- #61
Sizing Decoupling Capacitors for 240AC to 5VDC Conversion
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Discussion Overview
The discussion revolves around the sizing of decoupling capacitors in a circuit converting 240Vac to 5VDC using a step-down transformer, bridge rectifier, and a voltage regulator. Participants explore various aspects of capacitor selection, including input and output capacitor values, load characteristics, and the implications of different configurations.
Discussion Character
- Technical explanation
- Debate/contested
- Mathematical reasoning
- Experimental/applied
Main Points Raised
- One participant inquires about the correct sizing of decoupling capacitors for their circuit setup.
- Another participant suggests that C2 should be 50 to 100 times larger and C1 should be 10 times larger than initially sized.
- A rule of thumb is provided for the smoothing capacitor, suggesting a minimum of 1000uF for the input of the 7805 regulator based on expected current.
- Discussion includes the importance of considering load characteristics when sizing capacitors, with a request for clarification on maximum expected load.
- Concerns are raised about reverse biasing the 7805 regulator and the necessity of a protective diode across it.
- One participant explains the need for a diode to prevent reverse bias due to charge retention in output capacitors.
- Another participant discusses the voltage headroom and droop calculations for determining minimum capacitance values.
- There is a suggestion that the chosen bridge rectifier may not be suitable for the expected load current.
- Participants discuss the role of high-frequency capacitors in conjunction with larger electrolytic capacitors for transient response.
- One participant shares simulation results indicating that a 500uF capacitor may be insufficient for maintaining voltage under load.
- Clarifications are sought regarding the frequency of voltage cycles in relation to the rectifier's operation.
- Questions arise about the implications of grounding configurations in the circuit.
- Further inquiries are made about sizing output capacitors and considerations for different load types, including resistive and inductive loads.
Areas of Agreement / Disagreement
Participants express various opinions on capacitor sizing, with no clear consensus on the optimal values. Multiple competing views on the necessary configurations and considerations remain unresolved.
Contextual Notes
Participants mention limitations in load specifications and the need for datasheets to determine component properties. There are also unresolved questions about the impact of different load types on capacitor sizing.
Who May Find This Useful
Electronics enthusiasts, engineers, and hobbyists involved in power supply design, particularly those interested in capacitor selection and circuit stability in DC voltage regulation applications.
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