Emergency or Inverter Bulb Working Principle

Some of you may have seen these bulbs in your home. At first glance, they seem like your regular LED bulbs, with a string of LEDs powered by AC input. But here’s the twist: they hide a rechargeable battery inside, often an 18650 lithium cell. When the power goes out, this battery springs into action, illuminating a subset of LEDs.

You can get a pretty good idea of what’s packed inside these LED bulbs. It typically includes a standard bridge rectifier and an LED driver for the regular lighting component. There will be a battery charge controller, ensuring the battery gets juiced up during operation. When the power goes out, the battery takes the wheel, supplying power to the LEDs. It’s a straightforward setup, nothing too complex

But here in lies the interesting part, how will the LED know to turn ON when there is no power? Because it can also be that its switch to it is turned OFF. How does it differentiate? It uses a clever mechanism, it can turn ON only when the input AC terminals Live and neutral provide a momentarily closed path. This cannot happen in the normal case and can occur only if another load in the same circuit is also turned ON. It could be any device. This will provide a path for the battery to turn ON power to LED till the actual AC power kicks in.

Consequently, you can perform a seemingly magical feat by merely touching the LEDs with your hands (a reasonable resistance between the AC input pins triggers the battery to power the circuit). It’s a neat party trick that can impress your guests when you effortlessly illuminate an ordinary-looking LED bulb with no external power source, using just your hands! ?

If you liked the post, Share it with your friends!

Teardown and Repair: 20W Tubelight LED Driver

Tubelight Internals

Tubelight at home randomly stopped working this week for no apparent reason. Opened it up to see what went wrong. Started probing and saw that the input fuse (Supposedly a fuse but they put a 61Ohm resistor) was blown. Replaced that and the 41Ω, 1W resistor on the return path, and the tube light was back to normal. Not sure what caused the blow-up. Maybe the resistor degraded with time.

20W Tubelight Schematic and circuit
20W Tubelight Schematic and circuit

I went through and traced out the circuit for anyone wanting to know its schematic. The AC Input side is current limited with a “fuse” and fed into a classic full wave bridge rectifier(MB105) IC to convert to high voltage DC. There is space given in the input for filtration/safety capacitors but they have not populated it. Probably to save a few cents, might have put it there to pass the testing. The output of the rectifier contains a CY400 capacitor and Metal Oxide Varistors(MOV). This is fed to SIC9762 which is a high voltage LED driver with an integrated MOSFET. It’s a constant current LED driver with the drive current set at around 200mA with a sense resistor. On the output LED side, you have a large electrolytic capacitor of 100uF with a bleeding resistor to discharge the tube light immediately(else you will get a ghosting effect of the LED). There are 100 LEDs in total at the output.

All in all, it’s a penny-pinched circuit optimized to minimize the BOM to the lowest price possible. That’s exactly the reason you get these tube lights in the $3-$4 range per unit in the retail market.

If you liked the post, Share it with your friends!
1 2 3 19