Making the LED knee region larger and more easily usable

 In an earlier blog post, I talked about the knee region. This is the area of an LED's voltage-current curve, where the current begins to increase sharply. For most 3v LEDs, the knee region is a fairly small voltage range. Controlling the knee region requires a very careful adjustment of voltage.

 There is a way to expand the knee region: connect multiple LEDs in series !

 All the LEDs voltage aspects are cumulative, when connected in series. So the overall vFwd increases by the number in series, but so does the voltage range of the knee region. The basic assumption here is that all the LEDs you are connecting in series have the same data sheet part description, and also they were binned together voltage wise. A single LED might have a knee region that is only 0.1v-0.2v wide. By placing five in series, the voltage rage of the knee region now becomes a range of 0.5v-1.0v wide.

An additional advantage of five (or six) in series, is that you can power it with a 15-20 volt supply, by way of a single LM317/AMS1117 style regulator (implemented as a two terminal current controller). A 5s lithium-ion source (which is what a typical 18v or 20v tool battery contains) is an ideal power supply.

 Certain LEDs rated at higher voltages (e.g. 6v, 9v, 18v, etc) are actually multiple LED emitters in the same device, and wired in series. One such example is Cree J Series® 2835 6‐V, 9‐V & 18‐V LEDs. The 18v version is actually six emitters in series. These have been successfully driven in the 15.6v area, and at low currents (0.8-1.2 mA). Running them at the low current generates less light, but also generates almost no heat.

 

Enjoy !