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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: 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
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
Platinum
Re: Dimmable LED lamp
12/5/2012 11:11:09 PM
NO RATINGS
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
Platinum
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: Non polarized cap
12/5/2012 1:44:07 PM
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Here is a 2.2uF, 250v cap from Allied. 70189744 \$1.75 in single quantities. I used it in one of my other projects.

User Rank
Iron
Non polarized cap
12/5/2012 1:13:06 PM
NO RATINGS
You can take 2 polarized caps of double the desired value and connect them in series such that both -Ve terminals are connected together. The resulting equivalent capacitor can be used as a Non polar capacitor of desired value. I would use rated voltage on each to be safe and also I have always connected only -ve terminals togther although it may work with both +ve terminals connected as well.

User Rank
Iron
Re: Overly Complex
12/5/2012 1:05:49 PM
NO RATINGS
Always look on eBay for everything.

The following is just \$6.63 POST FREE:

User Rank
Platinum
Re: Overly Complex
12/5/2012 12:39:26 PM
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Collin55 and anyone else -- Does anyone know a source for non-polar capacitors in the 2uF to 10uF range? I'm having trouble finding them?

User Rank
Platinum
An even simpler LED driver
12/5/2012 12:28:26 AM
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If you want to drive just a few LEDs, here is the link to a 20mA LED ballast circuit that I created many years ago and still use nightly to drive three white LEDs. It's even simpler than the featured circuit, but it's not suitable for large strings of LEDs. Please put a 100 ohm, ¼ watt carbon composition resistor in series with it to limit in-rush current and to act as a fuse in case of a short. Mine has one, but I don't know why it's not in the schematic. The website has not been responsive to changes or new submittals. Put a low-value pot in series with R2 and R3 if you want dimming. Like the featured circuit, this circuit is not isolated from the power line.

http://www.discovercircuits.com/Andy/20mALEDballast.pdf

The SCR can be deleted if the LEDs are permanently wired and you don't need dimming. In my application, the LEDs were switched off after the ballast circuit, requiring the SCR voltage limiter to keep C2 from exploding. You can replace the SCR with a zener diode, but then the circuit would consume even more power when the LEDs are turned off. The LED current will be approximately 20mA per microfarad of C1. The LED current will go down, requiring a larger value for C1 if a significant number of LEDs is added. Also, the peak SCR current will increase as more LEDs are added.

User Rank
Platinum
Re: It's just what I need.
12/4/2012 9:40:03 PM
NO RATINGS
Hi, John, You don't seem to realize that Radio Shack is not "making money."  It's losing money and its stock has fallen from \$13 to \$2 a share in the last year. It's fighting for its life!  It's business model needs to change fast. People can buy cell phones in lots of places.  I want Radio Shack to *be there* in two years as a brick and mortar store and that's why I offered suggestions.  They need to mix Sparkfun, Adafruit and brick and mortar into a lively model.

Keep up the good work on your electronic designs.  I'm working on something new myself.

Jerry

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