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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 26th Aug 2022, 3:36 pm   #1
Wendymott
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Default Failing led strips

Hi Peeps. Some of you may know, I am an advocate of Illuminating my bench magnifier using white LED strips, rather than the old circular Flu tube. Basically the idea is sound, however I noticed, quite often singular leds would either go out or go low light out. When more than two failed in any section, it was cut out and another section inserted. Eventually I ran out of unused led strip, and I started to wonder why ?? New leds would last maybe a month, then start failing. I had changed the led mounts to aluminium, to heat sink the leds, as they do run rather warm. The leds are supposed to be rated at 12V DC and that is what the wall wart was supplying. So a two fold attack was required. I ordered another reel of white leds and these arrived this am "Friday". They are smaller in area and more closely spaced, and at 12V, the whole lot "100" ran at 3A. As I only use about 40 in total that = 1.2A ish.
I also decided that the wall wart needed attention.. Reduce the DC Out to 11V, .....11V to the leds gives no real reduction in light output, well not noticeable to me anyway. Switching SMP's have a "Live" area and an isolated area. The DC voltage is set by a circuit on the isolated side of the transformer, usually set via an opto coupler and a 3 pin variable reference... TL431.. The voltage setting resistor is ALWAYS a high tolerance type, mine was a 4641 Which I guess was 46K. Anyways, to set to 11V I kept on padding this resistor till I got 11V. I finished up with 43K. in parallel with the original. With the wall wart case suitably sealed, I fitted the new Leds... WHAT a difference. Time will tell if the new leds survive. Watch this space
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Old 26th Aug 2022, 5:07 pm   #2
line sync
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Default Re: Failing led strips

I can hardly believe the short life of those LED,s , the circular flo tubes last much longer than that.
I bet reducing the voltage makes the LED,s last a lot longer.

Robin
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Old 26th Aug 2022, 5:12 pm   #3
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Default Re: Failing led strips

I wonder if it's the reduced current in the LED or the reduced heating effect of the current limiting resistors that is of benefit?

Steve.
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Old 26th Aug 2022, 5:43 pm   #4
G6ONEDave
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Default Re: Failing led strips

Could it be the difference between a high frequency LED supply and a fixed constant voltage supply? Most of the LED supplies that I've seen have been of the high frequency type.

Dave
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Old 26th Aug 2022, 11:28 pm   #5
Cruisin Marine
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Default Re: Failing led strips

Perhaps the LED'S need a literally switched/ on and off with appropriate mark space ratio power source where they are not on all the time and switched to not make them work so hard?
Please do not confuse this with a switch mode PSU.
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Old 26th Aug 2022, 11:58 pm   #6
McMurdo
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Default Re: Failing led strips

maybe change the LED supplier!

I fitted 12V led strips under HWMBO's kitchen cupboards, it was over 5 years ago and they're on every night from dusk til bedtime, not had a single LED fail. I bought 2x 5m reels and used one, with one in storage for colour-matched spares which has never been called upon.

They're the 5050 high density type and have the plastic coating on them supposedly rated for outdoor use.
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Old 27th Aug 2022, 3:20 am   #7
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Default Re: Failing led strips

I was given a vinyl pipe LED light and iron wall wart some years ago by a landlord who was clearing a house out.
It has been used as an all day and night light in a windowless corridor for some years without fail.
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Old 27th Aug 2022, 8:50 am   #8
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Default Re: Failing led strips

When powering strings of LEDs I would always go with constant current rather than constant voltage. The PSU then adjusts the voltage across the load to keep the current at the designed / desired level.

You can crudely achieve a similar effect by making the PSU voltage manually adjustable and adjusting the voltage so that the standing current through the load is correct, but the PSU will not then compensate for changes in the load, for example if the current drawn by the LEDS increases or decreases as they warm up.
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Old 27th Aug 2022, 10:29 am   #9
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Default Re: Failing led strips

An LED will give the same light output but run much cooler if it is fed with, say a 50% duty cycle high frequency supply at twice the peak current than a DC supply of a given current, assuming it is within it`s ratings since light output is more or less proportional to current whereas the voltage drop is more or less constant over a wide current range.
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Old 27th Aug 2022, 3:33 pm   #10
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Default Re: Failing led strips

Think you've just been unlucky. I've installed LED strips in three locations (using the same original roll) that have been working for few years with not a single failure. The quality of budget-grade stuff from the far east is always something of a "Lucky dip" experience.

B
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Old 28th Aug 2022, 6:40 pm   #11
Wendymott
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Default Re: Failing led strips

Hi Peeps. I have read all the comments....Bazz I think your right... When you buy from ebay its a lottery. Kevin. Yes the commercial fittings are more reliable. I have a string illuminating my dressing table, bought from Ikea.
These led strips on a 8 mm film reel are an unknown, I take the idea that maybe a switched cycle supply may be better. I will see how the current setup works, and go from there. Thanks for your interesting comments.
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