Capacitors in practice
This page compiles the kickboard embedded lecture on capacitor uses, covering four practical applications: bulk electrolytic voltage stabilization, decoupling/bypass, parallel combination, and RC filtering. All four derive from a single property — impedance drops as frequency rises — and the unifying view is that a capacitor only works as a filter in the region left of its resonance point. Two practical fundamentals dominate: place the capacitor as close as possible to the IC power pin, and start with 0.1µF (100nF) as the default value, then tune after measurement. Mixing different capacitance values in parallel can introduce multiple anti-resonance points and actually worsen the response.
Usage map — everything stems from the frequency characteristic
- A capacitor's impedance is high at low frequencies and falls as frequency rises, and this single property is the common root of all four practical uses.
- However, past the resonance point a capacitor turns inductive (behaving like an inductor), so only the region left of resonance is usable as a filter.
Bulk electrolytic vs decoupling/bypass
- The kickboard board's DC battery input uses two 470µF electrolytic capacitors (C56, C57) in parallel for a total of 940µF, since parallel capacitance simply adds up.
- When the motor accelerates hard and draws a current surge, the battery alone is too slow and the voltage sags; the bulk input electrolytic instantly dumps its stored energy to hold the voltage steady.
- Decoupling places a capacitor right next to the IC as a local energy tank that instantly supplies transient current; placed far away it is, in the lecture's words, 'as if it were not there at all'.
| Decoupling | Bypass | |
|---|---|---|
| Purpose | Stable power to the IC | Divert supply noise to GND |
| Noise source | External / supply | IC internal switching harmonics |
| In practice | MLCC, nearest the IC power pin | Treated the same |
Parallel combination and value selection
- Combining equal values in parallel (e.g. 22µF×3 = 66µF) sharply lowers the impedance at the resonance point, effective when targeting a specific band.
- Mixing different values (22µF + 0.1µF + 0.01µF) lowers impedance across the band, but multiple anti-resonances arise between resonance points and the response can actually worsen.
- Where measurement is impractical, the default is 0.1µF (100nF), then tune if noise proves severe — 100nF filters a broad mid-frequency band and is a sound starting point.
RC filter — fc = 1/(2πRC)
- An LPF puts R in series and C to GND, passing below fc; an HPF puts C in series and R to GND, passing above fc.
- The cutoff fc is where output is 0.707× input (-3dB), and in a first-order system the phase lag there is -45°.
- For example, with R = 1kΩ and C = 0.1µF, fc ≈ 1.59kHz: an LPF passes below it and an HPF above it.
- cutoff frequency (Hz)
- series/GND resistance (Ω)
- capacitance (F)
Do not blindly mix different capacitance values in parallel — multiple anti-resonance points appear between resonance points and can worsen the response. Decoupling/bypass capacitors must be placed as close as possible to the IC power pin; placed far away they are 'as if not there at all'. The raw transcript contains many STT mis-recognitions, so consult the correction table first when referencing it.