CFL on PIR

old FAQ material

LOW-ENERGY LAMPS AND TIMER/DUSK-DAWN/PIR SWITCHES

By Matthew Marks 20/1/1997

There is often confusion as to whether low-energy light bulbs (fluorescent lamps designed to replace ordinary incandescent lamps) are compatible with devices such as timer switches, dusk-dawn switches or passive infrared (PIR) movement detectors. The problem occurs because these types of switches require a small operating current even when they are not activating the lamp. Ideally, there is a neutral connection so that the operating current can flow from live to neutral. However, some of these devices are designed to replace a conventional light switch, where there is no neutral present. They thus have to pass their operating current through the lamp.

A normal light bulb is quite happy to pass very small currents without protestation. However, low-energy light bulbs (both electronic and non- electronic, and old-style fluorescent lamps too) are highly non-linear, and will not pass any current at all unless a certain voltage is across them. This will result either in the switch malfunctioning, and/or (in the case of electronic lamps) the lamp periodically flickering when it is supposed to be off, as its internal smoothing capacitor is charged by the small current.

If the switch explicitly requires a neutral connection, or will operate without a load (plug-in timer switches, both electronic and electromechanical), it should be happy to run a low-energy lamp. However, devices which have a built- in load (such as PIR switches with lamps) may possibly still pass the operating current through the load, but gain no advantage from this, so are badly designed!

If you wire a conventional bulb in parallel with an energy-saving one, the switch should operate, but of course this compromises the energy efficiency of the set-up. A resistor could also be used, but it is difficult to give advice on this, because its value and power rating would depend on the switch.