Safe Constant Current Source

In the above circuit diagram, a CMOS operational amplifier (op-amp) controls the current via a P-Channel HEXFET power transistor (IRF9520) to maintain a constant voltage across R1. The current is given by: 1 = V_REF/R₁. The benefits of this configuration are:

• Within the event of a component failure, the load current is restricted by R1
• The overhead voltage required by the op-amplifier and also the HEXFET IRF9520 is very low.