An LED bulb makes light by passing current through a semiconductor junction, which converts some of the electrical energy directly into photons instead of heating a filament until it glows. That is the reason it uses far less power for the same light and why it runs cooler than the bulb it replaced. But the bulb is not only a light source: it also contains a power supply, and that power supply is what usually fails first.
From mains AC to light
The electricity arriving at the socket is a high alternating voltage, and the diode inside the bulb needs a low direct current. Between the two there is a driver: a small circuit that rectifies the mains, converts the voltage, and holds the current steady. The light itself is produced at a junction inside a semiconductor chip, and the colour is set by the material and by a phosphor coating that shifts part of the blue emission into other wavelengths to make white light.
The comparison that helps is a water wheel driven through a gearbox. The wheel is what does the visible work, and the gearbox is what makes the input usable. Remove the gearbox and the wheel will not turn properly; damage the gearbox and the wheel stops, even though it is still perfectly sound. In an LED bulb the driver is the gearbox, and it is the part that most often ends the bulb’s life.
Why LEDs need a driver
A light-emitting diode is not like a filament, which will accept whatever current the voltage produces. It has a threshold voltage and then a very steep relationship between voltage and current, so a small change in voltage produces a large change in current. Left to itself that would mean the light either barely glows or destroys itself, and the fix is to control the current rather than the voltage. That is what a driver does.
The other job the driver performs is converting the mains voltage to something the light can use, which means rectification, smoothing and a conversion stage. Every one of those steps costs a little energy, so the driver’s efficiency is part of the bulb’s efficiency. This is why the same light output can be produced by bulbs with different power ratings: the difference is often in the driver rather than in the chip. The device-level choices behind that circuit are discussed in LED driver rectifier selection.
Heat sinks you cannot see
An LED bulb still produces heat; it just does not produce it at the filament. Most of the waste appears in the driver and in the chip, and it has to leave through the body of the bulb. That is the real job of the metal fins and the heavy base that people notice, and it is also why the shape of a bulb is not only decorative: a bulb designed to run in a confined fitting is not the same product as one designed for open air.
There is a technical reason this matters more than it seems. The lifetime of the components inside a bulb depends strongly on temperature, and the relationship is not gentle: as a rule of thumb used in the industry, the life of an electrolytic capacitor roughly halves for every ten degrees Celsius of additional operating temperature. A bulb installed in an insulated ceiling fitting runs hotter than one in a lamp with air around it, and its components age accordingly.
Two failure modes: chip versus driver
The chip itself degrades slowly. Its output falls over many thousands of hours, and the standard way of describing this is the point at which the light has fallen to a defined fraction of its initial output. That is a gradual process, and a bulb that is merely dimmer after years of use is showing it.
The driver fails suddenly. An electrolytic capacitor dries out, a solder joint cracks under thermal cycling, or a small component fails because it has been run hot for a long time. The result is a bulb that flickers, strobes, buzzes or simply goes dark while the light-producing chip is still intact. This is the explanation for the most common complaint about the category: a product rated for a very long life that stopped working in two years, in a fitting that cooked it.
What the labels really promise
Three label figures are worth reading carefully. The power in watts tells you what the bulb draws, which is what you pay for. The light output, usually given in lumens, tells you how much light it produces, which is what you actually want. And the rated life in hours describes the point at which a defined proportion of bulbs would still be working under defined test conditions, which is not a promise that every individual bulb will reach that figure.
There is also a colour figure, which describes how warm or cool the light appears, and a colour-rendering figure, which describes how faithfully colours appear under it. Neither affects the life of the bulb, and both matter more to satisfaction than the wattage. Comparing bulbs on lumens per watt is the fair way to compare efficiency, since it removes the question of how bright the bulb is.
| Figure | What it describes | Effect on life |
|---|---|---|
| Power in watts | How much electricity it draws | Indirectly, through heat |
| Light output | How much light it produces | None |
| Rated life in hours | A test-based expectation, not a guarantee | It is the outcome being described |
| Colour appearance | Warm or cool white | None |
| Colour rendering | How faithfully colours appear | None, but it varies with the phosphor |
Dimming and flicker simply
A dimmer works by interrupting the mains waveform, which is exactly what a driver finds hardest to cope with. That is why some bulbs buzz or flicker on a dimmer while others behave, and why the compatibility between a specific dimmer and a specific bulb is a real question rather than a marketing detail. A bulb that is not designed for dimming may flicker, drop out at low settings or fail early.
Flicker is a separate issue and worth naming because people notice it without knowing what it is. If the driver does not smooth its output well enough, the light pulses at twice the mains frequency, which some people perceive as a shimmer and which is more visible in peripheral vision. Good drivers smooth it out; cheap ones sometimes do not, and the effect is worst on camera.
How to make bulbs last longer
Four habits extend the life of a bulb, and all four are about heat. Choose a bulb whose body can lose heat in the fitting you have, because a bulb in an enclosed shade runs hotter than one in the open. Match the dimmer to the bulb if you use one. Avoid fittings where the bulb sits directly above a heat source. And buy the light output you need rather than the highest available, because a bulb run at full output for hours every day ages faster than one that only lights a small area.
FAQ
How long do LED bulbs actually last?
The rated figure describes the point at which a defined fraction of tested bulbs still worked, under controlled conditions, and it is not a promise for one bulb in one fitting. In practice the driver tends to fail before the light-producing chip, and its life depends on temperature, so the same bulb can last many years in an open fitting and far less in an enclosed one.
What are the downsides of LED bulbs?
Three are real and worth knowing. Some bulbs are sensitive to the way a dimmer interrupts the mains, which can cause flicker or early failure. The light quality varies, so colour rendering is worth checking rather than assuming. And the electronics inside mean a bulb can fail suddenly, which is a different experience from the gradual dimming of an older technology.
Can I replace ordinary bulbs with LED bulbs?
In most cases yes, provided the fitting provides enough space for the bulb’s base and enough air around it, and provided the light output and colour are suitable for the room. The main compatibility question is dimming, which depends on the specific dimmer as well as the bulb.
Do LED lights need to be plugged in?
Screw-in and bayonet LED bulbs fit the existing socket and take mains power through it, so nothing new is plugged in. Strip lights, panels and some fixtures use a separate driver box, which does have its own connection. The distinction is between a bulb with the driver built into its base and a fixture with the driver outside it.
Why does my LED bulb flicker or buzz?
Usually because a dimmer is interrupting the mains supply in a way the driver cannot follow, or because the driver is under load at a low dimming setting. It can also be a sign of a failing driver, particularly if the flicker started suddenly on a circuit that has not changed. The first check is whether the bulb is rated for dimming and whether the dimmer lists the bulb type.
What to do next
Compare the bulb you are considering on lumens per watt and on where the heat will go, and treat the rated life as a test expectation rather than a guarantee. The device-level side of the driver is in LED driver rectifier selection and how lighting drivers fail first, and the thermal principle behind both is in why diodes get hot.
Background is available in our overviews of light-emitting diodes, switched-mode supplies and electronic components, with energy programmes from the US Department of Energy, electrical standards from IEC and assembly expectations from IPC. Efficiency and safety background is published by NIST, ISO and the Semiconductor Industry Association.
Tell Good-Ark the input voltage, the output current and the space inside the fixture, and we will point you at the rectifiers, switches and protection devices that suit the design.
Email sales@goodark.com, or start from the application design centre.
This article is published by Good-Ark, a manufacturer of discrete semiconductor devices used in lighting drivers. It quotes no efficacy or lifetime figure as a promise for any product, and the temperature rule of thumb is given as an engineering approximation rather than a specification.