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Sadly, it does not work very well in real life.
A simple adjustable constant current circuit that uses only an inductor, capacitor, shunt, diode, mosfet, two op amps, some resistors, and a potentiometer.
Does it buck, though? I don't know.
Notes:
The voltage dependent voltage source is in place of an op amp in a differential amplifier configuration.
If you start the circuit simulation with the power switch closed, it will say "cannot find solution". You need to start the circuit first, then close the switch. After that the constant current circuit will work properly.
The comparator now has some hysteresis due to a positive feedback resistor. This makes it buck instead of be a linear regulator.
How it works:
The comparator turns the mosfet on because the noninverting input voltage is higher than the inverting input voltage (which is 0 because there is no voltage difference across the resistor, which makes the output of the "differential amplifier" 0)
This causes current to rise through the inductor, shunt resistor, and LED. The diff amp raises its output voltage to the point that the comparator turns the mosfet off because the diff amp voltage is higher than the potentiometer voltage. This causes current to fall as the inductor's magnetic field collapses. This causes the diff amp output voltage to fall, which makes the comparator turn on the mosfet, and the cycle repeats.
I've breadboarded this circuit to drive a COB LED I scavenged from a light, and it works pretty well. It's more efficient than the linear version without hysteresis because the mosfet does not heat up as much when the current is set to about half.
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