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Gadget Freak Case #230: The Inexpensive Dimmable LED Desk Lamp
11/30/2012

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The dimmable LED driver circuit inside the desk lamp.
The dimmable LED driver circuit inside the desk lamp.

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mrdon
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Gold
Re: Have you solved the power dissipation problem?
mrdon   12/3/2012 1:48:14 PM
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armorris, Sometimes the best solutions for today's tech problems are the "old school" circuits. Agreed, occassionally digital technology can create a problem as opposed to solving it.

armorris
User Rank
Platinum
Re: Dimmable LED lamp
armorris   12/3/2012 2:05:57 PM
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Hi Dave,

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.

78RPM
User Rank
Gold
Re: It's just what I need.
78RPM   12/3/2012 2:58:59 PM
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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.

dbell5
User Rank
Platinum
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.

 

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.

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
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.

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.

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.



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.

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