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Gadget Freak Case #230: The Inexpensive Dimmable LED Desk Lamp

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colin55
User Rank
Iron
Re: Overly Complex
colin55   12/4/2012 5:54:13 PM
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"Potentially destructive current spikes flow as you switch from one capacitor to another."

We switch incandescent globes with switches all our life and I have never changed a light-switch.

"Also, a circuit with several non-polarized, high-voltage capacitors will get pretty large, especially when powering a large string of LEDs."

A 22n  200v X2 capacitor is not very large. It will deliver 10mA for low illumination.



armorris
User Rank
Platinum
Re: It's just what I need.
armorris   12/4/2012 5:01:44 PM
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John Duffy,

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.

John Duffy
User Rank
Gold
Re: It's just what I need.
John Duffy   12/4/2012 4:56:03 PM
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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. 

armorris
User Rank
Platinum
Re: Overly Complex
armorris   12/4/2012 4:42:21 PM
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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.

colin55
User Rank
Iron
Overly Complex
colin55   12/4/2012 2:49:30 PM
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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.



78RPM
User Rank
Gold
Re: It's just what I need.
78RPM   12/3/2012 9:26:28 PM
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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.

armorris
User Rank
Platinum
I found a cheaper isolation transformer
armorris   12/3/2012 3:48:33 PM
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Allied has a cheaper 50VA isolation transformer. 70218526. It costs just $11.66. I didn't see that when I wrote the article. That's the cheapest isolation transformer they have.

armorris
User Rank
Platinum
Re: It's just what I need.
armorris   12/3/2012 3:17:03 PM
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78RPM,
BTW, Radio Shack sells round circuit boards that may be ideal for converting an incandescent or CFL lamp to LEDs. The SKU# is 276-004.

armorris
User Rank
Platinum
Re: Dimmable LED lamp
armorris   12/3/2012 3:11:39 PM
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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.

dbell5
User Rank
Gold
Re: Dimmable LED lamp
dbell5   12/3/2012 3:04:21 PM
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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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