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

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armorris
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Worn pot may destroy LEDs
armorris   9/9/2016 8:07:01 PM
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When the dimmer pot starts to wear out or oxidize, the wiper might lose contact with the resistive element, causing the LED current to flow uncontrolled, damaging the LEDs. To prevent this, add a silicon signal diode, such as 1N4148 between the wiper (anode) and the high side of the pot (cathode). In normal operation, the diode will be reverse-biased and will have no effect on the circuit, but if the wiper loses contact with the resistive element (assuming the resistive element is intact), the diode will become forward biased, limiting the amount of current that can flow through the LEDs. They will glow brighter than normal, but probably will not be damaged in the short term.

armorris
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I found an error
armorris   3/31/2016 11:16:18 AM
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If you use a zener diode as the voltage reference instead of an LED, put a forward-biased signal diode like a 1N4148 in series with the zener diode to block a sneak-path that slows down the startup of the circuit. The LED would be back-biased during start-up, blocking the sneak-path. Note that this turn-on delay might not be noticeable unless the electrolytic capacitor is discharged. It does not have a bleed resistor, so it can hold a charge for days, once the voltage has dropped below that required for the LEDs to conduct.

Cadman-LT
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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
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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
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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
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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
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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
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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
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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
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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.

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