I have a really cheap COB based lantern. It has three COB strips in parallel, each with 10 individual LEDs within. It doesn't appear to have any current limiting resistors built in, unless they're built into the cob strips and I just can't see them. It takes 3 AAA batteries. When powered, the voltage across the batteries reads about 3.2vdc. Did they just make it knowing about what the typical voltage drop from batteries is? There are billions of these lanterns in every store and online so maybe they created cob strips that were purpose built to be driven directly from 3 cells? I would like to add a DC coaxial barrel jack to power it and use a standard usb cell phone supply and a usb to 2.5x5.5 barrel plug cable. When I power the lantern from my benchtop supply, it pulls 800ma at 3.2v. So if I start with a 5v usb supply, that ends up being a really big resistor, like 2.5 watts. That seems like a lot. So one thing I'm wondering is when I calculate the supply voltage for a current limiting resistor, do I start with the open circuit voltage of the power supply or the voltage when applied to the circuit? I would assume the open if this already had a limit resistor and I was just upsizing it to step up to a 5 volt supply, but since there are no resistors already, I'm not sure how to tackle this. I'm sorry for being such a noob!

 

LED manufacturers bin LEDs ;according to forward voltage at a specified current.



You can see from the chart below that white LEDs look a little more like resistors than those for longer wavelengths. This characteristic means that white LEDs experience a lower rate of current increase with increasing voltage, letting the manufacturer get away without current limit resistors. Besides, if the appliance is not used very often and can last a past the warranty period, that's good enough for manufacturers.

 

Interesting, I didn't know there was a difference based on wavelength. Thanks for the info.