I made this and I used an old HP 721 power supply set at 31 volts to power the LEDs. The 8 LEDs on each side of the middle are in series and add up to about 25 volts. The two sides are then connected in parallel. The 721 PS is set for 30 mA constant current and the power supply adjusts the voltage to give about 15 mA per LED string.
I mounted the wood strip on a piece of 8 gauge solid copper wire, which I clamped to the edge of the desk. I can bend the copper wire like a gooseneck lamp to direct the light to wherever I want. The individual LEDs can be positioned (bent) to adjust the light pattern to an area a few feet away.
About the only complaint I had when I made it several years ago was that the price of good quality white LEDs was at about a dollar apiece. So this light cost $16 just for the LEDs. I think I paid less for the HP 721 PS if the shipping isn’t included. Well, actually, I did pay more for the 721 at first. I won the bid on eBay, and the seller claimed it worked. But the seller sold me a lemon. The voltage on the meter could be adjusted with no load on the output. But when I put the slightest load on it, the voltage would plummet from 9V for example, down to just a volt or so. It wasn’t regulating. I opened it up and put a 470 uF electrolytic capacitor across the main filter capacitor, and connected the load. Bingo! It was regulating great. So I soldered the cap in, and put some silicone seal to hold it in place. That was the nice thing about that ancient power supply: there was plenty of space inside for an extra capacitor.
I complained to the seller. I think he knew all along that there was a problem – in fact that was most likely the reason he sold it. So after a few haggling emails back and forth, he agreed to refund some of the money. So I guess the final price was a bit less than what the LEDs cost.
I checked eBay recently and I can’t believe that there are sellers trying to sell these ancient power supplies for a hundred dollars, and shipping brings the cost up to $120 or more. And to top it off, they stopped putting these up for bid; instead they sell at a fixed price (“buy it now”), with the “or make offer”. I see some that have more than one offer, but I’m certain that the offers are far below $100.00. Also, they say “The item may have some signs of cosmetic wear, but is fully operational and functions as intended.” and the power supply has a knob that is broken or missing. That’s a bit more than cosmetic wear!
The 721 also had a good, strong aluminum case. Then HP came out with the 6214 and 6216, etc. series of power supplies with a plastic case. These HP 6214 power supplies are built with all discrete semiconductors (no chips), and are well designed, but they have one fatal weakness. After 10 years or so the plastic turned brittle and if you try to snap off the plastic ring to open the case, it would most likely shatter. That’s why you might see these power supplies held together with duct tape(!) The case would break too. I won a bid for one 6214 from a stupid seller, who packed the power supply in a cardboard box that was not much bigger than the power supply. There was no padding around the power supply, so when I opened the package, I found a hole in the case big enough to put three fingers through, and plastic pieces all over. I took a picture and emailed it to the seller, and I sent it back and got a refund.
I won a bid on another 6214 power supply from a local seller, and I made arrangements to go over to his house and pick it up and pay for it. While I was there, I talked to him about the brittle plastic problem, and he said he knew about it, too. He showed me a couple power supplies that had a shattered case. We made a deal, and he sold them to me for some extra cash I had in my pocket. So I now have a couple spare power supplies laying in pieces in a box. Those are my spare parts if something goes wrong with the ones that are still in one piece. The problem is that it’s most likely that if I try to take one apart ti fix it, the case will splinter into pieces.
Other HP power supplies that I’ve bought on eBay have had the same electrolytic capacitor problem. After a dozen or so years the electrolyte leaks out and/or dries up and the capacitor acts like an open. A few dollars worth of electrolytic capacitors fixes the problem. But the sellers lie about the power supply working, and you won’t know until you plug it in and test it with a load.
Back to experimenting – and swearing that I will never, ever buy another used item from eBay.
