The Safe constant current source.

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Figure the top is a Safe constant-current source circuit, how it works?
A cmos op-amp (number ICL 7611) controls the input current through a P-channel Hexfet power transistor (No. IRF 9520), then to keep up a constant voltage across the R1.
As they are connected in a serial form, so use the together current by: I = VREF /R1 , while the Vref to be defined by the IC2 is 1.25V.

The advantage of this outline are:
1. The load current is limited by R1 when the load is too heavy.
2. The op-amp and Hexfet there are the overhead voltage very low.