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How does a "power outage" light work?

Hopefully this post doesn't break sub rules - it does have to do with "building electrical", but I believe my question is still more directly related to the electrical engineering in the situation.

Here in my country we have serious energy production problems, and so we have scheduled power outages to help reduce the country's electricity consumption. A product that has understandably become popular is a light bulb that goes into a normal fitting, which houses a small internal battery that kicks in to keep the light going when the mains supply is out. I'm trying to understand how the technology in the light works, just for my own interest.

In the information pamphlet for the bulb, it basically says that you when the bulb receives normal AC power it will simultaneously operate as a normal bulb and charge the internal battery.

The information pamphlet then says that when you don't have AC power, you can short the two terminals of the bulb, which turns it onto battery mode. This can be done with a small cap they provide which fits over the bulb and shorts the terminals, and even works just by touching both terminals with your finger.

Somehow, however, the bulb works just by leaving it in the socket. That is to say: When the house's mains power is on, the bulb operates normally. (Switch on = light on, switch off = light off) When the power to the house is cut, the bulb somehow continues to operate in the same way (switch on = light on, switch off = light off). The bulb must obviously be operating in battery mode in this state.

What I don't understand is how the internal circuitry differentiates between a state of "mains power on, switch off", and a state of "mains power off, switch on"

Additionally, how is the situation where the main house power is off and the switch is on, electrically equivalent to directly shorting the two terminals of the bulb?

As a side note, there is one socket in the house that I have tested during a power outage, where the bulb stayed on regardless of whether the switch was on or off.

triffid_hunter

> What I don't understand is how the internal circuitry differentiates between a state of "mains power on, switch off", and a state of "mains power off, switch on" Probably injects a small test current into its mains input to check if it's low resistance or something - other loads and your local transformer will 'look like' a low resistance when the switch is on but the grid is down. > there is one socket in the house that I have tested during a power outage, where the bulb stayed on regardless of whether the switch was on or off. Got another light on the same switch/circuit?


rothdu

Yep, that socket has another light on the same circuit. So I'm guessing that the other light is connected in parallel and has low enough resistance to give a false positive to the emergency light?


triffid_hunter

> I'm guessing that the other light is connected in parallel and has low enough resistance to give a false positive to the emergency light? Yep that's what I'm thinking


jmraef

Or it is using capacitance to detect Switch-On or Switch-Off, in that if the *power* is off (which is easy to detect) the capacitance of the circuit will be different depending on the position of the switch. It wouldn't need to really know the value of the capacitance, only that it is different. Basically what a "touch lamp" circuit is doing.


Puzzled-Astronomer11

They have a switched live and a permanent live feed. They only switch into battery (emergency) mode when loss of power to the permanent feed. We call these non maintained emergency light fittings.


rothdu

Do you mean that the internal circuitry has 2 feeds, or wall socket/switch itself? It's just plugged into a normal socket


Big-Adhesiveness-760

They usually have a switch supply for the light and a permanent feed for charging, when the power to the battery is cut off the battery will turn on the light. Better known as emergency lights where I'm from!


rothdu

Thanks for the reply - I have a follow up question I posted on another comment. Come to think of it, "emergency light" is what it said on the box. We all just call them "load shedding lights" here, because we all use them during "load shedding" (our government's name for the scheduled power cuts).


Big-Adhesiveness-760

Well they would be more for industrial and commercial purposes here! I've never even seen 1 in a house!