--- title: "Decoupling capacitor and transient current" subject: "Instrumentation II" chapter: 7 key: "semester_5/instrumentation_ii:decoupling-capacitor-large" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/decoupling-capacitor-large site: BCT Engineering --- # Decoupling capacitor and transient current Instrumentation II · Chapter 7: Circuit Design - Asked in: 2076 Asoj, 3+2 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/decoupling-capacitor-large - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7 ## Question Explain how decoupling capacitor can be used to suppress the transient current. What effects do you observe when very large decoupling capacitor is connected in your circuit? ## Short answer A decoupling capacitor placed across IC power pins supplies transient current locally, preventing voltage dips from trace inductance. A very large capacitor causes high inrush current at power-up, slow voltage rise, lower self-resonant frequency making it ineffective at high frequencies, and possible resonance with other capacitors. ## Answer #### Suppressing transient current When a digital IC switches, it draws a short pulse of current (di/dt very high) from the supply. The inductance of the supply trace gives V = L·di/dt, a voltage dip or spike on the rails, and a ground bounce. A **decoupling capacitor** (e.g. 0.1 uF) placed right across the IC's power and ground pins stores charge. During a transient it supplies the current locally (I = C·dV/dt), so the demand on the distant supply is not sudden; the voltage across the capacitor changes little (ΔV = I·Δt/C). The capacitor also gives a low-impedance path to ground for the high-frequency noise present on the supply line. Hence supply ripple and ground bounce are reduced. ```text Vcc ----+------- IC Vcc | === C (0.1 uF, close to pin) | GND ----+------- IC GND ``` #### Effects of a very large decoupling capacitor 1. **High inrush current at power-up:** the uncharged capacitor behaves as a short circuit, so I = C·dV/dt becomes large, stressing the supply, fuse and connectors, and may trigger the supply's protection. 2. **Slow rise of the supply voltage** (longer start-up), which may cause improper reset. 3. **Lower self-resonant frequency:** a large capacitor has higher parasitic inductance (ESL), so it is ineffective at high frequency (inductive above its resonance), and does not remove fast noise. 4. **Possible resonance** (anti-resonance) with other capacitors and supply instability with regulators. 5. Larger size, higher cost, more leakage current and board area. Therefore, a combination of a small capacitor (0.1 uF / 0.01 uF) for high frequency and a moderate bulk capacitor (10 uF) for low frequency is used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a decoupling capacitor reduce supply dips during switching? A. It supplies the transient current locally (I = C·dV/dt), so the distant supply sees no sudden demand B. It increases the supply inductance C. It stops the IC drawing current D. It converts AC to DC Answer: A. The stored charge feeds the pulse, with only a small voltage change (dV = I·dt/C). (Key point: Transient suppression) 2. Why does a very large decoupling capacitor cause high inrush current at power-up? A. It has high resistance B. An uncharged capacitor acts like a short circuit, so I = C·dV/dt is large C. It raises the supply voltage D. It disconnects the supply Answer: B. Large C with fast dV/dt at start-up draws a big current, stressing the supply and fuse. (Key point: Inrush current) 3. Why is a very large capacitor ineffective against fast noise? A. It has lower ESL B. It has no capacitance at high frequency C. Its higher parasitic inductance gives low self-resonant frequency, so it is inductive above resonance D. It has too little leakage Answer: C. Above self-resonance the part behaves as an inductor, so it cannot shunt fast noise. (Key point: Self-resonance) 4. What combination is recommended in practice? A. One huge capacitor only B. Small (0.1 / 0.01 uF) for high frequency plus moderate bulk (10 uF) for low frequency C. No capacitors D. Only electrolytic capacitors Answer: B. Different values cover different frequency ranges without the drawbacks of one very large part. (Key point: Practical combination) ## Related in Instrumentation II - [Working principle of decoupling capacitor](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adecoupling-capacitor) — Instrumentation II, chapter 7 - [Ground bounce, decoupling and crosstalk](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-decoupling-crosstalk) — Instrumentation II, chapter 7 ## Related in other subjects - [Classical: inductor current and capacitor voltage, 100 V](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Aclassical-inductor-current-capacitor-voltage-100v) — Electrical Circuit Theory, chapter 3 - [Laplace: switch a to b, capacitor current and voltage](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Alaplace-switch-a-to-b-two) — Electrical Circuit Theory, chapter 4 - [Laplace: capacitor voltage and current, switch a/b](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Alaplace-switch-a-b-v0-r1) — Electrical Circuit Theory, chapter 4 - [Large starting current; 3-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-motor-large-starting-current-3point) — Electrical Machine, chapter 4