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Here's an intriguing project that comes with the usual cautions about not working with AC house power unless you know what you're doing, and an extra caution about the risk of burning your house down by trying to roll your own light bulbs. This builder decided to risk all that to reuse the "ballast cases" and Edison bulb screw connectors from dead CFLs (Compact Fluorescent Lamps) to create his own 7-bulb LED lights. Pretty cool, if you don't electrocute yourself, cut yourself, expose yourself to Mercury, or burn your house down. Oh, and that's not a standard or safe way of using or powering a breadboard. This guy obviously like to live on the edge. (Check out some of his other projects, like the broken incandescent bulb lamp.)
It has a certain cuteness. I'd like to see a lot more LEDs since they wouldn't actually use more power, there would just be less waste, and rather than full-wave rectifying why not have half the LEDs in each direction so each is on half time, it would be the same apparant brightness on half the power.
Or, if you don't mind losing the light bulb form factor, there's always the option of a 12VDC switching power supply (and yes, I realize that kills the cool factor, but better than killing a less skilled maker ;) ).
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More LEDs *would* use more power. The voltage drop now is through the capacitor which changes the phase of the current so no (significant) power is lost in the current limiting. Adding LEDs would change that phase relationship so the added LED power would come from the socket - just as conservation of energy would imply.
Full wave rectifying to one string versus two strings in antiparallel gives exactly the same light output and power consumption (except for the voltage drop of the diodes which is small compared to the LEDs). Since four rectifiers are less expensive than another string of LEDs, the present design makes sense.
One precaution of using a capacitor as a current limiter is the assumption that you are using a sinusoidal 50 or 60 Hz supply. If you have a lamp dimmer or run off a 120V inverter (with a "modified sinusoid" output), the harmonic energy will go through the capacitor and increase the LED current, perhaps to destruction. The "Kill-a-Watt" power meter uses a series capacitor in its power circuit and many have been destroyed this way.
LEDs - really cool light in more ways than one...
Jim Horn, WB9SYN/5 Bingen, Washington USA
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So if this way of snatching DC power from an AC line is dangerous or unhappy - does anyone have a bead on a better way? Or something to google to find an inverter (i think that's the word) with a similarly-sized form factor?
- A software guy with a soldering iron
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Try a switched-mode power supply with a high-frequency transformer. Try searching for TNY274GN and you will find some datasheets and possibly app notes.
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The datasheets are slightly over my head - i'll have to do some reading to understand it all. Thanks for the lead!
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As an experiment you can try running the led directly off 110VAC.
I think this might work. Just size the resistor accordingly, you want 10-20 mA. V=IR.
It will flicker at 30 Hz, because it will only light for half of the AC cycle. If you put a pair of LED's anti-parallel you could probably even that out to 60 Hz.
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How do you modify this to run on 220volts? Thanks
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For 220V, cut the capacitor value in half and double its voltage rating (i.e. a 0.22uF 600V non-polarized one should do it).
"you can try running the led directly off 110VAC" - and wonder why it doesn't light up. Most LEDs have a very low reverse voltage rating, so the half cycle when the voltage is reversed they will likely fail permanently. Also, the vast majority of the voltage will be across the resistor, so the vast majority of the energy used will be to heat the resistor, not make light.
But then again, that's how some Feya LED night light bulbs I recently bought work (though they have the 1N4004 diodes for bridge rectification).
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Hey... well it took me a while to notice this blog entry. Although it seems a bit overwhelming in the pessimism department, I'd like to say thanks for the interest and write-up about my project!
Some random "insights" from the creator:
- The seven "ultra-bright" green LEDs I used in (most of) these lights turned out to be way more than enough light output for my intended application of accent and night-lighting. My living room is well illuminated by a single one of these at night.
- The full-bridge rectification was done because it is the "right" way to power LEDs from an AC source. Some people (including myself) are extra sensitive to flickering light, so having the LEDs flashing at 30 to 60 Hz is very irritating and undesirable (unfortunately most designers of LED lighting do not take this into account)
- The capacitor-based "power supply" is indeed only intended for use with 50-60Hz AC power, and these lamps are not intended for use with dimmer switches.
- The use of perforated board (it's not breadboard) in this application is believed to be safe and within the ratings of the material. All of the connections to the perf. board are on the low-current/ low-voltage side of the circuit. All the remaining AC wiring is point-to-point and very well insulated. Also, since both the perf. board and the old CFL ballast casing are constructed of fire-retardant materials, there is little chance of combustion in the event of components failing.
- The goal of this project was to generate LED night/accent lamps that could be constructed very *inexpensively* and *easily*, and used in all sorts of places around the house as "drop-in" replacements for less efficient light bulbs. The original unit I made over 6 months ago now has over 2100 hours of "run-time" on it, and is still working great! Although not the most high-tech, eloquent, or even safest method for making LED lights, these hand-made LED lamps have met my requirements and exceeded my performance expectations.
Hope that helps,
-Nick
http://www.tehhouse.us/
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Introducing the world's first 5 spectrum hybrid led grow panel, the ISIS family of LED grow lights.
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