| The front end of the circuit in most emergency luminaires is a transfomer from 230V to some SELV, typically 8V or so for Fluorescent emergency packs. Dont know how much for LEDs but i'd expect about the same (with batteries typically being around 4.8..6V in both FL and LED, as this is what determine the needed transformer output)
The SELV is rectified to DC with a bridge, the resulting DC is still proportional to the line voltage. It goes to the battery charging circuit and to the voltage detection circuit
The voltage detection circuit have some threshold. If voltage is higher it is assumed to be normal and the emergency light off, if voltage is lower it is assumed to be outage and the emergency light is on
Sometimes the voltage during a brownout is fluctating around the threshold, which can cause the lantern to erratically go on and off
Sometimes the threshold itself is not well defined : The design of the voltage detector is based around some transistor, that will for sure be open at 0V, for sure be in saturation at near full line voltage, but not much thought was given to what will happen inbetween. At some intermediate voltage, it is possible that the transistors of the detector are in active mode where they were not meant to be (by the design of the luminaire), and that might cause the light output to be dimmer than normal (the power transistor before the LEDs being active instead of saturation, or base drive for the Royer FL driver getting lower voltage than intended etc)
This may be bad for the luminaire, if the transistors are dissipating power that they were not meant to. Likely not an issue to the low current transistors of the detector (the "logic"), but may overheat and burn the high power ones (the one controlling the LEDs or the inverter transistors in the FL)
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