Capacitor aging in power supplies is a gradual chemical process, not a sudden event, and the visible bulge everybody looks for is the last chapter rather than the first. Electrical properties drift for years beforehand with no outward sign at all. A supply can be measurably worse than it was when new while looking perfectly healthy inside.
What is actually changing inside the component
An aluminum electrolytic capacitor stores charge across an oxide layer formed on a wound foil, with a liquid or gel electrolyte carrying ions to that surface. The electrolyte is the part that ages. Over time it escapes slowly through the rubber seal at the base and its chemistry changes, so there is progressively less of it doing progressively less good work. Two measurable consequences follow. Capacitance falls, because less active electrolyte means less effective surface area. Equivalent series resistance rises, because the remaining electrolyte conducts ions less freely. Neither change announces itself. The component looks identical, the machine still boots, and the owner has no reason to suspect anything until a symptom finally appears. Solid polymer capacitors use a conductive polymer instead of a liquid electrolyte and age far more slowly, which is why better supplies use them in the positions that run hottest.
Why heat is the dominant variable
The chemical processes involved speed up markedly with temperature, which is why capacitor manufacturers quote an expected life at a stated temperature and why that figure improves substantially when the part runs cooler. This has a blunt practical implication: a supply operating in a hot case, in a hot room, with a clogged intake filter, ages its capacitors far faster than an identical supply in a cool, clean machine. It also explains why the capacitors nearest the heatsinks and rectifiers usually fail first while those at the cool end of the board remain fine. Ripple current adds its own self-heating on top of ambient, so a capacitor working hard in a heavily loaded supply is warmer than its surroundings even before the room is considered. Ripple current rating is the specification that describes how much heating a capacitor can tolerate from its own working current, and undersizing it is a common cost saving in cheap units.
The symptoms that show up before anything looks wrong
Rising series resistance means the capacitor filters less effectively, so ripple on the output rails grows. Falling capacitance means the supply’s ability to ride through a brief mains dip shrinks, so the machine becomes more likely to reset during a momentary sag that it previously ignored. In the input stage, a degraded bulk capacitor can cause the supply to behave oddly at startup, particularly under a heavy initial load. Owners typically report these as unrelated software or memory problems: random reboots, occasional failure to start, instability that appears only under load or only in summer. The seasonal pattern is a useful clue, because a fault that worsens in warm weather often has a thermal chemistry behind it. Hold-up behavior degrades gradually rather than failing outright, so a machine that used to survive a flicker and now reboots has given you a measurable clue.
What visible inspection can and cannot tell you
Bulged tops, a crusted deposit around the vent score, or brown residue at the base of a capacitor are unambiguous signs of failure and mean the unit should be retired. The absence of those signs means nothing. Many capacitors decline steadily to uselessness without ever venting, particularly smaller ones, and solid polymer types can fail with no visual change whatsoever. Proper assessment needs measurement of capacitance and series resistance at the right frequency, which means removing the part or using dedicated equipment, and it means opening a supply that holds a dangerous stored charge after being unplugged. This is service work, not an owner task. Solid polymer parts in particular can fail with no visual change at all, which makes appearance an especially poor test in a modern supply.
How to get more years out of a supply
Since heat drives the process, everything that lowers operating temperature extends life. Keep intake filters clean, especially the one under a bottom-mounted supply. Give the machine room to breathe rather than tucking it into a sealed cabinet. Avoid running the unit permanently close to its rating, since load produces both internal heating and higher ripple current in the output capacitors. If a machine runs continuously in a warm room, expect a shorter service life and plan for replacement rather than waiting for a failure that takes data with it. Long manufacturer warranties usually reflect better capacitor selection and are a reasonable proxy for expected life. Manufacturers sometimes specify a shorter warranty on units intended for warm or continuous duty, and that distinction in the product line is worth reading.
What to check before you act
- Track whether instability worsens in warm weather or under load.
- Clean every intake filter and clear space around the machine.
- Retire any supply showing bulging, crust, or residue immediately.
- Treat a long warranty as a signal about expected component life.
Common questions
How long does a computer power supply last?
It depends far more on operating temperature and hours than on age alone. A cool, lightly loaded unit can outlive a hot, heavily loaded one several times over.
Can I just replace the capacitors myself?
Stored energy inside a supply can be lethal after unplugging, and matching ripple current and temperature ratings matters. This is qualified service work.
The practical takeaway
Do not wait for a bulge. Treat rising instability in a machine that has run hot for years as a plausible supply symptom, keep the unit cool and clean to slow the chemistry down, and replace rather than repair when the evidence points that way.
References and further reading
- CPSC: recognizing failing electrical equipment at home
- NFPA: electrical equipment and fire safety research
- Department of Energy: computer equipment operating guidance
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
