Electronics > Beginners
constant current source design
Arjunan M R:
Hi, I am designing a PSU using LT3080. My requirement is that the minimum voltage of the PSU should be 1V or lower. It requires a minimum output current of 1mA. So to meet the minimum current requirement of the LT3080 ,
I need to put a 1mA constant current source at the output.The constant current source have to able to draw 1mA even at 1V or lower.I first decided to use lm334z but it is only available in higher quantities.So I decided to make a constant current source myself.I'm having problem that the current decreases when going lower than 1.1V.Why is it so?
Is it because the LM358 is not rail t rail?
Here is the circuit ↓↓↓↓
This is based on a topic "14.4V ground reference for ADC" in projects design and technical stuff.
tszaboo:
No, the LM358 is not rail to rail. If you have a question, that involves:
-Can the LM358 do x?
The answer is typically no.
The only thing the LM358 has is cheap price.
Zero999:
The LM358 can't sink much current at output voltages under 1.5V. It doesn't have a true 0V capable output stage, just a constant current source pull-down attached to an ordinary output stage.
Use a current mirror, which will work down to 100mV or so. Select R1 so it draws at least 4mA.
Arjunan M R:
--- Quote from: NANDBlog on June 03, 2019, 10:36:22 am ---No, the LM358 is not rail to rail.
--- End quote ---
I said that LM358 is not rail to rail.
--- Quote --- If you have a question, that involves:
-Can the LM358 do x?
The answer is typically no.
--- End quote ---
So which opamp can I use?
--- Quote ---The only thing the LM358 has is cheap price.
--- End quote ---
I need to build it functional but cheap.
Arjunan M R:
I am having some issue with the current mirror, even i set it to 4mA at 19V using 4.7K resistor.It is drawing 40mA at the load side.
I am using 2N3904.
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