78xx and 79xx Voltage Regulator Circuit Working Operation

All voltage sources cannot able to give fixed output due to fluctuations in the circuit. For getting constant and steady output, the voltage regulators are implemented. The integrated circuits which are used for the regulation of voltage are termed as voltage regulator ICs. Here, we can discuss about 78xx and 79xx series of ICs.

78xx Voltage Regulator Circuit

The voltage regulator IC 7805 is actually a member of 78xx series of voltage regulator ICs. It is a fixed linear voltage regulator. The xx present in 78xx represents the value of the fixed output voltage that the particular IC provides. For 7805 IC, it is +5V DC regulated power supply. This regulator IC also adds a provision for a heat sink. The input voltage to this voltage regulator can be up to 35V, and this IC can give a constant 5V for any value of input less than or equal to 35V which is the threshold limit.

Following are the PIN details of 7805 IC.
• PIN 1 ( INPUT) : It should be in the range of 7V to 35V. An unregulated voltage is applied to this pin for regulation. For 7.2V input, the PIN achieves a maximum efficiency.
• PIN 2 (GROUND) : Connect the ground to this pin. For output and input, this pin is equally neutral (0V).
• PIN 3 (OUTPUT) : Regulated output is available at this pin.

The figure-2 depicts voltage regulator circuit diagram which produces regulated 5V from AC mains.

Following are the components used in this circuit.
1. 230V-12V Step Down Transformer
2. Bridge Rectifier (or 4 PN Diodes – 1N4007)
3. 1A Fuse
4. 1000µF Capacitor
5. 7805 Voltage Regulator IC
6. 0.22µF Capacitor
7. 0.1µF Capacitor
8. 1N4007 Diode

Let us understand voltage regulator circuit working operation based on 7805 IC.
➨The AC power supply from mains first gets converted into and unregulated DC and then into a constant regulated DC with the help of this circuit. The circuit is made up of transformer, bridge rectifier made up from diodes, linear voltage regulator 7805 and capacitors.
➨As shown, the working of the circuit can be divided into two parts. In the first part, the AC Mains is converted into unregulated DC and in the second part, this unregulated DC is converted into regulated 5V DC.
➨Initially, a 230V to 12V Step down transformer is taken and its primary of transformer is connected to mains supply. The secondary of the transformer is connected to Bridge rectifier (either a dedicated IC or a combination of 4 1N4007 Diodes can be used).
➨A 1A fuse is placed between the transformer and the bridge rectifier. This will limit the current drawn by the circuit to 1A. The rectified DC from the bridge rectifier is smoothened out with the help of 1000µF Capacitor.
➨So, the output across the 1000µF Capacitor is unregulated 12V DC. This is given as an input to the 7805 Voltage Regulator IC. 7805 IC then converts this to a regulated 5V DC and the output can be obtained at its output terminals.

79xx Voltage Regulator Circuit

The 79xx series voltage regulators are negative voltage regulators, meaning they provide a stable and regulated negative output voltage. One of the commonly used components from the 79xx series is the LM79xx IC, where "xx" denotes the specific output voltage, such as LM7912 for a -12V regulator.

The PIN details are as follows.
Pin-1 (Input) :This is the unregulated DC input voltage that comes from an external power source. For LM7912CT IC from Texas Instruments, Input voltage range from -7V to -30V.
Pin-2 (Ground ) : Common ground reference for the regulator circuit.
Pin-3 (Output ) : This is the regulated DC output voltage. The voltage level is determined by the specific part number. For LM7912, output voltage is -12V.

Let us understand voltage regulator circuit working operation based on 7912 IC.
➨Similar to the 78xx series, the LM79xx series voltage regulators have an internal reference voltage. This reference voltage is typically set at the desired negative output voltage (e.g., -12V for LM7912).
➨The internal circuitry compares the reference voltage with a fraction of the output voltage through a feedback loop. If the output voltage tends to rise above the desired level, the regulator takes corrective action to reduce it.
➨The IC includes a control element, often a pass transistor, that regulates the flow of current from the input to the output based on the feedback mechanism. This control element adjusts itself to maintain a stable negative output voltage.
➨The feedback loop helps in continuously monitoring and adjusting the output voltage to compensate for variations in input voltage and load conditions.
➨Voltage regulators may generate some heat during the regulation process. To dissipate this heat, heat sinks are often used, especially when dealing with higher voltage differentials or larger currents.

Conclusion : In summary, the 78xx series provides positive voltage regulation, ensuring a constant positive output voltage, while the 79xx series provides negative voltage regulation, delivering a stable negative output voltage. Both series operate on similar principles, utilizing an internal reference voltage, feedback loops, and control elements to adjust the current flow and maintain a consistent output voltage despite variations in input voltage or load conditions.

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