Magic LED Bulb
I found this LED bulb for sale locally for about US$7.50. It was interesting and ingenious at the same time.
The LED bulb has built in battery backup which charges during normal use when AC mains is available. When AC power is off, the bulb has about 3.8V at 5uA available on the terminals. This is used so that it can detect if the AC switch is ON or OFF. When there is a load or resistance of about 100k or less connected between the terminals, the LED bulb turns on running on battery power. It is difficult to explain so look at the next images for how it works.
With AC mains available and the switch off, the LED bulb understandably, is off.
With the switch turned on, the bulb works as would any bulb normally do.
The internal battery is also being charged during use.
When the AC mains fails but with the switch turned off, everything does not function.
The LED bulb has about 3.8V at 5uA available between its terminals. This tiny current will be used to know the position of the switch.
With the switch turned on, the tiny current finds a path around the existing house wiring to any loads still connected and then back
to the bulb completing the circuit and as long as the total loop resistance is 100K or less, the bulb turns on running on battery power.
The idea is quite novel and simple, I'm wondering why I haven't thought of it. And now, for the teardown pics.
The bulb as I got it.
The description on the box is the generic stuff for common LED bulbs.
There is no mention of its special abilities.
The LED bulb does not look anything special.
Popped the cover off.
LED array uses 14pcs in parallel and mounted on FR4 PCB not the aluminum core type.
At 4.2V battery voltage, the LED array runs at 530mA. This will decrease when the battery voltage drops and I got about 130mA near the end of the battery life.
The LED current is limited by two 3.3 ohm resistors in parallel. No constant current circuitry is used.
530mA at about 3V is 1.59W which is way below the spec of "5 watts" but it is still plenty bright and useable.
The innards pulled out.
The battery is a single 18650 Li-Ion. As far as I can tell, there is no integrated protection PCB.
I discharged the cell at 300mA and I got 1819mAh. Must be a low capacity or used cell. But it took almost 6hrs at that rate so it would still have a decent runtime.
I'm a little concerned about the LED array heat going to the cell affecting its lifetime but only time will tell.
I was expecting something simpler but it has an SM7505P based switching powersupply and lots of discrete circuitry in it.
Charge current starts at 450mA from an empty cell and tapers down as it charges.
Charging does not terminate but the voltage stops rising at 4.22V when battery is fully charged.
Back of the PCB.
The bulb could even light up if there is sufficient conductivity of the skin.
It seems to have a decent runtime of over 5hrs on a full battery and gradually dims as the battery runs down. Charging time from empty is about the same duration. It runs at the battery voltage all the time even on AC power so it starts out dim (when the battery is run down) then gradually increases in brightness as the battery charges when on AC power. I will probably have this installed somewhere to test for long term durability.
Page created and copyright R.Quan ©13 Nov 2015.