Voltage Regulator Circuit
Fixed 9v Power Supply
There are a wide variety of 3-terminal voltage regulators available in a standard TO-220package with the most popular fixed voltage regulator being the 78xx series positive regulators which range from the very common 7805, +5V fixed voltage regulator to the 7824, +24V fixed voltage regulator. There is also a 79xx series of fixed negative voltage regulators which produce a complementary negative voltage from -5 to -24 volts but in this tutorial we will only use the positive 78xx types.
The fixed 3-terminal regulator is useful in applications were an adjustable output is not required making the output power supply simple, but very flexible as the voltage it outputs is dependant only upon the chosen regulator. They are called 3-terminal voltage regulators because they only have three terminals to connect to and these are the Input, Common and Output respectively.
The input voltage to the regulator will be the +12v yellow wire from the PSU (or separate transformer supply), and is connected between the input and common terminals. The stabilised +9 volts is taken across the output and common as shown.
So suppose we want an output voltage of +9 volts from our PSU bench power supply, then all we have to do is connect a +9v voltage regulator to the +12V yellow wire. As the PSU has already done the rectification and smoothing to the +12v output, the only additional components required are a capacitor across the input and another across the output.
These additional capacitors aid in the stability of the regulator and can be anywhere between 100nF and 330nF. The additional 100uF output capacitor helps smooth out the inherent ripple content giving it a good transient response. This large value capacitor placed across the output of a power supply circuit is commonly called a “Smoothing Capacitor”.
These 78xx series regulators give a maximum output current of about 1.5 amps at fixed stabilised voltages of 5, 6, 8, 9, 12, 15, 18 and 24V respectively. But what if we wanted an output voltage of +9V but only had a 7805, +5V regulator?. The +5V output of the 7805 is referenced to the “ground, Gnd” or “0v” terminal.
If we increased this pin-2 terminal voltage from 0V to 4V then the output would also rise by an additional 4 volts providing there was sufficient input voltage. Then by placing a small 4 volt (nearest preferred value of 4.3V) Zener diode between pin-2 of the regulator and ground, we can make a 7805 5V regulator produce a +9 volts output voltage as shown.
Increasing The Output Voltage
So how does it work. The 4.3V Zener diode requires a reverse bias current of around 5mA to maintain an output with the regulator taking about 0.5mA. This total current of 5.5mA is supplied via resistor “R1” from the output pin-3.
So the value of the resistor required for a 7805 regulator will be R = 5V/5.5mA = 910 Ohm. The feedback diode, D1 connected across the input to output terminals is for protection and prevents the regulator from being reverse biased when the input supply voltage is switched OFF while the output supply remains ON or active for a short period of time due to a large inductive load such as a solenoid or motor.
Increasing The Output Voltage
So how does it work. The 4.3V Zener diode requires a reverse bias current of around 5mA to maintain an output with the regulator taking about 0.5mA. This total current of 5.5mA is supplied via resistor “R1” from the output pin-3.
So the value of the resistor required for a 7805 regulator will be R = 5V/5.5mA = 910 Ohm. The feedback diode, D1 connected across the input to output terminals is for protection and prevents the regulator from being reverse biased when the input supply voltage is switched OFF while the output supply remains ON or active for a short period of time due to a large inductive load such as a solenoid or motor.
Then we can use 3-terminal voltage regulators and a suitable Zener diode to produce a variety of fixed output voltages from our previous bench power supply ranging from +5V up to +12V. But we can improve on this design by replacing the fixed voltage regulator with a variable voltage regulator such as the LM317T.