Hello all. I am trying to setup a constant current source using a 2n3904 and a Motorolla LM358N. I want to ensure 4mA collector current regardless of my load. Here is a screen shot of the circuit, an explanation of my rationale, and the problem I am having.
My Rationale
Using the datasheet, to the best of my ability, it looks like my DC current gain would be 70. By that logic my base current, 3.68V/23k = 160µA, multiplied by 70 should give me a collector current of 11.7mA (NOTE: The LM356N does not go rail-to-rail so the output is 3.68V). When the voltage across my sense resistor exceeds 4V, 4mA total current, the LM356N will output low unbiasing the 2n3904. I know I may have to implement some hysteresis into this circuit.
My Problems
1. I am pretty sure I'm interrupting the datasheet incorrectly. Could someone explain this section to me?
How I see it if I'm fully on, what would my collector current be? Well in this case it would be 5v/1k which is 5mA.
With that information I assumed I would reference the 3rd entry for the behavior I could expect. Also, Vce is listed as 1V here.
How does this factor in?
2. As it stands right now, I get 2.8mA of total current through my sense resistor. What exactly am I screwing up here?
3. Lastly, I've heard it is poor practice to use an op-amp as a comparator without some sort of hysteresis. How can I implement this here?
Thank you all so much for any input. Hopefully, I was clear in my explanation.
My Rationale
Using the datasheet, to the best of my ability, it looks like my DC current gain would be 70. By that logic my base current, 3.68V/23k = 160µA, multiplied by 70 should give me a collector current of 11.7mA (NOTE: The LM356N does not go rail-to-rail so the output is 3.68V). When the voltage across my sense resistor exceeds 4V, 4mA total current, the LM356N will output low unbiasing the 2n3904. I know I may have to implement some hysteresis into this circuit.
My Problems
1. I am pretty sure I'm interrupting the datasheet incorrectly. Could someone explain this section to me?
How I see it if I'm fully on, what would my collector current be? Well in this case it would be 5v/1k which is 5mA.
With that information I assumed I would reference the 3rd entry for the behavior I could expect. Also, Vce is listed as 1V here.
How does this factor in?
2. As it stands right now, I get 2.8mA of total current through my sense resistor. What exactly am I screwing up here?
3. Lastly, I've heard it is poor practice to use an op-amp as a comparator without some sort of hysteresis. How can I implement this here?
Thank you all so much for any input. Hopefully, I was clear in my explanation.