Negative Shunt Regulator

Using just a few components and the circuit shown (Figure 1) it is possible to build a regulated negative supply with an output voltage of –2.5 V to –36.0 V with an output current in the order of 100 mA. The TL431 shunt regulator from Fairchild Semiconductor has good accuracy and only requires an unregulated negative voltage source. The Drop-out-voltage (Vin – Vout) of the regulator appears across resistor R3. The shunt regulator must always have a load connected at its output.