Hi All,
In history, I used 1/2 of an LM358 to create a constant current source for an LED. The circuit was powered by +12V. The opamp was configured as a voltage follower, driving the base of a 2N6386 darlington transistor with a 5 ohm resistor in the emitter to ground and connect as feedback to the minus input of the op-amp. The LED was connect between +12V and the collector. The positive input was a 0-5V signal which would control the 0-1 amp current through the LED.
The circuit worked fine until you set the input to zero. The LED would remain barely lit. With an oscilloscope, the output of the op-amp was oscillating around the 1.4V Vbe cutoff of the darlington transistor. Even though there are internal resistors across the base-emitter junctions of the transistor, the op-amp’s output would go low enough to bias the darlington in cutoff, but then increase until current would appear in the feedback and then dive back into cutoff.
I solved the problem by adding a pull-up/pull-down resistor somewhere in the circuit, but forgot where. I seemed to remember that it was adding a weak feedback signal when the transistor is in cutoff and the feedback from the 5 ohm resistor is absent. Probably not the right solution, but I did have a real world problem.
What is the right solution when adding a power element to the output of an op-amp circuit? When more than one output voltage from the op-amp satisfies the feedback requirements (such as saturation or cutoff of the pass power element). It there a resource that reveals practical biasing of op-amp control circuits for situations like this?
Thanks in advance for your help and guidance.
In history, I used 1/2 of an LM358 to create a constant current source for an LED. The circuit was powered by +12V. The opamp was configured as a voltage follower, driving the base of a 2N6386 darlington transistor with a 5 ohm resistor in the emitter to ground and connect as feedback to the minus input of the op-amp. The LED was connect between +12V and the collector. The positive input was a 0-5V signal which would control the 0-1 amp current through the LED.
The circuit worked fine until you set the input to zero. The LED would remain barely lit. With an oscilloscope, the output of the op-amp was oscillating around the 1.4V Vbe cutoff of the darlington transistor. Even though there are internal resistors across the base-emitter junctions of the transistor, the op-amp’s output would go low enough to bias the darlington in cutoff, but then increase until current would appear in the feedback and then dive back into cutoff.
I solved the problem by adding a pull-up/pull-down resistor somewhere in the circuit, but forgot where. I seemed to remember that it was adding a weak feedback signal when the transistor is in cutoff and the feedback from the 5 ohm resistor is absent. Probably not the right solution, but I did have a real world problem.
What is the right solution when adding a power element to the output of an op-amp circuit? When more than one output voltage from the op-amp satisfies the feedback requirements (such as saturation or cutoff of the pass power element). It there a resource that reveals practical biasing of op-amp control circuits for situations like this?
Thanks in advance for your help and guidance.