Capacitor (aluminium electrolytic)
Capacitor
A polarised capacitor that packs a lot of capacitance into a small can, using a thin oxide layer as the dielectric. The default choice for power-supply smoothing and bulk energy storage.
Schematic symbol
Capacitor (polarised)
How capacitor (aluminium electrolytic) appears in a circuit diagram.
Types & variants
Key specs
Capacitance
Farad (F)Usually 1µF to 10,000µF+. Large compared with ceramics.
Voltage rating
VMax DC working voltage. Never exceed it; pick at least 1.5-2x your rail.
ESR
ΩEquivalent series resistance. Lower ESR = less heating and better filtering in SMPS.
Ripple current
AHow much AC ripple it can carry before overheating.
Lifetime / temp
hours @ °CRated e.g. 2000h @ 105°C; life roughly doubles per 10°C cooler.
Markings
Value and voltage printed on the can. A stripe with '-' marks the NEGATIVE lead; the longer lead is positive.
Standard values
Common: 1, 10, 22, 47, 100, 220, 470, 1000, 2200, 4700µF at 6.3/10/16/25/35/50V.
How to choose
Smoothing: bigger µF = less ripple. Match/exceed the rail voltage with margin. For switching supplies use low-ESR 105°C types. Watch ripple-current rating.
Pinout & package
Radial — the stripe marks the negative leg
Check first: Longer leg is positive; the printed stripe (and shorter leg) is negative. Reversing it can make the cap fail — sometimes violently.
2 leads, POLARISED. Radial cans most common; longer lead = +, stripe side = -. Get polarity right.
Example circuits
- Reservoir/smoothing cap after a bridge rectifier
- Bulk decoupling on a power rail
- Audio coupling cap (large value)
Common failures
Dries out with age/heat (capacitance drops, ESR rises); bulged top or vented bottom; explodes if reverse-connected or over-volted.
How to test
Bulging top = replace. Use an ESR meter in-circuit, or capacitance mode out of circuit. High ESR/low value = bad.
Substitutes
Equal-or-higher voltage and equal-or-lower ESR; same/slightly higher capacitance. 105°C can replace 85°C.