After reading a lot of messages on here from recent weeks as well as my own troubles with LED christmas lights, I am left with one question. If I was to build a QUALITY 120 VAC light, and want to provide minimal flicker, bulb protection, and maximum light output, what would be the ideal circuit design?
I may be wrong, but it seems that I'd need a bridge rectifier and an electrolytic capacitor in the power supply. Then comes the question of whether there should be a resistor for each LED, and each wired separately in parallel (which would mean a stepdown transformer), or to use series wiring, but with this filtered power supply input.
I'm actually considering building one by hacking apart a set of white xmas lights and rewiring them onto a board, with a reflector from an old auto headlight surrounding it.
But putting cost aside, what is the best way to build this?
Please post a schematic to one of those free photo websites.
Schematics do not just trip lightly from one's fingers -- they take work. And work ain't free.
An off-line switcher circuit, modified to provide constant current to a string of LEDs, would be one candidate for 'best', although not for 'cheapest'.
The biggest part of any 'best' design would be part selection -- most of the schematics are probably OK if you don't use the cheapest of Chinese parts.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Hey, this aint 1950. All I have to do is place the parts in a container, connect my USB powered drill and soldering iron to my computer, and the computer will read the schematic, etch and drill the circuit board, and attach the parts. Then all I have to do is mount the board on a reflector, and plug it in. My computer running Windows98 can do this. Just dont try it with Windows7. You'll end up with a circuit board containing 300 more parts than are needed, just randomly mounted to the board and doing nothing. So much for the latest Microsoft bloated operating systems.
On a serious note, I have built things from schematics. I dont mind the work, but need something that uses available parts. The hardest part of any project is finding parts. However this should not require any unusual parts.
An off-line switcher circuit seems a bit excessive. Kind of follows the MS bloat theory. Lets stick with simple but effective....
The FIRST thing to do is firm-up your requirements. The most important issues are not specified.
Do you want to learn about technology or provide LIGHT? You can learn most of what you need without building anything.
Assuming you want light...
Do you care what it costs? If money is no object, go to Home Depot and buy LED lights. If cost is an issue, go to Home Depot and buy CFL lights.
How much are you willing to invest in a few percentage points of efficiency? Do the math.
Are we done??? Guess not... So, you're determined to build something...
If your leds are all the same and their reverse voltage rating is at least as high as the forward drop, you can make two strings, hook them in reverse parallel. Put that in series with a capacitor sized for the peak current and frequency you want/have. Voila...light.
(note that if you buy unknown leds from an unknown vendor with unknown specs, you have no guarantee that all the lights in the batch will have the same characteristics. 'Bout all you can count on is that they were probably swept off the floor of the "reject devices" room with the same broom...maybe...)
The first design has serious problems with line spikes. Anything that increases the dV/dT of the AC can pop the string in a heartbeat. So, don't turn off your blender.
You can mitigate this with some series resistance. You trade simplicity and efficiency for reliability.
This light will flicker. If that is a problem for you, you're gonna have to go to DC or higher frequency AC Drive. White leds have a phosphor with some persistence. I've never tried to measure the light to see how much that helps.
For DC, you already stated the plan. Split the parallel strings and use a resistor for each string and put them in parallel (not reversed). Use a cap and a rectifier. You fixed the flicker and made the cap or the rectifier the casualty of line spikes. More series resistance on the input side.
By now, you have a working light with poor efficiency, poor reliability, poor safety...the list of "poor" goes on.
Go to eetmag.com. Browse the pages and you'll find advertising for different kinds of chips to power LED lights. Go to the vendor sites and get reference designs.
You can probably build a breadboard of a pretty nice light. But it has problems.... How are you gonna package it so that it's safe, (doesn't electrocute your kids or burn down your house) doesn't radiate noise that shows up on your TV or causes your X10 devices to malfunction, fits in the place you want to put it, dissipates the waste heat safely?
The other problem is where are you gonna get parts? Want 10,000/month, no problem. IF you want 1 part, even if you can get it, by the time you add shipping and handling for three different components from different places, you're gonna spend more than the cost of a REAL commercial LED light.
If you want to have an interesting discussion, call up your fire insurance carrier and explain how you're gonna fill the place with untested, unapproved, uninspected lights tied into the electrical grid. Ask if that affects your fire coverage.
Lights of America has some interesting devices. Can get 'em at Walmart or Costco for about $6 each. They're 2 Watts 110 lumens or so. Stock varies by store, so call ahead. You'll find lots of reliability complaints for LOA stuff, but my experience has been good.
Last year's design has 20 old-style axial leaded LEDs pointing straight up. This year's design has a stalk in the center with 4 vertical strips of surface mount leds along the stalk pointing sideways and 3 more on top pointing up. Somewhat more light output, and much better light dispersion for a typical incandescent replacement. It also has a heat sink in the base suggesting that it's actively managing the power. The light output is probably as good as you're gonna get with a DIY design. Also worth the price to take one apart and see how they do it. I just use 'em, so don't know or care what's inside. I'll take one apart when it quits working.
I have three of them burning 24/7 to light my house. My neighbors call me the "mole", but I find I can easily get from my computer/tv chair to the kitchen for coffee and the bathroom to dump coffee. Only have to turn on the CFL if I want to read something on paper.
Bottom line, building a custom LED to learn about it can be a fun time waster...aka hobby. Building a custom LED lamp or two to get light is a fool's errand. As always, YMMV.
If you read through the relevant parts of the thread on 120 VAC LED lamp =
innards, you will see a number of circuits in LTSpice format that you = can=20 build. But my final post there was to note that there is a Supertex part =
that has flicker-free HF buck converter operation and PWM dimming, = available=20 for less than a dollar at Mouser:
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The data sheet pretty much gives you the schematic and parts list.
X10 devices malfunction in normal operation anyway. The ancient control protocol is too weak to stand up in a noisy environment.
Actually even given a working schematic the design of a mains powered LED based lamp unit to emit useful amounts of light and manage the waste heat thermal issues during operation is something of a challenge.
Only after the house burns down if they trace the fault to your unit.
The one thing they do well on a good day is simulate a spotlight - which is virtually impossible to do with a CFL.
Can you be more specific on exactly which cells you bought? I tried to rebuild a 7.2V B&D screwdriver using cells from a laptop battery. Open cells seem to plague this product. At 5 amps, it takes very little internal resistance to trip the overload and shut it down. Yes, at least part of it has to do with the age of the cells. Researching the subject, It appears that there exist lower capacity
18650 cells with better ESR designed for this type of use, but I couldn't find any specs that I could correlate to the DealExtreme cells.
I'd also be interested in how you rebuilt the VersaPacks. When I contemplated such a project, it wasn't obvious how to get 'em apart without damage.
That's what I was looking for (the data sheet). I did read thru a good part of that thread "120 VAC LED lamp innards". I must say that a thread of over 100 messages and filled with a whole string of bickering and flames is not the easiest thing to digest. I also found a link in there to some wersite that stated that I needed to pay or be a member to download their damn .jpg picture schematic. Of course I know this is usenet and this is expected. amd its still 1000 times more useful than facebook or any of that trash that seems to have become the whole reason most people use a computer these days.
Anyhow, I have learned from all of this, and now pretty much know that the LED light string on my christmas display contains no parts except the actual LEDs and the wiring/sockets. It's no wonder they dont hold up well, particularly when I'm connected to a rural line, connected to many large horsepower motors on farm equipment along with heating high power drawing heating devices, pumps, and more. Maybe what is really needed is a surge supressor and filter cap on the plug end of the lights.
It appears that LED lighting is the future of lighting. Incandescent is being phased out, CFL has always had it's downfalls, (particularly they wont work in unheated buildings in cold weather), not to mention they can catch on fire and contain mercury.
The LED flashlights have really come a long way since they first appeared on the shelves, so why not use them on 120vac. But then again, why not just take them to DC. I actually did some major house remodeling using a 30 LED worklignt, in a house that did not have power. When that worklight got weak, I would plug it into my truck's cig lighter socket to recharge. When I got home, I plugged that same light into a 120v outlet with a wall-wart xformer. I've had that worklight for at least 5 years now and it works great. That is the light setup of the future, it works in a house, car, or can be used in the wilderness when camping. It seems to me that the defect in the current 120vac lights are a lack of converting to DC before it enters the light string. Maybe that's where an improvement can be made. Running them off a wall-wart seems to be the solution, and at the same time a 12vdc auto adaptor can be used.
LED is still in it's infancy. Now it needs to grow.
Unless you have magic eyes, rectify and feed to one of the parts that is designed to provide constant current to LEDs (in series, up to the available voltage). If you can see 120hz "flicker" you have magical eyes, and must go nuts at movies or TV or computer monitors (I'd suppose you must do usenet on a DECWriter terminal or the like...no flicker.)
For free, try this one. Add a bigger capacitor if you need to deal with your magic eyes.
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Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.
To be totally honest, I dont see flicker on even the cheapest LED xmas lights. However, I'd think that using a capacitor would make the whole string brighter since it's continuous, and for the cost of a cap, well worth it.
I still have a CRT tv set, and the flicker does not bother me, but the high voltage high freq. noise does bother me at times. My computer monitor is a LCD. I used to get sore eyes if I was on the computer for hours, this no longer happens. I know it's because I sit closer to the monitor than the tv.
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