led lights

[Brian Gaff]

To the person who sent me a message about why they do not make one big led. There was no valid return address on the email so its got to be a reply here. For all those who also or only send replies directly, its very bad set up wise to not include the valid email address as people cannot reply privately. so you said that lots of small ones can be coupled in series across the mains. Not quite as the will only work on DC, and a half wave rectifies is no good as it loads just one half cycle, you need a bridge, also leds need to be current limited otherwise they wiill burn out. Brian

[The Natural Philosopher]

which is an ideal use for a capacitor as a lossless way to current limit.

You could - though I don't know that they do - use a pair of diodes in parallel back to back lighting on alternate half cycles. But I prefer the bridge.

[Martin Brown]

Unfortunately if you just do that the first mains transient kills them.

Plenty of efficient regulated switched mode devices can make Dc volts or constant current source from mains voltage. The main problem is shunting away the waste heat from the LED die itself.

There may be, but you have to protect them from too much reverse voltage or the magic smoke comes out.

[The Natural Philosopher]

Well a bit of a resistor or another capacitor will sort that.

Indeed.

The other diode does that, stoopid.

[Andrew Gabriel]

You need a resistor which will limit the peak current at switch-on at the mains cycle peak (340V) charging the initially discharged capacitor, to the max non-repetitive forward current. There is actually an even worse scenario - that you switch it off when the capacitor has max voltage across it and then you switch back on when the mains is at the opposite peak, which will give you 680V difference (minus the forward voltage drop of your LED string). You might try to mitigate this second case by using a bleed resistor across the capacitor, but a momentary supply interrupt won't give that time to operate, so you still have to cope with this 680V problem.

Basically, this is why capacitive droppers are not easy to design. They can only be used with loads that don't care about massive surges, or you need a resistor to do a portion of the dropping.

Any mains spikes are transmitted straight through too.

I've done that with indicator LEDs, but some power LEDs have a lower max reverse voltage than their forward voltage drop, and some datasheets quite strongly hint they aren't designed for reverse biasing.

[Andy Dingley]

Why (apart from some stroboscopic effect) is that a problem?

My LEDs at present are either one string (60), or two anti-parallel strings (108 or 130-something). They're strobing at 50 or 100Hz, but it's not a big deal.

They don't - if you only feed them their forward voltage.

[DerbyBorn]

Andy Dingley wrote in news: [e-mail]:

I saw some amazing stage lighting at the weekend which I assumed used LED Lights - colour change and really bright. What are they doing that is different to the domestic crap?

[John Williamson]

Spending money. The stage lighting stuff isn't cheap. It uses high output, single colour LED chips, with big heat sinks to keep them cool. Life is still not great, though. Red plus green plus blue gives almost any colour you wish. They also compromise on the colour rendition, as you effectively have three big narrow spikes in the spectrum, not a broad distribution of wavelengths as as you get with the domestic white ones.

[Martin Brown]

Very risky to do that. Though you can get away with it on AAA or AA batteries where the internal resistance of the cells limits the current.

Even so under the right conditions AAs can source 10A into a dead short!

They are cost no object purpose designed units with heatsinks to match. Individual LEDs are 3, 5 or 7W in a grid and some are self collimating allowing a very shallow unit to have a highly directional beam.

Domestic LED lamps are compelled to fit standard bulb sockets and lamp shades. They could be made a lot brighter if you were allowed to put a decent forced air aluminium ventilated heatsink on them.