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OPA377-Q1: Some questions about using OPA377 to build a current limiting circuit

user6222316
Prodigy 90 points
Part Number: OPA377-Q1
Other Parts Discussed in Thread: OPA377

I use the OPA377 as  a current limiting circuit.

The voltage of the 3 pin of OPA377 is 0.19V，but  the voltage of the 4 pin of OPA377 is 0.72V。

The voltage of the 3 pin and the 4 pin should be equal is 0.19V.

The voltage between the 2 pins of the connector P3 also should be 0.19V，but it is 0.72V。

What is the problem?

thanks！

  • 0
    Guru 140200 points

    Hi,

    your circuit should work:

    opa377.TSC

    Take care, if the turn-on voltage of LED is too high the circuit could oscillate. You can check this by replacing the LED with a 1N4148 for a test.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 90 points

    I simulated it in TINA, it works normally。

    But in the actual circuit, it works abnormally。I have checked all the value of the resistors and the capacitors，it‘s all right。

    On the PCB，the LED works，but the voltage of the testpoints VF3 and VF1 are too high。

  • 0 in reply to user6222316
    Guru 140200 points

    Hi,

    what is the voltage drop across the LED?

    Have you checked the circuit with a small signal diode (1N4148 .e.g.) replacing the LED?

    Do you have a scope?

    Try not to directly touch the circuit nodes with the multimeter or scope probe. Insert a 100R resistor first, to isolate stray capacitance.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 57241 points

    Hi ,

    The problem is not in the simulation, but it is likely on your board. Are you using breadboard? 

    Per your description, the issues seem to reside at the Op Amp's feedback loop. Pin4 has to follow Pin3, if OPA377 is working properly. Since Pin4  is measuring 0.72V, the voltage across RT1 resistor will be approx. 0.792V, which is a wrong voltage value. 

    Please follow Kai's suggestions and check the  voltage across the constant current drive branch, where 3.3V = V_LED1 + V_NPN(collector to Emitter) + V_RT1

    V_RT1 voltage across RT1 needs to be ~0.2V in order to generate 20mA of constant current.

    If you are confident about all the physical connection on the board, then replace the OPA377 as last  resort. 

    Best,

    Raymond 

  • 0 in reply to kai klaas69
    Prodigy 90 points

    The voltage drop across the LED is 1.3V. 

    I use a 100Ω resistor to isolate stray capacitance.The test results are the same.

    The voltage I actually tested is as shown below：

    I use 1N4148 instead of LED,the voltage I actually tested is as shown below：

  • 0 in reply to Raymond Zhang1
    Prodigy 90 points

    I am not using breadboard.I use the print circuit board. For test voltage, see my reply to the kai klaas69。thanks

  • 0 in reply to user6222316
    Guru 140200 points

    Hi,

    hhm, I think something is damaged in your circuit. I would start from scratch with total fresh parts and check again.

    By the way, what transistor do you use? Any wrong connections?

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 57241 points

    Hi ,,

    If you have 2.4V above T1 transistor, you will have not 1.4V at the bottom of R6. Below the collect of T1, there is only 0.9V remaining. I agreed with Kai that something is wrong in your circuit. 

    Have you replaced OPA377 as I suggested? 

    Best,

    Raymond