Single Power-supply Circuits
An issue with the previous circuit, is that Q1 will not be saturated unless its base is biased about 0.5 volts above Vcc.
The circuits at right and left operate more efficiently from a single (Vcc) supply.
In both circuits, R1 allows Q1 to turn on and pass voltage and current to the load. When the current through R_sense exceeds the design limit, Q2 begins to turn on, which in turn begins to turn off Q1, thus limiting the load current. The optional component R2 protects Q2 in the event of a short-circuited load. When Vcc is at least a few volts, a MOSFET can be used for Q1 for lower dropout-voltage. Due to its simplicity, this circuit is sometimes used as a current source for high-power LEDs.
Read more about this topic: Current Limiting
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