Low voltage Leds for serial loop.

[MuntyScruntfundle]

Hi folks. I'm guessing this forum is for larger electrical applications than this, and if anyone can point me to a more relevant site please do. But I thought I'd give you guys a punt.

I have a need to put 5 leds in a 1:10 remote controlled vehicle. There isn't room (without it looking really naff) to run5 leds each with it's own resistor, so I need to put them together in serial. I have no problem with this, but the kicker is; I have to run them from a 7.2v battery.

The lowest voltage leds I have tucked away in the toolbox are 3v. So that isn't going to work. I can't use those strip leds as I need to put a single led into a single miniature model light.

I also have to add there is next to bugger all space in this model where I can tuck things away unnoticed.

Anyone have any ideas?

Many thanks.

 

[Lucien Nunes]

The forward voltage of an LED depends on the colour - shorter wavelengths need higher voltages, and white LEDs are based on blue chips. Therefore they need >3V and there is no way around this. You will either need to produce somewhere around 20V with a step-up converter, and run them in series, or better use something like two MAX1916 3-channel PWM programmable constant current drivers. See: https://datasheets.maximintegrated.com/en/ds/MAX1916.pdf

This would involve running one wire back from each LED that could be tiny - an enamelled 30SWG wire would do - to the driver. They are only 3 x 3 x 1mm thick, so not difficult to hide, and the programming resistor can be tiny - an 0201 size would be fine (so small you will struggle to solder to it).

There are many alternative drivers, falling into two main groups - ones for single-cell applications that include a charge-pump step-up, and those for higher voltages that run direct from the supply which is the flavour you want.

 

[HandySparks]

Surely a ¼W resistor for each LED won't take up much space? The resistor doesn't need to be located near the LED.