This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
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!
Hi,
your circuit should work:
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
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
Hi user6222316,
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
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