Hello everyone! I'm wanting to convert 40 fairy lights to USB and need to know what resistor I should use and where to place it as I am a noobie! I have 3 seperate sets (120 fairy total LEDs) of these that will plug into a 4 port USB outlet. Here are the specs of the USB charger outlet: total output of 5V/5.8A; quad (4) Type-A USB ports; up to 2.4A per USB port.

I bought three USB-A pigtails already with a two wire connection. Fairy lights are also two wire.

 

What are the specifications of the leds, like voltage, current rating , are they all wired to one plug ?

 

I couldnt find any specs for the fairy lights. There are 40 fairy lights connected together to a USB plug already but I want to cut the ends off because the PCB is programmed to only turn on by pushing a small button. I want them to always be on as long as there is power at the outlet. Im using a sensor switch that auto turns on/off. Fairy lights from Amazon are linked here.

 

Welcome to AAC!

I'm wanting to convert 40 fairy lights to USB and need to know what resistor I should use and where to place it as I am a noobie!

If the load isn't fixed, resistors are a bad idea. Do these LEDs blink or dim?

How are the lights normally powered?

 

We must GUESS that the warm white LEDs are about 3V and are all in parallel. Also guessing that 10mA on each LED is bright enough then the current for 40 LEDs will be 400mA and a series resistor for 5V USB is (5V - 3V)/400mA= 5 ohms. The heating in the resistor is 2V x 400mA= 0.8W then a 1W resistor will be very hot, use a 5.1 ohm 2W resistor.

 

THE GUY who is an expert on this subject is Big Clive on YouTube. He has converted fairy lights and battery LED strings to work off USB converters and power banks many times, and explains choosing the required resistor value clearly. Education, informative and for to watch.

Spend a few minutes searching for his videos and watching them on YouTube and you'll know everything you need to do this

 

I would use a constant current source like a LM317 regulator .

 

I would use a constant current source like a LM317 regulator .

That probably won't work with a 5V source. The regulator will drop a couple volts (depending on current) and the current setting resistor will drop another 1.25V.

 

Thee. TS has already mentioned that the strings run on a USB connection, and so it is quite likely that the power required is five volts. But then the TS also mentions removing the existing control PCB because that requires a button press to switch on.
So to provide any advice that is not just a guess we need for the TS to measure the voltage that feeds the LED string after that PCB, and also to note the polarity, to avoid damaging the strings.
So the TS needs to tap in to the wires on only one string and measure the voltage, and note which wire is positive. THEN there can be some useful advice. Until then it is all random guesses.