Electrolytic capacitor
An electrolytic capacitor delivers high capacitance (10µF to thousands of µF) in a compact polarized can. It buffers power rails against load steps, smooths rectifier ripple, and stores energy for brief holdup.
An aluminum anode is etched to maximize surface area, then anodized to grow a thin oxide layer that acts as the dielectric. A liquid electrolyte makes intimate contact with the etched surface, effectively becoming the second plate. The huge surface area is why the µF/dollar is so high.
In plain terms
A larger, slower bucket than a ceramic. Cheap per microfarad, but you must pour it in the right way — the wrong polarity ruins it.
Why designers use it
- Hold up the input rail of a switching regulator long enough between cycles.
- Smooth the lumpy DC coming out of a bridge rectifier into clean rail-like volts.
- Couple audio between stages where exact value matters less than bulk capacitance.
- Provide energy reserve so a brown-out doesn't reset your microcontroller.
Best for
- Bulk power smoothing
- Audio coupling
- Holdup energy
- Rectifier output
Key specifications
- Range: 1 µF – 47,000 µF
- Voltage: 4 V – 600 V
- Tolerance: ±20 % typical
- ESR: 30 mΩ – 5 Ω
- Ripple current: 20 mA – 5 A
- Lifetime at rated T: 1,000 – 10,000 h (Halves every 10 °C above rating)
When not to use it
- On high-frequency rails — ESR is much higher than ceramics, so they don't bypass GHz noise.
- When polarity could be reversed — they pop or vent if installed backwards.
- In hot environments at full voltage — life halves every 10 °C above rating.
Common mistakes
- Installing it backwards. The stripe on the can marks the negative leg; getting it wrong vents electrolyte (and sometimes flame).
- Picking a 105 °C part rated 1000 h and putting it inside an LED bulb that runs 90 °C continuously — it'll dry out in months.
- Trusting an electrolytic alone to bypass a chip — pair it with ceramics for high-frequency coverage.
Where you will find it
- A defibrillator paddle is built around a high-voltage electrolytic: the paramedic charges it slowly from a 12 V battery over several seconds, storing tens of joules, then dumps the full charge through the patient's chest in about 10 ms to restore heart rhythm — a task no battery alone could achieve at that speed.
- A PC's motherboard uses clusters of electrolytic capacitors near the CPU voltage-regulator module to absorb the massive load steps when the processor wakes from idle — without them, the rail would sag and the system would crash every time a game started.
- An audio amplifier's power supply uses a 10 000 µF electrolytic as its reservoir: the capacitor refills from the mains rectifier 100 times per second and smooths out the lumpy DC into a steady rail so the speaker never hears a 100 Hz hum.
A short history
Aluminum electrolytic capacitors remain useful where substantial capacitance is needed for smoothing and energy storage. Compared with small ceramic parts, their selection places more emphasis on ripple current, ESR, temperature and service life. A familiar cylindrical case is not a promise that two capacitors have the same performance.
Good to know
- Lifetime halves for every 10 °C above rated temperature — a 105 °C/2,000 h cap running at 85 °C lasts ~16,000 h.
- Reverse-bias an aluminium electrolytic and it can vent dramatically; the 'K' or stripe marks the negative leg.
- The 'capacitor plague' of 1999–2007 was traced to an industrial-espionage incident where a stolen electrolyte formula was missing key stabilisers.