led driver question for the electronics experts

[ajh]

My replacement 4ft t8 strip light failed after a few years. I took it apart and other than some slight discolouration on the driver board no obvious reason.

I was surprised the driver was so complicated with what looks like a transformer for a switched mode power supply and an IC amongst loads of other discrete components and the strip of 81 leds.

Why is it not possible to use the leds across the mains input with the sum of forward voltages equaling the mains voltage, one strip attached live to neutral and another the other way round and a current limiter?

[Andy Burns]

Flicker.

[nib]

It is done sometimes in quick-and-dirty, or cheap-and-nasty, low-power lamps, where you can sometimes see a pair of strings of LEDs in opposite polarity fed direct from mains via a current-limiting capacitor.

But flicker, and more difficult to stabilise the current with varying supply voltage and temperature.

nib

[nib]

Flicker. Does anyone else notice how LED lights can split up into dashes if you move you head quickly? It's quite noticeable to me, especially with car rear lights in the dark, I can often tell if they are continuous or pulsed.

As a spectacle-wearer, I have another way of checking, effectively using the retina as a sort-of oscilloscope - I just waggle the lenses rapidly in front of my eyes. Continuous lights, like an incandescent, show as smears, pulsed sources as a series of dashes. You can even estimate the frequency and mark-space ratio!

Anyway, back to the original topic. I have an ancient plug-in LED, the very first I bought, many years ago, which is about 100 small LEDs arranged in a shape reminiscent of a pineapple. The waggle test shows that is flashing right on an off. That is one that I know is just a capacitor dropper from the mains. It lives in the bathroom where its crudeness is not important.

nib

[Cursitor Doom]

Would you get flicker at 50Hz? I have to say I've done this and can't notice any, but that might just be me. LEDs are "currrent orientated" devices so your current limiter would need to keep the current through the chain at 20mA (or whatever each LED is rated for). There are potential dangers with doing this, however. I wouldn't recommend it to someone unfamiliar with electrical safe tea.

[SteveW]

Definitely. Where LEDs are driven at 50Hz, without smoothing (such as the red and green men on some pedestrian lights), I get a horrible, momentary strobe effect as I drive past or turn my head while waiting, giving multiple very short lasting after-images.