Often we have to adjust the tension of a negative supply and do not have the ideal component hand, this circuit in addition to solving this problem, serves up a Power Adjustable Negative. He use the regulator 9v negative, 7905, the envés to use with fixed voltage, we can use it as a variable regulator. The 7905 in this configuration can adjust the output from -5 to-30V with an input of up to-35V. See table below. Example : Vin = -20 V Radj Vout 33 -5,74 100 -6,99 330 -11,03 680 -18,20