High power constant current circuit

[Coza]

I've been looking to make a constant current circuit of 6.5A at 14V (looking to reconfigure for 4A as well).

I have tried using a darlington pair to limit the current flow. This was controlled by a shunt resistor and sense points feeding in to a op-amp current amplifier. I am confident this works fine. The current amplifier then feeds in to an op amp current comparitor and using a reference voltage (2.5V). From here is where I think I am getting a little stuck.

My current comparitor feeds into a base of an npn transistor with the emitter to ground. At the point the current comparitor reaches the desired current I want the npn transistor to start switching and turning the darlington pair off to reduce the current.

So far upon testing the whole circuit works fine up until I look at adjusting a constant resistance load. I still seem to be able to draw more than my desired current.

Have I missed anything obvious?

Many thanks

 

[Pjdd]

I haven't had time to fully analyze your circuit but you have your output transistor reverse biased.

 

[Coza]

I have tried moving the 220K resistor to the emitter side of the darlington pair.

I am aware of the massive heat dissapation given out from doing this and have got it attached to a large heatsink.