The output current is set by the resistor R in the collector of Tr2, which may be varied to offer a range of output currents from 100 mA to 2 A with a fine control by means of VR3 which varies the reference voltage to the non-inverting input of the op amp.

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The feedback path from the output to the inverting input of the op amp maintains a constant voltage across R, equal to (Vcc-Vin) and hence a constant current to the load given by (Vcc-Vin)/R.