There are many components that can be reused from old desktop PSU(Power Supply Unit). I have written about many components that can be salvaged from SMPS(Switch Mode Power Supply) like PC817 optocoupler that can be used to build Arduino inverter and TL494 PWM Controller IC based Inverter.
Here I wanted to write about another component that can be salvaged from older SMPS which is the TL431 precision adjustable shunt regulator IC. It can be used in variety of applications in electronics circuit such as voltage regulator, precision voltage reference, voltage detection and monitor, over-voltage protection, constant current source and comparator.
The TL431 as a adjustable voltage regulator circuit diagram is shown below.
The input is 9V DC from like battery and the output can be adjusted from 0 to 7.5V DC using the 5kohm potentiometer R1. The value R1 and R2 resistor divider network depends on the desired output voltage. The equation to be solved for resistor values is,
\(V_{out} = V_{ref} (1+\frac{R_1}{R_2})\)
Now the reason to use the capacitor C1 of 47nF in the above shunt regulator is that, inside the TL431 an op-amp is used and within its internal circuit there is feedback loop. An op-amp with feedback loop can lead to oscillation which can be caused by factors like the load has large capacitance, the total phase shift is 180 due to wiring or due to using pass transistor that can add extra phase shift. In short, the capacitor C1 of 47nF is used in the above circuit to prevent oscillation or feedback loop stability that might be accidentally introduced in the circuit. It is known as compensation capacitor.
The series resistor R3 of 220Ohm is used to limit current that flows into the TL431 IC. Without it, if the TL431 starts conducting it will draw maximum current of 100mA TL431 cathode rating from the battery and burn the TL431 IC instantly. Beside this main function, it does additional things. It regulates the load voltage as R3 controls current through both the load and the TL431. When load is heavy, then most current flows through the load and when the load is light, then TL431 shunts the extra current to main constant output voltage.
But this shunt voltage regulator design with TL431 is not recommended. It is better to use series pass TL431 based voltage regulator.
