hi

I would like to understand the working principle of the circuit. The purpose is to use a current source(2.5mA) to check the 1 form C relay(1 Open, 1 Close, 6, 5, 4) status. Another relay is use to represent the status(right down corner). The input voltage is 24V at pin 7(left up corner). It seems R4 is used to control the current. Any unclear, please let me know.

Thanks in advance!

  • 0
    TI__Guru*** 140696 points

    user4090349,

    Try redrawing the U2 pin 8 and U3 pin 1 amplifiers with the feedback resistor (output to IN-) placed directly above or below the op amp. 

    This step alone will make it much easier to understand. If the circuit operation is still unclear, post the new drawing and the questions about sections yet to be understood.