Holiday project; fixing LED holiday light string!

Repairing string of LED Holiday Lights

First of all, unless you (like me) prefer repairing to discarding stuff, this does not make any sense. These holiday lights are so cheap in USA that it is probably not worth spending 20 minutes working on the problem. But I am undaunted by such practical concerns!

I have one string of LED lights on which one half has gone dark. These are solid color with no color changing, etc. I think that it is extremely simple, with 1/2 string of LEDs oriented one direction across the AC mains and the other half string oriented in the opposite direction. Whereas a good engineer would put a limiting resistor in series between an LED and a voltage source, I think that these lamps simply rely on the series resistance of the LEDs. If this discussion is of interest I can draw a schematic of what I think…but sorry I will not include all 50 or 100 or (??) LED lamps.

In days of old the holiday lamps had incandescent bulbs which had a shorting feature. The idea was that if a bulb failed it would short out, thus slightly increasing the voltage to the series connected bulbs, thus hastening their demise also! :slight_smile: But I do not know whether these LED lamps are designed to permit the rest of the string to light when one LED dies. I do have some other fancier strings (these are a different product from a different store) on which a few have died (the LEDs seem less reliable) but the rest of the string stays lit.

Anyway I see the name of the game as finding the bad LED lamp without maybe having to test every one in the dark half string.

I know that many of you are in Europe or otherwise in a land of ~230 VAC outlets. I do not know if your 230 VAC holiday lights are very similar (except maybe double the string length) to those in USA and the lands of 115 VAC.

I am thinking of removing a lamp from the midpoint of the half string, then doing some sort of test to determine on which side (of the removed lamp) has the open connection, thus narrowing the field of suspects by half. Then going to the middle of that half half string and do another similar test. Rinse and repeat. With this method I think I ought to be able to find the guilty lamp with 6 tests in a half string of 64 for example (2^6 = 64) or that is the general idea. Call me crazy but a key objective will be to have enough voltage to test the series lamps but not kill myself in the process. And I am thinking of practical ways to put voltmeter probes into the socket. If I had a known bad lamp I could take it apart, and use its base to connect to my meter and use it as a plug test.

So what are the thoughts from our community? Anything to take our minds away from COVID and towards some holiday fix it spirit!

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They exist, but because the 230V world is also pretty much the 50Hz world they’re on the edge of visible flickering and really crappy.

Thanks, yes. I can see mine blink also. Very obvious if you scan your eyes across them while lit at night.

Another observation. In USA the 120 VAC outlet has a hot side and a neutral side. If I plug the timer switch in wrong so that the switch switches the neutral side, there is enough leakage current so that the lights light dimly. (I have the lights on a shrub which is likely to be damp this time of year.) But the brightness is amazingly uniform. Sort of pretty, until I go out and plug the plug in correctly. Then they go dark when the switch is “off.”

I decided to draw a schematic diagram…

So you have an indicator light telling you the setup is still working :wink:

BTW, our timer switches are required to include DP switches, exactly because our (type F) sockets aren’t polarised.
The ones from Aliexpress are exempt, of course :slight_smile:

I do know if your LEDs are detachable, but I remember doing this a lot at my parents house and the way to go was to take a know working lamp (from the part that was working) and then do the search as you mentioned replacing a lamp with the functioning one until it works, no measurement needed :relaxed:

Yes the lamps unplug. But:

  1. The problem might be the LED or the socket or maybe the socket contact. So I am not sure but I can imagine somehow going right past the problem by putting a good lamp into a bad socket or something…

  2. I would rather spend 4 hours thinking of how to fix it in 1 hour, instead of spending 2 hours fixing it. Not that I am lazy (but that too) but more for the challenge. BTW I never had any patience for crossword or jigsaw puzzles. IMHO life presents me with plenty of real puzzles; don’t need the concocted ones.

So nothing has changed!
Back in the good/bad old days when the kids were little, I reserved a 1/2 hour to assemble the Christmas tree and 2 hours to make the lights work. There were always at least a couple of dud globes.

My method was to remove approximately every sixth globe and then check for series continuity from one empty socket to the next. Checking LED systems that way would also work, bearing in mind that, being diodes, resistance is only one way.

I agree with your comments about crosswords. I hate them…tried a couple, years ago, but the clues were always wrong or there were never the right number of squares for my answers.

PS Is your drawing the new BobZ inspired “Neon” colour scheme that will be available in Kicad 6?

One other thing: are the LEDs keyed, or can they be plugged in the wrong way around?

Gosh that would really be a wish to make our lives difficult. There may be some designs in which the LEDs are actually 2 connected antiparallel (so bi-directional and not polarized). But these are keyed. I hope to spend some time on this this weekend. I hope that maybe a 50V DC source will produce enough leakage current through the string (of good LEDs) to be detectable with my DMM and yet be able to cause only minor electrocution. :expressionless:

50 volts is fine, even at hundreds of amps.
Back in the day, when telephone exchange computers were made with relays… millions of 'em, that’s the voltage they ran on. 50 volts gave distance without frying customers.

Not a wish, just would believe pretty much anything regarding some quality Chinese merchandise.

Just don’t be touching the phone wires in your house when a call comes in… That 80VAC bites. :wink:

Who still has such a hardline?
Anything can happen with those. Haven’t you seen the Matrix?

Even the phone connection coming out of your cable or Fios box should still have the 80VAC ring tone to keep compatibility with older telephones. My mother still uses her POTS phones with her cable service. I have POTS through my Fios service, but don’t use it. (I only have it because the bundle combining internet and phone (I don’t have cable) cost less than the internet alone… Don’t ask me how VerizonFios does their pricing, I don’t understand it either…) If I did use my POTS service the only place I would run the phone line to would be my all-in-one printer to send my once-in-a-decade fax…

I have not seen the Matrix. I do not like crossword puzzles. :slight_smile:

Ah, ring current; there’s nothing like a little thrill early in the morning to wake you up. Managed to get caught out many a time while messing about on distribution frames.

[quote=“paulvdh, post:11, topic:26180”]
Who still has such a hardline?

We are still the proud users of copper to the house. Good ol’ ADSL2. The joys of country living seven kilometers out of town. The phone company can’t be bothered running fibre out here and we can’t be bothered trying that new fangled airborne stuff.

What matrix?
But do you still like LED puzzles? :slightly_smiling_face:

I like some electronic puzzles…

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