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

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Andrew Morris designed a dimmable LED driver circuit that is simple and energy efficient. He then installed
the circuit into a portable fluorescent lamp.

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User Rank
Iron
Re: Dimmable LED lamp
12/5/2012 6:07:08 PM
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I didn't follow up on your last comment, about adding a complement to R8 and D9 to paralleled strings. This would just be a series LED and resistor, not tied into the FET drive, right? To balance the voltage applied to the strings?

Dave

User Rank
Platinum
Re: Dimmable LED lamp
12/5/2012 11:11:09 PM
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Use the circuit in figure 2 as a reference. The strings are essentially tied in parallel. R8, D9, D10, etc is the primary string and R10, D46, D47, etc are the secondary string. Both strings connect to the source of the MOSFET, Q2. Therefore, the voltage applied to all strings would be the same and if the strings are identical, the current through them will also be the same, even though only the current through the primary string is sensed (by the voltage across R8). D9 is being used as the voltage reference to control the voltage across R8. All added strings would be connected in the identical way as the string containing R10.

The voltage on R8 is sensed by Q1 through R2 and R3 while Q2 is switched on. The switching threshold is set by the voltage on Q1's emitter.

User Rank
Iron
Re: Overly Complex
12/6/2012 9:47:01 AM
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Sure- here are some caps in that range (rated for atleast 120VAC) http://tinyurl.com/aj8v59b

User Rank
Iron
Re: Overly Complex
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/

User Rank
Platinum
Re: Overly Complex
12/6/2012 12:11:21 PM
NO RATINGS

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.

User Rank
Blogger
Re: You could create your own lamp.
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.

User Rank
Platinum
Drawbacks?
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?

User Rank
Platinum
Re: Drawbacks?
12/7/2012 10:06:54 AM
NO RATINGS
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.

User Rank
Gold
Re: An even simpler LED driver
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).

User Rank
Platinum
Re: An even simpler LED driver
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.

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