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

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

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。

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

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：

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

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

Hi user6222316,,

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