constant current source with potentiometer

Thread Starter

pager48

Joined Nov 25, 2018
160
What needs modifying to make this constant current circuit vary the current with the entire turn of the pot?

Currently the pot only changes the current up to ~5.8K and beyond that the pot doesn't effect current. Tried changing the shunt and pot values but in every case the pot doesn't completely vary the current.


 

LesJones

Joined Jan 8, 2017
2,325
When the output current is 7.5 mA you have 7.5 volts across the 1K resistor so you only have 4.5 volts between the mosfet source and positive rail. The output voltage of the LM358 cannot get all the way up to + 12 volts and you need a few volts between the source and the gate of the mosfet to turn it on. I think this is the reason for the limitation of the output current. You could probably get to a slightly higher output current using a bipolar transistor as this would only require about 0.7 volts between emitter and base to turn it on.

Les.
 
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