Creating a stable 5V DC supply out of 6 V (RMS) AC

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SUMMARY

The discussion centers on creating a stable 5V DC supply from a 6V RMS AC transformer using an LM7805 voltage regulator. The participants emphasize the importance of selecting an appropriate transformer voltage, with recommendations to use at least a 9V transformer to ensure sufficient input voltage for the LM7805, which requires a minimum of 7V. They also highlight the necessity of a large reservoir capacitor, suggesting a minimum of 4700 µF to maintain voltage stability under load. Additionally, the use of Schottky diodes is recommended to reduce voltage drop in the rectification process.

PREREQUISITES
  • Understanding of AC to DC conversion principles
  • Familiarity with LM7805 voltage regulator specifications
  • Knowledge of capacitor sizing for power supply applications
  • Basic circuit design skills, including schematic representation
NEXT STEPS
  • Research the specifications and applications of LM7805 voltage regulators
  • Learn about Schottky diodes and their advantages in power supply circuits
  • Study capacitor sizing calculations for power supply stability
  • Explore the use of DC-DC buck converters for efficient voltage regulation
USEFUL FOR

Electronics hobbyists, circuit designers, and engineers working on power supply design for microcontrollers, particularly those using ATmega 8A microcontrollers.

  • #61
First of all, I would like to thank @Borek who, in post #2, mentioned the DSO138 oscilloscope. The device has really been very helpful.

Next, I have performed the experiment, based on the circuit I had posted here. These are the results from the oscilloscope:

WhatsApp Image 2021-05-17 at 2.22.03 PM.jpeg

Output of full wave rectifier bridge
WhatsApp Image 2021-05-17 at 2.22.03 PM (1).jpeg

After passing the output through the voltage regulator
with necessary filter capacitors

I can see that the average voltage is 4.43 V, the maximum being 4.83 V. 5 V is not reached. Is this an accuracy error of the oscilloscope?
 
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  • #62
Does the first picture look like full wave rectification? Check your diode orientation.
 
  • #63
pbuk said:
Does the first picture look like full wave rectification?
Yeah, I have seen that. Trying to figure out the issue.
 
  • #64
Update: Fixed it.
WhatsApp Image 2021-05-17 at 4.44.28 PM.jpeg
WhatsApp Image 2021-05-17 at 4.44.28 PM (1).jpeg
 
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