Repair - Multi-purpose Fault finding method for LV LEDs

Here I'll show you a second example of fault finding method for LED strings supplied from a transformer and rectifier, ie low voltage. This shows the method's veratility and effectiveness.

General info:

• LEDs only work on DC so there must be a rectifier to convert AC to DC;
• Usually the supply voltage is 24V, so each series section will contain between 7 to 11 LEDs depending upon LED colours (remember red, orange, yellow LEDs about 2.0V each; remaining colours about 3.3V each). So if there's 70 LEDs in total, there could be 7 parallel groups of 10 LEDs in series;
• To determine the number of series LEDs, energise the LED string and mark the unlit LEDs with tape and count them. This will help in fault finding.

Method 3: Example 2

Summary of Divide & Conquer method:

The principle is to connect a resistor to bypass about 50% of lights in the suspect group and see if the remaining group of lights glow when light string is energised.

How does it work: the resistor makes a parallel path to the bad LED and electricity can flow in remaining 50% of lights which obviously light up (the red line in the string below shows the current path.). This process is repeated until the bad light is found.

Here's a detailed example of how to use this method. For simplicity, the good, parallel sections are not shown. The diagram also shows how to test a LED with a DMM. Note don't test a LED with a battery - it needs a current limiting resistor to stop it blowing up!

Note these diagrams show a LED fitted into a base fitting. Before removing LED, make an alignment mark on the LED holder and its base (see photo). When inserting the LED holder into base, ensure the 2 marks align. Otherwise the LED and its section WILL NOT work because the LED is reversed connected.

Now if the LEDs are directly wired, you have the choice of cutting wires (and resoldering) or inserting a sewing pin into the wire to get a connection (afterwards apply silicone to hole to seal it up). The choice is yours.

Red line is direction of current flow.

Resistor test lead fitted with 3 metre lead + crocodile clip each end (to suit):

–     12V LEDs – 2 of 1200 Ω 1/2W connected in parallel (= 600Ω)

–     24V LEDs – 1 of 820 Ω 1W

Refer to Replace LEDs section for info on replacing LEDs. You will do it right and not have more blown LEDs.