LED strings and power supply voltages

[David_AVD]

Over the past few years, there have been many discussions about what voltage to run various brand / colour LED strings from. LEDs are current driven devices and can go from off to smoking debris with only a small increase in applied voltage.

Some time ago, I flagged the pitfalls of using LED strings on power supplies of much higher capacity than what they shipped with. It's my belief that a lot of LED set manufacturers use the output impedance of the tiny plug packs to limit the LED current. Substitute a grunty PSU (with a much lower source impedance) and suddenly there's nothing to limit the LED current!

The other way manufacturers limit LED current is via the thin wiring used. Once you cut off the "lead in" cable and substitute something heavier / shorter, you've removed some of the resistance they were probably counting on.

LED strings really need a series resistor for each "section". This will help equalise variations in LED voltage due to temperature, colour and batch parameters. Some LED strings have these resistors inserted inline with the wiring and some don't.

Anyway, just my ramblings to provoke a bit of thought and discussion.

 

[AAH]

What this really means is that if you are not using the original plugpack that came with the set of lights you are going to have to check to see if there are series resistors per led or per led segment or you will possibly have catastrophic and possibly combustible failures. A string that works well on the original plugpack when modded up to shorter strings or when hooked up to a dimmer pcb may blow up because there is no or insufficient current limiting. If you're going to mod strings from original then you're going to have to check the suitability. Popping into the chat and asking a few questions will possibly save you some potentially expensive heartbreak.
As a starting point if you're going to do some mods then you may want to add a series resistor to each channel as a safety fuse for short term testing. Maybe add a 100ohm 1Watt resistor. The resistance an wattage will actually depend on the number of leds in the strings but 100 ohms will give you some protection as a starting point.