Some months ago a local store had a whole pile of these AAA battery powered LED lamps that you could mount pretty much anywhere:
They weren’t all the same – some had contacts for three AAA batteries, some had contacts for four, and there were a couple of different brands. They all looked pretty much the same, though, and were surprisingly bright (some had cheap batteries in them from the factory I guess) and really, REALLY inexpensive.
I bought a few of them, took them home, and puttered around with them. Some had a resistor in them and some didn’t. I tried running one off my bench supply set to the right voltage and they started to smoke. Even with a set of AA batteries instead of AAA they got pretty hot. So… not only were they cheap, they were counting on folks using AAA batteries with a particular current capacity/internal resistance. Not sure if that’s clever engineering or just super cheap.
I was really impressed with those Chip on Board (COB) LEDs, though. Even with the batteries from the store in them they were crazy bright. I went back to the store and bought out the rest they had, thinking I’d take them apart and use the LEDs for other purposes.
And then they sat.
Fast-forward to a couple of days ago. I have this snazzy little gooseneck-mounted magnifier and wanted to turn it into an illuminated one. I tried a couple of different approaches and wasn’t happy with them. Then I remembered those switch lamp things.
Taking them apart is easy, it’s just six screws on the back (two are under the sticker), and then the whole thing pretty much falls apart until you’re left with this:
A slide switch, some wires, two COB LEDs mounted to a plastic reflector, and sometimes a resistor (this one didn’t have one).
Snip the wires and the plastic “rivets” and the LEDs come off easily:
Notice how there is one red wire, one blue wire, and two white wires? There is a positive and negative terminal on both ends of the LEDs. Both positive terminals are connected, as are both negative terminals, so you can chain the LEDs together however you want. In this lamp’s case, they were wired in parallel.
There are two markings that I could find on the LEDs in this lamp and on several others that I tried – “10YQM” and “NT64”:
Each COB has ten LEDs in it and the back of the COB is a nice little aluminum strip that will carry away some heat. Unfortunately, I had no idea how much heat, or how much current would generate how much heat, because I couldn’t find any datasheets (or even information) on these devices.
So, bench time! I set up an LM317 as a constant current source and, starting from 50mA, slowly increased the current until the voltage across the device stabilized, then increased it some more until the aluminum got just slightly warm to the touch. What I ended up with were the following numbers (tested on six of these devices):
10YQM/NT64 COB LED
Forward voltage: 2.7-2.8V @ 150mA
Forward current (sustained, no external cooling required): ~150mA
Maximum current tested: ~350mA for less than 10 seconds (got hot pretty quick)
I’ve had them running for five hours at a time at 125mA with no problem, and at that power, four of them are plenty bright enough for a desk lamp:
They’re wired in series and held on with some masking tape (you can see it in the picture) but I’m printing a proper holder for them right now. Maybe it’ll be the subject of another post.
Anyway, if you have any of those LED switch lamps or some of those 10YQM/NT64 COB LEDs, good news – they’re nice and bright and easy to use!