Short Answer: The input capacitor stabilizes the supply (especially with long wires). The output capacitor prevents oscillation and improves transient response.
Detailed: Linear regulators like the 7805 are feedback amplifiers. Without proper capacitors, they can oscillate (output jitters) or respond poorly to sudden load changes.
| Capacitor | Typical value | Why it's needed |
|---|---|---|
| Input | 0.33–1 ยตF (ceramic) + 100–470 ยตF (electrolytic) | Prevents voltage dips from long supply wires; filters input ripple |
| Output | 0.1–1 ยตF (ceramic) + 10–100 ยตF (electrolytic) | Prevents oscillation; supplies transient current when load changes suddenly |
What happens if you skip them:
No input cap: Regulator may oscillate or fail to start
No output cap: Output voltage may "bounce" under load; some regulators (e.g., LM1117) require output cap for stability
Wrong capacitor type: Using electrolytic where ceramic is needed can cause high-ESR oscillation
Pro tip: Place a 0.1 ยตF ceramic capacitor as close as possible to both the input and output pins of any IC. This is called decoupling and solves 90% of noise problems.
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