Plainwire / Guides / Desk lamp LED driver
Desk lamp LED driver
Cheap LED desk lamps fail in the driver, not in the diodes. The emitters are rated for tens of thousands of hours and are running well below that rating; the electrolytic capacitor next to them is rated for two thousand hours at 105 °C and is sitting in a sealed aluminium tube with a warm heatsink.
Reading the symptom
| Symptom | Most likely cause |
|---|---|
| Visible flicker at low brightness only | Output capacitor losing capacitance; ripple now visible |
| Flicker at all brightnesses, worse when cold | Same capacitor, further gone |
| One segment or strip dark, rest normal | An open emitter in a series string, not the driver |
| Nothing at all, no glow, no click | Input side: fuse, bridge, or the switch itself |
| Comes on, dims over about a minute, cuts out | Thermal shutdown — check the heatsink path before blaming electronics |
The bulk capacitor on the mains side of one of these holds enough energy to hurt several minutes after unplugging. Discharge it deliberately: unplug, wait sixty seconds, then bridge its terminals through a 1 kΩ resistor for five seconds and confirm with a meter that it reads under two volts. Do not short it with a screwdriver — that pits the terminals, welds the tip and startles you into dropping the board.
Getting the driver out
On a lamp with a weighted base the driver is almost always in the base rather than in the head, because that is where the mass and the space are. Look for four screws under the felt pad. On a lamp with an in-line brick, the brick is the driver and it usually splits along a glue seam that yields to a thin blade and a lot of small cuts.
Photograph the wiring before disconnecting. Output polarity matters and the colours are not standardised across manufacturers; there are lamps with a black positive.
Identifying the failed capacitor
You are looking for the electrolytic on the low-voltage output side — usually the one physically nearest the output wires, and usually the smallest. Three checks, in order of how much you can trust them:
- Look at the vent. The scored cross or K on the top should be flat. Domed, split or crusted means the capacitor is finished and there is nothing more to establish.
- Look underneath for residue. A dark ring on the board around the can is dried electrolyte and is conclusive.
- Measure it out of circuit. In circuit readings on a driver are meaningless because of everything in parallel. Lift one leg. A capacitor reading more than twenty per cent below its marked value is out of spec.
A cheap capacitance meter is worth owning for this and nothing else. The important number it will not give you is equivalent series resistance, which is what actually kills these; a capacitor can measure the correct capacitance and still be useless. If the physical evidence says replace it, replace it regardless of what the meter says.
Choosing the replacement
| Specification | Rule |
|---|---|
| Capacitance | Match exactly. This is a timing and filtering component, not a bulk reservoir. |
| Voltage rating | Match or exceed. Going higher is safe if it physically fits. |
| Temperature rating | Buy 105 °C even if the original was 85 °C. It is the whole reason the first one died. |
| Lifetime rating | Prefer a 5,000 or 10,000 hour part. The price difference is pennies. |
| Physical size | Measure the diameter and height. The lid has to close. |
Fitting
Note the polarity stripe before the old one comes out, mark the board with a pen, and fit the new one the same way round. An electrolytic installed backwards will run for a few minutes and then vent, loudly, in your face. Clip the legs before soldering so there is no leverage on the fresh joints.
The one that is not worth repairing
Where the emitters are soldered directly to an aluminium-core board and the driver is potted in epoxy in the same housing, there is no repair. You cannot get the capacitor out of the potting without destroying the board around it. Those lamps are a sealed unit whatever the retail listing implies.
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