With the cost of high-brightness LEDs coming down, Andrew Morris decided he wanted a dimmable LED desk lamp. Yet he found the ones on the market were very expensive, and few of them were dimmable. So he decided to use his engineering skills to build his own.
He installed his circuit into a fluorescent desk lamp he had picked up years ago at a flea market. He also discovered the LED driver circuit was dirt cheap and simple to assemble.
Andrew Morris designed a dimmable LED driver circuit that is simple and energy efficient. He then installed the circuit into a portable fluorescent lamp.
The dimmable LED driver circuit inside the desk lamp.
No, I see no problems with what you want to do. If you are running the LEDs at 20mA and have no more than 126 volts worth of LEDs, you can put them in series and probably don't need to tweak anything. If you can, bypass the series resistors in the prefab LED strings to save the wasted power. You would probably use the circuit in figure 1 of the article. If you don't mind running a third wire to the LED string, you could use one of the LEDs as the voltage reference as in the lamp and in figure 2 of the article. R4 has a lot of adjustment range to it. BTW, you can power less than 7 LEDs with the circuit, but the dimming operation will be very abrupt, due to the steep slope of the rectified sine-wave at this low voltage.
It's a practical device that anyone could use. Doesn't look too difficult to build, either. I think I'll modify my wife's floor-standing lamp. Her lamp as it is now either has a hot glaring incandescent or a lousy CFR. This might be just what she needs.
Not easy to bypass the resistor, but it doesn't drop a lot when powering modules at 12VDC. 36 LEDs would be 12, 50mm modules, so I'd put 6 in series for 72V, and run two strings.
A small (5VA or so) transformer with dual primaries costs less than $5 (see DIgiKey 237-1042-ND, e.g.) and cna be used as an isolation transformer, probably compact enough to build into the base. Leave the low voltage seconary winding(s) open, and use one primary as the output.
dbell5, Yes, if someone can touch the uninsulated LED strip, you definitely need some kind of isolation transformer. Thanks for the cheap alternative suggestion.
BTW, if you cannot bypass the resistors in the prefab strings, you will have to add a complement to R8 and D9 to all secondary strings. If you can tap into it, you may be able to use the first resistor in one string as R8 and the first LED as D9. Then you could just parallel all additional strings with no added components.
Thanks! I think Radio Shack should re-define itself as the Geek Shop or the Robo Shop. They should sell Sugru and 3D printers and quadracopters. Forget the consumer electronics I can buy at Best Buy. Radio Shack could hold weekend gatherings where geeks could teach geeks. In the 1970s I bought vacuum tubes to repair trashed TVs so I could resell them. Radio Shack and all similar companies should sell to us Makers. Real Guys don't do woodworking and auto repair anymore.
The circuit is far too complex. All it needs is a capacitor-fed supply with 3 or 4 capacitors and a rotary switch. Or you can use less capacitors and a full / half-wave set of diodes and some switches.
I've tried the technique you're describing and it has flaws. Potentially destructive current spikes flow as you switch from one capacitor to another. Even if you have an electrolytic capacitor across the LED string to absorb the current spikes, they will eventually damage the switch contacts. Solving this problem would make the circuit more complex than the one featured here. Also, you would not have continuously-adjustable brightness control. Also, a circuit with several non-polarized, high-voltage capacitors will get pretty large, especially when powering a large string of LEDs.
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