On 06/01/15 08:07, Tim Watts wrote:
On 06/01/15 06:10, John Robertson wrote:

As usual, heat is the enemy. Reading the technical specs for caps is
enlightening. They are rated usually at something like 2,000 to 5,000
hours at their rated temperature. So an 80C cap will die after something
like 2000 hrs at 80C or 4000 hours at 50C and 10000 hours at 40C (not
looking it up!), whereas a 105C cap will last 10000 hours at 85C, etc.
So, the better the grade of cap the longer it will last in warm to hot
environments. And there is the equivalent resistance and inductance to
consider as well. Some caps are much more tolerant of 50/60hz and others
are better at 20,000hz. Selecting those takes time and the cost
accountants slip in at some point...

Just out of interest, do any mains LEDs use simple reactive droppers, eg
capacitor or inductor, then bridge rectifier and smoothing capacitor?

I suspect not at this point in time.

The problem is that an inductor is large and expensive at 50hz.

Consider 25W of LED. that represents 100mA of current, or an inductor
impedance of around 2.4k ohms 7.6 henries.

That is a fairly large beast. Too large for a light bulb - and indeed
its pretty similar to what's inside a fluorescent ballast of similar
wattage.

Ergo these days we rectify, smooth and chop and transform at much higher
freqs than 50hz to get power at different voltages or to achieve current
limiting. And that gets rid of 100hz flicker too..or should,

And it gets rid of big iron cored chokes. smaller cheaper ferrites with
less copper involved are far far cheaper.

BUT you have the switching transistors and the rectification and
smoothing components operating at high power and mains voltage - bad
reliability especially when HOT.

Actually what is needed is the electronics independent of the bulb.
Maybe built into the light fitting, as it is with fluorescent. And
independently replaceable.

And do 12V LEDs have driver circuits which are less stressed than their
mains equivalents? ie are 12V formats more likely to last longer?

Probably. switching at 12V is not such a hard task for most electronics

The problems of LED bulbs are nothing to do with LEDS and a lot to do
with miniaturisation and cost.

Personally I think that we should be looking at LED lights that are
neither ES nor bayonet, but in fact sit on the end of a piece of wire,
and are large enough and expensive enough to be considered fittings, not
consumables, and simply get wired in when the house is built.

And last at least 15 years.

E.g. you replace a ceiling rose assembly with something remote from the
'bulb' that drives the 'bulb' over a piece of wire at LV. Or you have
integrated lumieres that replace conventional ones.

And indeed in-wall power supplies for designer lighting etc.


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Everything you read in newspapers is absolutely true, except for the
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