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This is the final version of my CC Source. My previous versions behaved peculiarly, because of which I didn't use them. Take a look at the previous versions and maybe try to come up with an explanation as to why they behave weirdly, even though they work perfectly on the simulator.
Version 1: http://everycircuit.com/circuit/5043444003897344
Version 2: http://everycircuit.com/circuit/4974413209141248
It's time to talk about this circuit now. This is probably the simplest way to make a CC Source/Load. Here's how it works. When a power supply is connected, the MOSFET's gate (an IRF540N) starts charging through the 10k resistor. As the gate charge builds up, the MOSFET starts to turn on. The current starts flowing through it, the LEDs and the current sense resistor. When the drop across the resistor (the 470ohm one) approaches, in this case, 705mV (it reaches 607mV for a 2n2222a), the BJT starts to become conductive and begins to clamp the gate voltage. The system reaches a steady state once the drop across the sense (or shunt) resistor turns on the BJT just enough to keep the MOSFET'S conductivity at just the right level. The two transistors work together to ensure a steady, constant current flowing through the load. On changing the load, the 2 transistors readjust and keep the current at the same value. The 100k resistor is a pull-down resistor to discharge the gate of the MOSFET. The constant current I = Vbe/Rs.
It's as simple as that.
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