Lighting device
In one embodiment of lighting devices and lighting systems, the lighting device has a connection for connecting to a primary power supply and has a secondary power supply, such as a battery. A measuring circuit is operable to measure an impedance of the primary power supply connection and to determine from the measurement if a main power supply has failed, and if so whether to power light sources using power from the secondary power supply.
1. A lighting device comprising:
one or more light sources;
a primary input power connection for receiving primary power from a primary power supply, for powering at least one of the one or more light sources;
a secondary input power connection for receiving secondary power from a secondary power supply, for powering at least one of the one or more light sources; and
electronic circuitry coupled to the input power connections and arranged to control power delivery to at least one of the one or more light sources using power received at the input power connections;
wherein the electronic circuitry comprises sensing circuitry connected to the primary input power connection and configured to sense an external impedance connected to the primary input power connection and an isolator for isolating the sensing circuitry from other components of the lighting device, the other components of the lighting device being electrically connected to the primary input power connection and presenting a low impedance across the primary input power connection at least when said sensing circuitry is sensing said external impedance.
2. A lighting device according to claim 1 , wherein the isolator comprises a semiconductor junction device isolating said sensing circuitry from the other components of said electronic circuitry.
3. A lighting device according to claim 2 , wherein said semiconductor junction device comprises a zener diode.
4. A lighting device according to claim 2 , wherein the semiconductor junction device is coupled to the primary input power connection and is connected to the sensing circuitry such that the semiconductor junction device is reverse biased when said sensing circuitry is sensing the external impedance connected to the primary input power connection.
5. A lighting device according to claim 2 , wherein the sensing circuitry is arranged to apply a measurement voltage to the primary input power connection and wherein an amplitude of the measurement voltage is lower than a breakdown voltage of the semiconductor junction device.
6. A lighting device according to claim 1 , wherein the isolator comprises a transformer.
7. A lighting device according to claim 1 , wherein said isolator comprises a relay device isolating one or more of said light sources from the sensing circuitry.