LED Lighting: High-Power Drivers

Luxeon LEDs, like all high-power LEDs, require constant current drivers to power them properly. This can be accomplished using resistors just like traditional LEDs, but the current draw created by these LEDs usually requires large resistors and ends up producing a lot of heat (wasted energy). For maximum flexibility, drivers with large input variances are desired but fixed input drivers can just as easily be used when a known fixed voltage will be used for the input.

DIY drivers:

LM317

The simplest of all constant current drivers is the LM317 driver. It's not very efficient, but if you are using a plug-in power supply it should not be a problem. I have yet to actually build this type of driver but my guess is if you stray too far from the target LED voltage + 3 volts the LM317 chip will get VERY hot. I may have to experiment with using this setup for driving LEDs in parallels of 3 for spotlights. My understanding is this is the principle circuit behind the Xitanium drivers. Input rages for this circuit are +3 vdc above the voltage requirement of your LED array to +35 vdc. Also needing consideration is that the back plate of the LM317 chip (the metal heat sink) is positively charged at the input level so if you are not using a circuit board you need to be careful what may touch this plate. If attaching a heat sink it's a good idea to use an isolation pad. For a more in-depth overview check out this publication by OnSemi.

NUD4001

The second driver I have to share is based on a NUD4001 driver IC. It is the second simplest circuit I have found to power high Power LEDs. Using the above values will produce a driver that has a constant current of 350 mA. Maximum input voltage is 18 vdc. The additional pins (J1-1, J1-2, J2-1, J2-2) are for adding an optional dimmer control (below). There is nothing proprietary to this board and the full details for this driver can be found Here. The downfall to this driver is, like the LM317 the larger the difference between the input voltage and the LED voltage, the hotter the TIP42C transistor. When using this driver to power a single Luxeon I LED with a 12 vdc power supply, a large heat sink is required on TIP42C. If using the same power supply but powering 3 Luxeon I LEDS wired in series, no heat sink is required. In fact, TIP42C can be eliminated completely. The other issue with this driver is the NUD4001 IC. All the parts for the LM317 driver can be found at your local Radio Shack. The NUD4001 IC is only available in small quantities (of 25) from OnSemi. Also, the IC is a surface-mount component. Not a suitable job for a novice solderer or circuit etcher. Even with these draw-backs, it is a much more stable, flexible, and efficient driver than the LM317.

Dimmer circuit for NUD4001

To add to the allure of the NUD4001 driver, OnSemi has provided a dimmer circuit example as well. After all, what good is a blindingly bright LED if you can't dim it down when needed? Since I have times when I know I will need to dim an LED lamp and times I know it will always be on full-bright, I have separated out the dimmer from the driver and made it an optional add-on board. If you want to use it, it simply plugs onto the header pins on the driver. If you don't need it, put a standard PC jumper on J2 of the driver.

So, what can we do with these circuits? To start off, the NUD4001 driver circuit has a bridge rectifier. This allows the driver to operate from either DC or AC power. This gives us the ability to use a standard $20 outdoor landscape light transformer to drive our LEDs, or a car battery, a smaller 12vdc alarm backup battery, or even a 6 vdc lantern battery if driving only a single LED. The dimmer allows us to do two things. 1) Obviously, control the intensity of the LED. 2) If we combine a Red, a Green, and a Blue LED (all driven by their own driver and dimmer), we can adjust the intensity of each LED and therefore change the resulting output to ANY color we want. With some minor changes, the dimmer can even be used as a strobe controller (more on that in the future - and maybe a board).

Commercial Drivers

Lastly, there are commercially available drivers. While these drivers perform exceptionally well, they are exceedingly expensive. The starting price for a driver that will power three Luxeon I LEDs from a 12 volt (ac/dc) source and offers no dimming control is around $20. That is enough to buy the components for at least 3 of the above NUD4001 (with dimmer) drivers and probably 6 of the LM317 drivers.

There are times when commercial drivers are much preferred. If you need to power a single LED from a low-voltage source (2 AA batteries) there is a commercial driver that can do that extremely efficiently and is about the size of a pencil eraser. If you need a driver that can drive more than 3 LEDs then a commercial driver is a good option. If you plan on powering your drivers with line voltage (110 - 120 VAC) BUY THE COMMERCIAL DRIVERS! The drivers shown above are designed to run off of 12 vac and below and are relatively safe (just don't go sticking powered wires in your mouth). 110 vac is deadly. Some of you may find that OnSemi makes a high voltage version of the NUD4001 (model number excluded on purpose). I do not suggest you experiment with it nor do I suggest you develop your own driver with it (even if following OnSemi's schematics) nor do I suggest you expose your guests to it. Doing so will expose you to lethal voltages while testing and could expose your guests to lethal voltages while in use.