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.
The circuit in it's current form is not suitable for high current LEDs. I think that at 120 Hz, the electrolytic capacitors involved will be way too large. Also huge voltage spikes will be produced when the MOSFET switches off. It would be far better to use a high-frequency switch-mode power supply for high-current LEDs.
I kind of disagree for two reasons. One, they are making most of their money selling cell phones and cameras and stuff (think about it, how many vacuum tubes pays for one cell phone?), plus almost everyone buys cell phones, but fewer people buy non-consumer electronics. Two, they ARE shifting back toward that sort of thing, recently stocking arduinos, dozens of kits, forrest mims books, etc, in addition to all the components, tools, etc. they had before (I heard that they might even be selling rasberry pi soon, but I'm doubt it). Anyway, the point is that they can't just suddenly change their already very profitable buisness model, but they ARE expanding in the way you mentioned.
I'm not affiliated with Radioshack, I'm just easily annoyed by people who assume people, websites and stores can and should change their whole way of doing buisness when it's already working.
Aside from that, I think the project is great, but I do want to know if it will work with high power LEDs, such as several 3W 750mA type, or even a 10W 3000mA type.
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.
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.
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.
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.
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.
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