Diagnosing LED flashing

If your lamp flashes when you think it should be off then you probably have one of two problems:

The lamp may be receiving an induced voltage, usually from two-way switching.

This flashing is the result of capacitive coupling between the live and switched live strapper wires that run between two-way switches. Alternatively, the flashing may be the result of other stray leaked voltage. The stray voltage causes a build-up of charge within the lamp components, which is then released when it reaches a level sufficient to overcome the impedance of the lamp circuitry. The result is a cycle of brief illuminations. These flashes are not a safety hazard, but are obviously not ideal. Historically, this process also occurred on traditional filament lamps – but the stray voltage was not sufficient to light the bulb.


First you should confirm the diagnosis: this is easy, simply replace one of the LED lamps with a filament bulb. The additional resistive load offers a dissipation route for the charge. In addition, with the switch off, you can usually check for a small voltage on the switched live wire. If the circuit has two-way switching, this voltage will be more apparent on one of the “off” configurations, rather than the other. Adding a suppressor (or snubber) to the circuit between the switched live and neutral will solve the problem. The suppressor should be installed in parallel with the lamps, and as close to one of the lamps as possible (possibly within the fitting itself or ceiling rose).This costs just a few pounds, a more expensive solution is to add a resistive load, which mimics the action of the parallel lamps.

The switching device (PIR sensor, light sensor, electronic switch etc.) may not completely turn off the supply voltage.

Some sensors or electronic switches, intentionally, accidentally or as a result of component tolerance, will leak a voltage through the lamps in the lamp ‘off’ state, leading to regular flashing of the lamp. These sensors or electronic switches have generally been designed with filament lamps in mind as the small voltage leaked through the lamp is not enough to energise a filament lamp. However, this small leaked voltage creates a cycle of energy build-up and discharge in low energy lamp components leading to flashing.


1) Check with the sensor or electronic switch manufacturer that the load is completely isolated in the ‘off’ state before use.

2) Don’t use two-wire sensors or electronic switches. These sensors or electronic switches are designed to replace a wall switch and do not use a neutral conductor. As they do not have a neutral conductor they will constantly and intentionally leak a small voltage across the load in order to operate. They will cause low energy lamps to flash unless the leaked voltage is dealt with.

3) Use four-wire sensors or electronic switches. These usually operate through mechanically switching an internal relay which has switch contacts that are completely isolated from the PIR sensor’s circuit and as such are extremely unlikely to leak a voltage across the load so no flashing would be witnessed.