LEDs, or light-emitting diodes, have been available for use in electronics since the early 1960s. In the decades following, their capabilities and uses have exploded, allowing these devices to light city streets, illuminate flashlights for hours under battery power, and power televisions and computer screens in ways that would have been unimaginable not that long ago.
What is a COB LED?
One way that LEDs have changed since the 1960s is their miniaturization, going from through-hole components to much smaller surface-mounted devices (SMDs), and now to COB, or chip-on-board LEDs. While these larger (or less dense) styles are definitely not going away any time soon, COB LEDs present an incredible “lumen density.” In fact they are able to put out nearly 40 times the amount of light per surface area as through-hole technology, and almost 10 times that of surface-mount LED tech.
The secret to the COB LED’s incredible lighting capabilities is that one discreet solder-on module houses multiple LEDs. This means that the process of assembling an LED lighting setup doesn’t need to attach each individual LED. This is both time-consuming and comparatively inefficient space-wise, as a COB doesn’t have to individually package each diode. Here one can simply place a single component to install multiple LEDs–often hundreds–in a confined space.
Where to Use COB Lights
As with any technology, there are a few considerations that may make the use of COB lighting inappropriate. First, with so many LEDs packed into a tight space, one of these units can produce a lot of heat. While LEDs are very efficient compared to their incandescent cousins, the sheer number of individual light/heat emitters do add up (as we’ll explore later). Driving these devices can be complicated as well, since you can’t just straddle two leads between a CR2032 battery. You’ll often need to produce 30+ VDC to get a COB light working.
Another consideration is that there aren’t as many colors available as with traditional LEDs, and addressable or color changing features seem to be (so far) unknown with this emerging tech. A COB is also less of a point source than a single diode, so properly aiming a beam can mean a very large reflector assembly.
Finally, if you simply haven’t gotten your hands on one–fair or not–you might be hesitant to use them. In the rest of this article, I’ll explore a bit of experimentation that I’ve done with COB LEDs.
COB LED Disassembled
Intact COB, and unit with coating partially scraped away to reveal individual diodes
If you’re wondering where the actual diodes are on a COB, they’re just beneath the surface of a special phosphor coating that makes a COB look somewhat akin to a fried egg when unpowered. This helps them to emit the proper shade–or wavelength mixture–of white. Scrape this off, and you can see the LEDs beneath, though after attempting this twice, doing so is more difficult than I imagined, and there’s a distinct possibility that you’ll damage the COB in the process. Nonetheless, you can see the difference above, and observe just how many little lights can be crammed into a circle that in this case is about the size of a US quarter.
PWM Controlled Brightness
As noted earlier, given the number of LEDs involved, these COB units do get extremely hot, something I didn’t appreciate when first experimenting with them. In fact, I quickly destroyed the connection on the first unit that was tested. Eventually I used one of the smallest COBs that I’d obtained for this article (with what in other circumstances would have been a hilariously large heat sink). Also notable is that these units can be extremely bright. After some initial experimentation, testing was done underneath a diffuser shroud while wearing sunglasses.
The other consideration is that my usual power supply is only rated to 30V, while the units that I tested were spec’d to roughly 35V nominal. This meant implementing a supply capable of this higher voltage. With that worked out, I experimented with limiting the current to vary the brightness. This works to some extent, but was somewhat difficult to control with my particular setup.
PWM control setup
I then turned to PWM for control via an FQP30N06L MOSFET and an Arduino Nano Every. This was loaded up with a lightly modified version of the built-in “Fading” example, using pin 3 as the PWM output, attached to the MOSFET gate pin through a 200-ohm resistor. Cycling this MOSFET with a PWM “analog” output, with the COB power supply set to 34.5V, allowed the COB LED to ramp up and down as if it were a normal LED. I also added a 5mm through-hole LED directly to the output pin to mirror what was going on with the more powerful COB.
This worked just as it should have, as both the voltage and current are well within the FQP30N06L’s specs of 60V and 32 continuous amps, respectively. While no problem for this type of MOSFET, it should go without saying that an Arduino’s outputs aren’t nearly up to the task of supplying the kind of power required here. Also notable is that while nominally a surface-mount part, hand-soldering wires to the COB units tested was fairly easy.
Conclusion
Getting my “hands dirty” with COB LEDs, it’s clear that they are an amazingly capable for producing a very bright light from a small package. On the other hand, power requirements are a bit more demanding than single LEDs, and there are fewer color and technology options. If you do decide to perform similar experiments, be sure to take the necessary precautions, including protecting your eyes, and using a larger heat sink than you likely initially anticipate needing!
· LED Multiplexing with the MAX7219 Module
· Charlieplexing Tutorial: Charlieplexing an LED Matrix
· What is PWM? Pulse Width Modulation Explained
· Arduino PWM: Pulse Width Modulation in Arduino
