Introduction: A Different Way of Fixing a Christmas 3-wire LED Lights

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About: I am a semiconductor engineer. I do a lot projects as a hobby, such as robotics, programming, microcontroller, 3D printing, CAD modeling, and CNC.

This is additional information of how to fix Christmas LED lights different from the instruction provided by the tool.


I purchased the LED Keeper tool and initially I thought the tool did not work as advertised. By understanding how LED worked, I figured out how to fix it differently from what the instruction.

Supplies

  1. The LED Keeper which you can get from a home improvement store or Amazon: https://www.amazon.com/LED-Keeper-Light-Repair-Tool/dp/B007H4I8HY/ref=sr_1_3?crid=3HG29CVNU47BG&keywords=fixes+christmas+led&qid=1699825952&sprefix=fixes+christmas+led%2Caps%2C150&sr=8-3

Step 1: Optional: Understanding How an LED String Works

  • The picture shows typical 3-wire LEDs, in this case there are two independent groups, left half and right half. When one (or more) LED in a group is bad, the entire half fails.
  • When you trace the wires, they are grouped by 3-wire, adjacent groups are separated by 2-wire.
  • LED is semiconductor diode device. The electrical current flows one way to light it.
  • The top of the 2nd picture shows the electrical from your home receptable, it is a full wave AC 120V (in US).
  • The bottom of the 2nd picture shows the electrical current flowing through LED. Because LED only allow current to flow one direction, it only gets half wave.
  • Let's say each half has 30 LEDs (total of 60 LEDs). Since they are in series that means each LED get 120V/30 = 4V. Then it is divided by two because it is half wave (50% duty cycle in engineering). In this case effectively 120/30/2 = 2V.



Step 2: The LED Keeper Instruction

The original instruction says, the way I understand it, starts from the first unlit LED next to a lit LED, see the picture. If probing it using the LED Keeper light the LED, then move to the right and so forth to find a bad one.

Step 3: What Is Wrong With the LED Keeper Instruction?

The LED Keeper method of tracing a bad LED assumes the current flowing to the left, see the picture. I found this was not the most efficient way. In my case the LED current flowing to the right instead.

Step 4: More Efficient Way to Trace Bad LED(s)

Without knowing which direction of LEDs currents are flowing, I found the best way is to probe from each end:

  1. In the picture, it starts from "PROBE 1". If it lights the LED, move the next right LED.
  2. If "PROBE 1" above fails, start from the far-right side. If it lights the LED, move to the next left LED.

Why is this method more efficient? It is because step 1 above, if you guess incorrect LED direction, you may be probing one by one all the way to right side. Or you may think the first LED of the step 1 above is bad.


In summary, on a bad half string of LEDs, start from one end. And if it fails, the next thing is to start from the other end.


To fix a bad one, you can follow the LED Keeper instruction, it could be the LED or corroded socket.

For a corroded socket alternatively, you could cut the socket wires off and short/tie the wires together. In this case the voltage of each LED will be higher therefore brighter. Do not short too many, you will burn the LEDs because the voltage on each LED may go too high. A little brighter LED may survive better in a colder climate.

Step 5: Alternative Starting Point

Instead of probing each end, another starting point is finding the direction of the electric current. The picture shows the LED bulb tester, (+) and (-). The current from from (+) to (-). The first LED to probe is the end with (+) sign.