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how to troubleshoot LED Christmas strings

Posted by w0lley32 (My Page) on
Mon, Dec 28, 09 at 18:27

Hi! I have several strings of blue LED Christmas lights that either don't work or are incredibly dim. I have a collegiate diploma in electronics even though I don't work in that field anymore, so I started troubleshooting them to discover why they fail so frequently. They consist of two 35-light strings end to end on a single cord, and have no electronics because I can "see the sine wave go through them" so to speak.

So I thought I could check them with a multimeter, but on the diode setting I get an open circuit both ways, and I checked with a regular LED and I get .7 volts direct and open circuit on reverse, so I know my meter is good. However, on these Christmas LED's, the only scale that registers anything is the 20M ohm scale, where I initially get 65 Megohms, that tapers down to 17.2 in one direction, and 10 Megohms, that tapers down to 3.8 in the other direction, which seems to indicate that they contain some capacitors. However I ground one socket down only to find it only contains a tiny LED molded in plastic with a lens fitted over the LED.

Is it possible that these LED's don't work as regular LED'S since they are made to work on AC circuits (neither the plugs nor the wires have any integrated components of some sort) and if so, would anybody know how I could troubleshoot these strings to replace the failed LED's? At $20 a string, it seems to me they should last more than three years. Thanks for your help.


Follow-Up Postings:

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RE: how to troubleshoot LED Christmas strings

I've been looking at a few different brands of LED Christmas lights.

Is this a Philips brand ?

Some strings don't use the single reverse blocking protection diode.
Some strings don't use the single current limiting resistor.

They will fail earlier if no protection.

LED Christmas Lights and How to Fix Them


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RE: how to troubleshoot LED Christmas strings

Thanks for your reply. They are Canadian Noma brand, and they contain no other electronic components than the LED's themselves. I've been on the site you are mentioning, but they don't really tell how to troubleshoot those LED's. Are they different from regular direct current LED'S? If not, how can I locate the bad one(s) in the string? Thanks.


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RE: how to troubleshoot LED Christmas strings

You could try one of these.
If pointed right at the Filiment of any Christmas String Bulb, it'll "sense" the AC if it's there.

Practice on a "good" string with a "bad" Bulb in a "known" location.


Christmas Light String Tester


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RE: how to troubleshoot LED Christmas strings

Diagram illustrates the GE 50 lamp using C7 lamps. The fullwave bridge rectifier and resistors are divided between the male and female ends of the string. Component failure results in loss of all lights. Testing of indivisual lamps is easy using the R1 scale on my ohmmeter which supplies 3 volts DC to the lamp and cause it to light.


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RE: how to troubleshoot LED Christmas strings

Do not keep single reverse blocking protection diode.


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