Capacitors

In one line

A capacitor stores energy in the electric field between two conductive plates — it charges up, holds, and releases. It can dump a big burst of energy fast, or smooth a noisy signal.

How it works

Apply a voltage and electrons pile up on one plate, leaving the other plate positive. No current flows through a capacitor at steady state — it just holds the charge. Two properties matter:

• Capacitance (farads, F) — how much charge it holds per volt. Bigger plates, closer together, store more.
• It resists changes in voltage. That makes it perfect for smoothing (filling in the dips of a wobbly supply) and filtering (blocking slow drift, passing fast signals — or vice-versa).

Where it shows up in digital health

• Defibrillators charge a large capacitor over a few seconds, then release ~150–360 joules in milliseconds — far faster than a battery could. The "whine" before the shock is the capacitor charging.
• ECG / EEG front-ends use capacitors in filters to strip mains hum (50/60 Hz) and baseline wander so the real waveform survives.
• Decoupling capacitors sit next to every chip on a device's board, acting as tiny local reservoirs that steady the supply when the chip suddenly draws current.
• Pacing and sensing circuits use them for precise timing.

Watch for

Electrolytic capacitors are polarised (a + and − leg) and fail — sometimes dramatically — if reversed. In a medical device, a degraded capacitor is a classic cause of drifting measurements and intermittent resets.

Key takeaways

• A capacitor stores energy in an electric field — charges, holds, releases; it resists changes in voltage.
• It can dump a huge burst fast (defibrillator) or smooth/filter a signal (ECG front-end, decoupling).
• Electrolytic types are polarised — reverse them and they fail, sometimes violently.
• A degraded capacitor is a classic source of drift and intermittent resets in devices.