What capacitors do in a driver
LED drivers contain several component types, each performing a distinct function. In a well-designed driver, electrolytic capacitors are the components that carry out energy storage for smoothing the DC output, power factor correction, and both input and output filtering. They are essential to the circuit and cannot be eliminated. In practice, they are also nearly always the first component type to reach the end of its useful life — not because of a design fault, but because of a fundamental physical property of their construction.
The failure mechanism: electrolyte evaporation
An electrolytic capacitor consists of aluminium foil electrodes separated by a liquid or gel electrolyte, sealed inside an aluminium can. The seal is not perfectly impermeable — over time, a small quantity of electrolyte slowly evaporates through it. As the electrolyte volume decreases, capacitance falls. When capacitance drops below the minimum required by the circuit, the driver either fails outright or produces ripple current that degrades the LED string prematurely.
This evaporation follows the Arrhenius relationship between temperature and reaction rate — a well-established principle in materials science. For electrolytic capacitors, the practical consequence is the industry rule of thumb known as the 10°C halving rule: every 10°C decrease in operating temperature approximately doubles capacitor service life.
The Arrhenius rule in practice
A capacitor data sheet shows a rated life at a maximum operating temperature — for example, 10,000 hours at 105°C. This is the maximum rating, not the target operating point. Applying the 10°C halving rule, the expected service life at lower operating temperatures is approximately:
- At 95°C: ~20,000 h
- At 85°C: ~40,000 h
- At 75°C: ~80,000 h
- At 65°C: ~160,000 h
- At 55°C: ~320,000 h
The implication is direct: a driver claiming 100,000-hour service life requires its capacitors to be operating well below their maximum rated temperature to achieve that figure in service. The data sheet life figure and the capacitor operating temperature in a specific luminaire installation are both necessary pieces of information — neither alone is sufficient.
In practice
- A driver's stated service life (often 50,000–100,000 hours) almost always applies at a temperature specified in the data sheet — typically a stated ambient temperature or a measurement point on the driver housing. Actual operating temperature inside a luminaire may be higher.
- Rule of thumb for electrolytic capacitors: service life halves approximately for every 10°C increase in operating temperature above the design point.
- MTBF and rated life (L value at a stated temperature) are two different metrics. MTBF describes the failure rate during the random-failure phase of a component's life and does not capture wear-out — a high MTBF figure therefore says nothing about how quickly a capacitor dries out.
The thermal chain: from ambient air to capacitor
The operating temperature of a capacitor is not determined by the ambient air temperature alone. It is the sum of several contributions: ambient air temperature, heat conducted from the LED module into the housing, heat generated by the driver circuit itself, and the thermal resistance of each interface in the heat path. Industrial environments with nearby process heat sources raise the baseline above what a standard data sheet assumes. Urban settings with heat radiation from road surfaces and buildings can produce higher night-time temperatures than open rural sites. A luminaire installed with restricted airflow around the driver compartment — tightly enclosed in a column top, for example — will run hotter than an identical luminaire with better thermal management.
Two identical drivers from the same production batch can therefore reach the end of their useful capacitance at completely different points in time, depending on where and how they have been installed — not just how far from the coast they happen to stand.
Overdriving: a compounding factor
Running an LED module at a higher drive current than its nominal design point increases LED junction temperature and driver loading simultaneously. Both effects raise the operating temperatures of the components that age fastest. A luminaire designed with conservative drive currents — operating the LED at a margin below the maximum rated current — produces somewhat fewer lumens per unit but delivers both the LED module and the driver at lower temperatures, extending the service life of the capacitors and the LED string in parallel.
VALDUR is designed with drive currents that balance luminous efficacy with thermal margin. The LED and driver operate cooler than at maximum rated current, extending both LED and driver service life.
What this means for procurement
Because electrolytic capacitor ageing is a predictable wear-out mechanism rather than a random failure, it can in principle be tracked over time — if the luminaire provides operating hours and fault reporting before total failure occurs. A single MTBF figure on a data sheet does not provide that information, and should not be read as a service life commitment.
When reviewing a driver specification, the following questions give more useful information than the headline hour figure alone:
- What capacitor technology is used in the driver — electrolytic, film, or ceramic?
- At what temperature is the stated service life specified, and how does that compare with actual operating temperature inside this luminaire?
- How does the luminaire's thermal management — housing design, driver placement, and heat path from the LED module — affect driver operating temperature?
- Does the driver provide operating hours and fault diagnostics that allow ageing to be detected before total failure, rather than relying on a single MTBF figure?
Next level of understanding
The capacitor's real service life rarely appears in the figure a supplier quotes as the luminaire's service life.
The 100,000 hours on a data sheet is almost always a projection of when the LED module's lumen output will fall to a given level — not a measure of the driver or its capacitors. A driver ageing exactly as this article describes can sit behind a headline figure that says nothing about that specific component.
Service Life & Reliability
Why street luminaires fail
What the hour figure on a data sheet actually measures — and why the driver often sets the limit before the LED module.
Summary
A driver's stated service life is not a universal guarantee — it applies at a specified temperature, and the electrolytic capacitors typically found inside dry out faster the hotter they operate. The 10°C halving rule makes operating temperature one of the most decisive variables for how long the driver actually lasts in a given installation, alongside the component selection itself.
MTBF does not describe this wear-out process, and a high MTBF figure is no guarantee against early capacitance loss. Film and ceramic capacitors avoid the drying-out mechanism but are not automatically superior in all respects — which technology a specific driver uses should be asked directly, not assumed from a generalised service life claim.
For procurement, questions about capacitor technology, the temperature at which service life is specified, and the availability of operating hour diagnostics provide a more useful basis for comparison than a single hour figure or MTBF value on a data sheet.