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

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armorris
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Platinum
Re: The lamp is still available
armorris   12/21/2012 12:29:45 PM
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I just modified the lamp from Amazon. The head is almost identical to the one in the video, but the design of the weight and the ballast coil in the base are different. You have a lot less room to work in the new base, but because the electronics is so simple and produces no heat, I was able to do it. Amazon is currently out of the lamps, but will email me when they come in. I'll keep this blog apprised. Then you can email me for a photo and a layout drawing of the new circuit board.

Cadman-LT
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Platinum
I want one
Cadman-LT   12/10/2012 3:05:12 AM
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I'd like to have one of those!

cvandewater
User Rank
Gold
Re: An even simpler LED driver
cvandewater   12/8/2012 12:56:03 PM
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Hi Andrew,

Yep - born & raised Dutch, but now Californian resident.

Clarification for others not familiar with the term - Elco means Electrolytic Capacitor. I was not aware that it is a typical Dutch expression.

Re: isolated, I referred to the housing, indeed the electronics are "hot" so it is required that it cannot be touched anywhere, including at openings for trimming pots to select sensitivity and timeout of the PIR detector. Unfortunately that are typical locations where (rain)water gets in, causing the demise of the detector. Usually it first starts acting weird, switching the light on and off randomly before finally failing completely.

"geisoleerd" is indeed the Dutch word for "isolated" and essentially means separated, so this can mean either the galvanic isolation from the grid or the protection of a "hot" circuit from touch by a non-conducting enclosure. So a further qualification is needed and I forgot to re-read my typing so I did not catch that my statement was ambiguous.

Most likely you found the same type of rope light that I had a section fail in.

armorris
User Rank
Platinum
Re: An even simpler LED driver
armorris   12/8/2012 9:37:12 AM
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cvandewater, I see from your name, that you must be Dutch. The last time I heard the word "Elco" used to reference an electrolytic capacitor was during my military service in Holland 38 years ago. I was in my early 20's. I learned the language and I had the time of my life. Holland was a great place to be if you are into electronics.

You mentioned that similar capacitor coupled power supplies are used in commercial products that are isolated. What do you mean? PIR motion detectors are not isolated from the power line (mains), but as with all electrical appliances, are insulated from the user. My Dutch is rusty, but I think the word "geisoleerd" would be used to describe both, but they are not the same in this context. The output of such a power supply is not galvanically isolated from the mains.

Also, I think I saw a Chinese-made LED rope light in a store that used a series string of white LEDs wired in anti-parallel configuration, like you mentioned. I couldn't see inside the molded-on power plug, but there wouldn't have been room enough inside it for anything but a series resistor. It had no visible electronics.

cvandewater
User Rank
Gold
Re: An even simpler LED driver
cvandewater   12/8/2012 6:49:58 AM
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Similar circuits are used in commercial products that are isolated, such as PIR motion detectors in outdoor lights (I had one that stopped working, a great way to learn how the darn thing is designed)

That circuit is actually simpler - after the 100 Ohm that limits inrush, it has a film cap and a bridge rectifier with a zener and Elco at its DC outputs, that is all.

With the right value film cap you can select the current, I once built a 40 LED light with the circuit, omitting the zener and using a 200V Elco.

I like to make things as simple as possible while retaining safety and functionality. This dimmable light not only has a cool factor but also re-purposes a desk light, one of the other things that I like.

Maybe I will try a design that I have been toying with for some time, by having 2 LED strings anti-parallel, so you'd only need the 100 Ohm limit resistor and foil cap. Maybe have a gas discharge tube (that I have in abundance from line protection units) across the LEDs to catch any spikes that could otherwise ruin them. These 3 component is about as simple as it gets while keeping things work reasonably well, unlike the cheap Christmas light LED string that simply has a series of LEDs hard-connected to the incoming AC, which explains why part of that string it already dead (one series string apparently did not survive a spike).

armorris
User Rank
Platinum
Re: Drawbacks?
armorris   12/7/2012 10:06:54 AM
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Tool_maker, Thanks for the kind words. White superbright LEDs really have gotten cheaper. Most, if not all, are made overseas. I've seen a 27 LED flashlight for $5. All the parts in this project came from Allied Electronics but few, if any, were made in the US. The LED manufacturer has a California address, but I've seen almost identical LEDs elsewhere, for the same price, made in Taiwan. The brightness control pot (R6) is made in Mexico. I think only the 10-ohm carbon composition resistor, R1 is made in the US. That resistor costs $0.45 at Allied, whereas the other (foreign-made) resistors cost $0.015 each.

We can't have it both ways. We can't have cheap products made in the US by people earning high wages.

Tool_maker
User Rank
Platinum
Drawbacks?
Tool_maker   12/6/2012 8:06:33 PM
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  That is one cool light you made and I am impressed by your apparent knowledge of all things electronic. What you did is way over my head. My questions: Whenever I read about alternative lighting LEDs are alway associated with high cost, yet you appear to be able to construct this light for a very reasonable sum. Are they coming down in price or are they only cheap for an EE that has the knowledge to build them? How many of the components are made in the USA and are American made components availble? If so, how would that impact on the bottom line?

Charles Murray
User Rank
Blogger
Re: You could create your own lamp.
Charles Murray   12/6/2012 4:52:37 PM
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I agree, Rob. A high percentage of today's light strings use LED technology. Clearly, it's gone mainstream.

armorris
User Rank
Platinum
Re: Overly Complex
armorris   12/6/2012 12:11:21 PM
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Excellent answer, Sparkz.

Adding series and parallel resistors was what I meant when I said that fixing the peak currents will complicate the circuit, especially if you have a lot of steps. And yes, the capacitors will not be very big unless you have a lot of them. While there may be many different ways to power and dim LEDs, I can think of no other continuously dimmable approach with this combination of simplicity and energy efficiency.

Sparkz
User Rank
Iron
Re: Overly Complex
Sparkz   12/6/2012 10:48:03 AM
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@colin55,

Using the rotary to switch through different current limiting caps would work but the author is right, the caps would hold a charge.  So when switching from one to another, the peak (in the US) is 170V and if you switch back to that cap when the AC is at the opposite peak, it's now trying to switch a 340V potential with a large rush of current, which will pit and eat contacts.

 

Just add a 15k resistor across each cap to bleed it down and you are good to go!

 

This could help...

http://planetcalc.com/1980/

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