LEDs and LED driving for RC and other applications

I bought a batch of 3W red, green, blue and white LEDs and lenses from China a few months ago to supply with RC LED controllers. But haven’t done so yet, mostly because explaining high power LEDs and how to drive them is more difficult than you would think.

Once the current gets up a bit, driving LEDs is much more complicated than it seems, especially when small-size, light-weight and a variable supply voltage are added in.

All LEDs are current, not voltage driven. The resistor is just to limit the current. In the process the resistor drops the extra voltage and gets hot. Volts dropped x Current = Watts. One or two watts will quickly cook small components.

A 3W LED running at 3W needs a pretty good heatsink; there is almost 3W of heat to get rid of just from the LED.

A LiPo varies from 12.6V down to say 10V. As the voltage drop across the resistor gets smaller, the resistor value and the power supply voltage become more critical. Low value resistors will result in the LED intensity varying as the battery voltage varies.

Putting LEDs in series reduces the voltage to drop across the resistor for the same current; reduces the resistor value and voltage varies intensity. Worst case (easy to do) with a low value resistor, the LED fails or something gets very hot.

Using a special LED driver (a constant current source) is best. But depending on the combination of LEDs and supply voltage is still not as simple as it seems.

Voltage regulators and constant current supplies need some voltage drop from input to output to work properly.

Assuming basic LEDs that need 2-3.5V ea. (depending on colour) and a 11V typical supply you can run 2 or 3 LEDs in series with a cheap adjustable UBEC type of voltage regulator and some low value resistors. The UBEC provides a constant supply voltage and allows smaller resistor values of lower wattage.

Voltage across the LED doesn’t matter, but you must apply more than it’s minimum required before enough current flows to light it up. The difference between just lighting up and going poof is very small when the resistor value is small; hence the need for a regulated supply voltage.

There are boards that look almost the same as these but are LED drivers.

Another problem is that if you put different colour LEDs in series, the same current flows through both, but they appear different brightnesses. You probably could put a resistor across the brighter one to shunt some of the current around it and reduce it slightly; but I haven’t tried this.

Highly recommend a current limited supply when testing LEDs etc. A LiPo will easily kill LEDs and drivers before you can react.