The circuit shown will provide three pre-set currents which will remain constant despite variations of ambient temperature or line voltage.

ZD1 produces temperature stable reference voltage which is applied to the non-inverting input of IC1

A full DC feedback is applied from the output to the inverting input holding the voltage at Q2’s emitter at the same potential as the non-inverting input.

The current flowing into the load therefore is defined by S1. With the values employed here a pre-set load current of 10mA, 100mA or 1A can be selected. Q2 should be mounted on a suitable heatsink