LED Christmas Lights - serial or parallel

Existing set of Christmas tree lights with multiple push in bulbs has stopped working and replacing the fuse bulb doesn't fix it. Rather than pull out, test and refix a hundred odd bulbs I am considering replacing the entire set with some of the newer LED based ones.

But, does anyone know whether these are wired in series or parallel - or more importantly if one LED fails will the whole set stop working?

Andrew

Reply to
Andrew May
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I suspect the LEDs failing need not be on your radar.

Reply to
Andy Burns

Not guaranteed. I have some of those garden lights that set into decking. Each has half a dozen LEDs in it and, in two years, I have already had to replace three of the lights because LEDs were failing. Admitted they were only Homebase ones but I would have expected a longer life than that.

Reply to
Tinkerer

No - you might lose more than one but not all of them. They're usually wired in a combination of series and parallel circuits.

Reply to
Dave Plowman (News)

...........or could it be the power supply thingy?

Reply to
John

LEDs only have a near infinite life when under-run. A bit like tungsten. ;-)

Reply to
Dave Plowman (News)

Andrew May wibbled on Thursday 26 November 2009 09:23

Do a binary chop - at most, assuming it's one failed bulb, you'd need to remove about 7 bulbs and test.

1) Pick a bulb half way along the test section (initially teh test section is the whole string) 2) Remove bulb 3) Test continuity between each bulb socket contact and L and N at the plug 4) If one contact is open circuit, determine which section that is (pull another bulb out if it's not obvious), and defined that as the new test section, marking your current position as the end of section (wrap of sticky tape). Goto 1) 4a) If both contacts are open circuit, you'll need to defined two test sections and goto 1. 5) Both contacts have continuity - throw the bulb away and put a good one in.

OTOH if you really (I know I would) arelooking for an excuse to buy an new set, delete this post and I won't tell :-)

Reply to
Tim W

They seem to come in multiples of 40 so it seems likely that loosing one will take out a string of 40 which is a substantial proportion.

What worries me is that although the mean time to failure for a single LED may be 10,000 hours I have no idea what the expected standard deviation from that figure is - and that is what will dictate the expected life of a whole string.

Andrew

Reply to
Andrew May

I have a Homebase 40-LED set in which 3 near neighbours are out. It may be because I crunched one between wall and ladder last year, but there's no visible damaged. The rest of the chain works OK, so I guess it is a series/parallel arrangement. My plan is to buy a reduced-to- clear set in the sales for spares, unless someone knows another source? I assume they're all the same voltage (no internal resistors) regardless of chain length? Of course, the package says "non- replaceable bulbs", but, hey, this is uk.d-i-y.

Tip: When you take them down, wind them onto a cable drum to avoid tangling.

Chris

Reply to
chrisj.doran

I don't know where you got a MTBF of 10,000 hours from. High efficieny LEDS operating at 25C have a typical MTBF(90% CI) of 5,000,000 hours. That gives a typical failure rate of 0.037%/1000 hours.

If you run the lights for six hours per day, you can expect to lose

0.08% of the bulbs per year. That's one bulb every thirty years (ish).

Given that they are usually wired in parallel, that's going to be over a thosand years before they've all gone or if you're only happy to have

50% of them working, about 500 years.
Reply to
Steve Firth

I doubt they are in strings of 40. That would require about 60V to operate them. The PSUs tend to be about 12V-24V so strings of about 8-16 is more likely.

Reply to
dennis

But that would imply a lot more cables than are actually present. In a string of 160 anything between 10 and 20 pairs by that reckoning. Unless, the current is so low that they are using very thin multicore cable.

I'm tempted to buy a set just to find out.

Andrew

Reply to
Andrew May

I'm not sure how they work the MTBF out, some I've seen quote like that are tested on a 50% duty cycle and as the temperature rises the life is significantly reduced. A friend of mine brought some of those strings of LEDs from china 24 or 36 per string and he put them around the house for lighting, but he's had about 20% of them failing and the power supply 12V burnt out too. If one LED is only slightly out of spec then if that goes open circuit (in parallel) then this will mean that all others will get a slight increase in voltage due to the drop in current, but obviously this depends on the quality of the PSU and it's regulation.

Sounds good in theory and on paper. I'd like to know the details of the ones used as indicators on London buses as they should, be reliable due to H&S

Reply to
whisky-dave

It's a figure that I was given by the gentleman in John Lewis when I enquired if he knew how they were wired.

If they are wired in parallel doesn't that mean that each lamp has to have a current limiting resistor whereas if they are in series it is only one resistor per string?

Andrew

Reply to
Andrew May

No, three will do it if they are all on at the same time. Two for +/- one for each string in series.

Reply to
dennis

Yup, tend to be series /paralell with 3 wires on a 24V wall wart.

MTBF is calculated with a roulette wheel, big brand white is now typically rated to 50K hours to 70% lumens output, off brand white might be 10% of the price and may or may not be worth the gamble depending on application.

ForeverBright, U.S., came up with first LED Christmas lights, 35 is series on 120V mains, have couple of sets , circa 2000/1 they set the now standard LED shapes of berry, pine cone and stave.

Aldi had some nice cone shapes in colours and ice white LED last week.

John Lewis have an excellent set of colour morphing LED lights , if you can find stock.

B&M Homestores, if you have one local, have a dazzling array , almost exclusively LED.

B&Q have nice set of colour changing acrylic jewels, have had a set from last year on 24/7 here with zero failures.

Commercial dispalys are almost exclusively LED nowadays,Glasgow George Sq tree , 2 500m, 2.5Km, of white LED, power draw 400W.

Life span for big brand really shouldn`t be issue nowadays.

Cheers Adam

Reply to
Adam Aglionby

On the web site, if not in stores

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they are suitable for use both indoors and ... errm ... only indoors

Reply to
Andy Burns

This might help:

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've got 'flu at the moment and all I can see is dancing equations.

Reply to
Steve Firth

Also depends on how well they're made. There are lots of poor quality ones going very cheaply, and guess the sort of thing they end up in...

Reply to
Andrew Gabriel

The LEDs used in christmas decorations are cheap chinese ones manufactured for the purpose, or rejects from batch tests. Life expentancy can be incredibly short, because they only need to work for about 45 days. (They only have to last one season - no one complains if they die the following year, they won't even remember which shop they were bought from, and if they do, it likely won't still be there anyway.)

These LEDs are a problem for real equipment manufacturers. There's an enormous glut of them, at less than 100th of the price of proper spec LEDS, and there's much opportunity for distributors to pass them off as such and make big bucks. Many respectable companies have been caught out in this way, and ended up manufacturing commercial products which don't last even 6 months before the LEDs are dead.

Reply to
Andrew Gabriel

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