Why LED Bulbs Fail At The Driver, Not The Diode

An LED is rated for tens of thousands of hours, yet replacement bulbs fail after a fraction of that. The diodes are usually fine when the bulb is thrown away.

A diode cannot run on mains electricity

Light-emitting diodes need a low, tightly controlled direct current. Mains supply is alternating, at a voltage far too high, and varies with load elsewhere in the building.

Every bulb therefore contains a converter that rectifies, steps down and regulates the supply. This driver is a small power supply squeezed into the base.

Its job is to hold current constant rather than voltage, because an LED's current rises steeply with small voltage changes. A few per cent too much shortens life dramatically.

The driver lives in the worst possible location

The electronics sit in the enclosed base, directly beneath the diodes that are producing heat, inside a sealed housing with almost no airflow.

Capacitors are the component least tolerant of that. Electrolytic types dry out steadily, and the rate roughly doubles for every ten degrees of additional temperature.

As capacitance falls, the output becomes unstable. The bulb flickers, dims or cuts out intermittently, all symptoms of a supply that can no longer smooth its own output.

Enclosed fittings shorten life further

A bulb in an open pendant sheds heat to the room. The same bulb in a sealed downlight or a glass shade recirculates its own heat back into the base.

Manufacturers rate bulbs for enclosed use separately, because the internal temperature can differ by tens of degrees between the two situations.

This explains why one room burns through bulbs and another does not. The fitting is doing more to determine lifespan than the bulb.

Cost pressure targets the driver first

The diodes are a small part of the cost. Reducing the price of a bulb means simplifying the driver, and the usual simplification is a cruder circuit with fewer components.

The cheapest designs pass a chopped version of the mains waveform straight through, which is why some bulbs visibly flicker on camera or at the edge of vision.

Better drivers cost a little more and run cooler. Because the packaging rarely describes them, price and stated lifespan are weak proxies for what is inside.

The diodes do degrade, just slowly

LEDs do not usually fail abruptly. They dim gradually as the semiconductor and the phosphor coating age, which is why ratings describe the hours until a stated fraction of brightness remains.

Heat accelerates that too, so a bulb driven hard in a small package fades faster than the same diode running gently.

Fixtures with integrated LEDs and separate drivers avoid much of this, at the cost of a light you cannot fix by unscrewing anything.