Doubt on rechargable LED emergency bulb

Question

I have recently seen a rechargeable LED emergency bulb on Kickstarter and I was wondering how it works.

In the event of a power failure, the LED bulb will turn ON automatically. That is, when the switch is ON and mains power goes off, the bulb will detect that and turn ON. If the switch is OFF the bulb will not turn ON. When mains are ON and the switch is ON the bulb will start recharging again.

It is just like a normal bulb. See the Youtube video

My question is, how does the bulb detect the event of mains failure? Can somebody give me a hint on this?

Answer 1

The Youtube video analysis is quite complete: The bulb tries to pass a tiny DC current (battery voltage through two 1 MΩ resistors) through the line connections, on the assumption that there is SOME DC path through the network at all times (even if it's through the pole transformer).

The light comes on if it receives full voltage from the line; this is "normal operation".

The light also comes on if there is no line voltage, but there's a DC path between its two contacts; this is "emergency operation".

Note the restrictions: The special bulb must be the ONLY bulb on the switched circuit. If there were other regular bulbs, they would provide the DC path and the special bulb would stay on all the time.

The demo video is actually using two outlet strips. One is plugged into the wall, and the black lamp is plugged into it, along with the second (switched) outlet strip that we see sitting on the table.

When he unplugs the first outlet strip from the wall to simulate the power failure, it is the black lamp that provides the DC path that allows the special bulb to switch on and off. Without that black lamp, the demo would have failed! (Also, if the presenter had switched off the black lamp during the simulated outage, the smart bulb would have switched off, too.)

Answer 2

If the power has failed somewhere on the grid, the residential line impedance is still low from the transformer secondary winding.

The AC light switch can now enable the internal LiPo battery to power the Shunt LEDs in the circuit. There is a slow rise time to charge up the capacitor before the DC battery can operate.

So the breakers and outside transformer must still be connected for this to work. Correction: Clive demonstrates in the Youtube video that even finger resistance across the Edison contacts is enough to enable the emergency light (with a power failure and external switch closed.)

Judging by the kickstarter investor feedback, they were not too happy with the results. The Youtube product had quality issues but at least worked.

Answer 3

When there is power the bulb works normally. If there is no power you have to short circuit the terminals for the bulb to burn on battery. When it is in the circuit and when power goes off the terminals of the bulb get connected together through other loads in the circuit and so it burns on battery.