Tiny Tuesday: LED Bulb

Recently I went round the house cleaning our light fittings, which meant that the bulbs had to be temporarily removed in order to allow me to remove the tinted glass “shades”. The vast majority of our lights are now fitted with LED bulbs, and we’re particularly keen on the sort which mimic old-fashioned clear filament bulbs and give off a relatively warm white light. In removing one of the bulbs I found that the cement holding the glass envelope to the metal base had failed: the only way to remove the bulb was to pull the glass part completely away from the base and then remove the base with pliers (with the power off!).

The silver lining is that this allowed me to investigate how the bulb works – and anyone who knows me will be aware that I hate not knowing how things work! Hidden in the base of the bulb was a tiny circuit board only about 12 mm long – the triptych here shows both sides of the board. Of course I then had to Google what the components do. The writing on the IC chips isn’t too easy to read on the photos, but with a magnifying glass I could make it out quite well. The one marked “MB10F” is a silicon bridge rectifier, to convert alternating to direct current; the one marked CYT1000A is a device which ensures that the LED “filaments” are fed with a constant current regardless of fluctuations in the mains voltage – seemingly this is critical for LED lights. I think the other two components are resistors. I've now corrected this according to cyclops's comment below: there's probably a resistor and a capacitor. Fascinating! (There’s a circuit diagram here if you’re a nerd like me.)

The other photo, at about the same scale, shows one end of one of the six LED “Filaments” in the bulb. Apparently each “filament” consists of a row of many tiny LEDs, the whole assembly being coated with a phosphor-impregnated polymer matrix which diffuses light and also alters its spectral composition (basically its colour).

There’s no room for a metal heat sink in these bulbs, so the glass envelope is filled with a gas which conducts heat quite well – usually helium. LED bulbs are generally about 50% efficient in converting electrical energy into light energy – about 10 times more efficient than old tungsten bulbs which waste about 95% of the energy as heat.

(Well done if you’ve read this far…sorry, I can’t help myself!)

Having found out all that it’s amazing how little they cost! Sadly I suspect that they all come from the Far East.

Many thanks to loisbiz for hosting TT this month and prompting me to research this.